GCSE Physics

Unit 1.5 – Features of Waves

Enw:

Dosbarth:

Athro:

Learner Expectations: Learners should:

• Stick all sheets into their books • Underline Gwaith Dosbarth/Cartref, the date and title with a pencil and ruler • Draw tables and diagrams using a pencil and ruler • Write in blue or black ink • Self or Peer-Assess in Red ink • DiRT and CTG Improvements are to be done in Purple • Score through incorrect work with a single line

Scientists should:

• Draw graphs with a sharp pencil and a ruler • Include correct units ONLY in the headings of tables • Show the full working out when performing calculations • Label graph axis fully including SI units

Marking Criteria – What does it mean?

Sp and underlined

Spelling Mistake Write the corrected spelling 3 times either in the margin or at the end of the piece of work

P and underlined Punctuation Error

// New Paragraph Gr and underlined Grammatical Error

^ Missing Word KV with some vocab

Key Vocabulary Missing Re-write the answer with suggested vocabulary used

O Incorrect use of/or missing Capital Letter RTQ

Read The Question You have clearly missed something key in your answer. Read through again.

EV Evidence is required Asterix and expand the answer SYW

Show Your Workings Go back and write how you got your answer.

? and underlined

Unclear/Confusing Re-write the sentence so that it makes clear sense

WWWT? What is Wrong With This? Check over your answer again and identify where you went wrong.

WW Wrong Word √ Good/Valid Point Made

Ex Develop your answer Re-write your answer with further explanation

√√ Exceptional Point

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________________________ Lesson 1: Features of Waves

Amcanion y wers… Progress

By the end of the lesson, you will… Start End Review

…define wavelength, frequency and amplitude. …label the wavelength, amplitude, crest, trough and rest point. …describe the features of a longitudinal and transverse wave and give examples of each. …calculate the wave speed from wavelength and frequency.

Allweddeiriau

The Mexican Wave Watch the clip of the crowd. How would you describe the movement? Do people move seats? Comment on the direction of movement.

Write some of your ideas below.

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Surfing the wave – life in the ‘Green Room’ In big surf, as the waves start to break, they can start to curve over the top of a surfer, forming what surfers call a ‘tube’. As a surfer moves along the tube, it’s often referred to as being the ‘Green Room’ due to the colour of the sea. This is the ultimate rush for a surfer. It only happens in big surf, and only the best surfers can get into this special place.

Figure 1. A surfer in a wave tube

Waves at sea are formed by a number of different factors, like the topography (shape) of the shoreline and the underlying seabed, but the most dominant factor is the direction and strength of the wind. The wind far out at sea causes the water to ‘peak’ and ‘trough’, forming a swell. As the swell moves onshore and breaks, it forms surf. The best surfing beaches, like Fistral Beach in Newquay, face into the prevailing wind and swell. When surfers are assessing the surf at a beach, they are actually doing some basic physics. The height of the surf is a measure of the amplitude of the waves. More amplitude means more energy, bigger surf and more fun. The time between each wave is related to the frequency of the waves. If the frequency is too high the surf becomes messy, and the wav fronts interfere with each other. The best surf happens when the frequency is very low and the time between the waves is very long – typically 12 to 18 seconds in the UK. The distance between the waves is called the wavelength, which is related to the frequency.

High frequency usually means short wavelengths, and vice versa. In the best surf, the distance between the waves can be up to 50cm. the frequency and the wavelength of the waves are always related to the speed of the waves.

Figure 2. Fistral Bay, Newquay

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How do we describe a wave? There are two types of waves, transverse waves, like water waves, where the direction of motion of the wave is at right angles to the direction of vibration of the wave, and longitudinal waves, like sound, where the direction of motion is the same direction as the direction of vibration of the wave. Transverse waves travel as a series of peaks and troughs, longitudinal waves travel as compressions and rarefactions. Both of these sorts of waves can be demonstrated with a slinky spring as shown below.

Figure 3. Longitudinal and transverse waves on a slinky

The frequency, f, of any wave is the number of waves that pass a point in 1 second. Frequency is measured with a unit called hertz (Hz). Waves at sea have a very low frequency, typically 0.1Hz – which means you only get one every 10s or so. X-rays and gamma rays have incredibly high frequencies. The X-rays given out by the black hole Cygnus X1, for example, have a frequency of about 1018Hz, that is 1000000000000000000 of them arrive every second!

Figure 4. Artist's impression of space around a black hole

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Wavelength, ℷ, is the distance a wave takes to repeat itself over one cycle. It can be measured from one wave crest to the next wave crest, or from one trough to the next trough (see below). The image shows the difference in wave length of two waves travelling at the same speed.

Figure 5. Transverse wave measurements

Because wavelength is a distance, it is measured in metres, m. The wavelength of radio waves used to transmit Radio Five Live on AM is 909m or 693m, depending where you live, whereas the wavelength of the gamma rays used to treat a cancerous tumour can be 10-12m, about a hundred times smaller than the radius of one atom!

Figure 6. These two waves are travelling at the same speed, so the one with the higher frequency has a shorter wavelength

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The amplitude of a wave is a measure of the energy carried by the wave – more energy greater amplitude. The amplitude is measured from the undisturbed (normal) position for the wave to the top of a peak or the bottom of a trough for transverse waves like water waves or the waves in the electromagnetic spectrum. The wave produced during the Boxing Day Tsunami in 2004 had an amplitude of 24m when it hit the shoreline at Banar Aceh in Indonesia!

Figure 7. These two waves have the same frequency and wavelength but different amplitudes

How fast can you go on a water wave? Do all water waves travel at the same speed or do they go faster near the beach than they do out at sea? Do water waves in the laboratory behave any differently? The speed of a water wave is easy to measure. Like the speed of objects, such as cars and runners, wave speed can be determined by measuring the distance travelled in a given time and then calculated using the speed equation:

𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤 𝑠𝑠𝑠𝑠𝑤𝑤𝑤𝑤𝑠𝑠 =𝑠𝑠𝑑𝑑𝑠𝑠𝑑𝑑𝑤𝑤𝑑𝑑𝑑𝑑𝑤𝑤𝑑𝑑𝑑𝑑𝑡𝑡𝑤𝑤

Wave speed is a general property of all waves. All electromagnetic spectrum waves travel at exactly the same speed – the speed of light, which is 300000000m/s (3 x 108m/s). Sound waves travel at about 330m/s at sea level, and ultrasound travels at about 1500m/s through flesh. The seismic waves produced during an earthquake can travel as fast as 5000m/s (5km/s) through hard rock like granite.

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Worked Example Question A surf canoeist takes 12s to travel 48m on a wave crest coming onto a beach. What is her speed?

Answer

𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤 𝑠𝑠𝑠𝑠𝑤𝑤𝑤𝑤𝑠𝑠 = 𝑠𝑠𝑑𝑑𝑠𝑠𝑑𝑑𝑤𝑤𝑑𝑑𝑑𝑑𝑤𝑤𝑑𝑑𝑑𝑑𝑡𝑡𝑤𝑤

= 48𝑡𝑡12𝑠𝑠

= 4𝑡𝑡/𝑠𝑠

Test Yourself 1. A water wave takes 20s to travel 90m between two buoys. What is the speed of the water

wave?

2. An earthquake occurs 16km (16000m) away and it takes 4s for the first seismic wave to

arrive. How fast is the seismic wave travelling?

3. In a thunderstorm, the lightening is seen almost immediately. The thunder, however, travels

much more slowly, at 330m/s. If the time delay between seeing the lightning and hearing the thunder is 6s, how far away is the storm?

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4. The Moon is 384403000m away from Earth. A radio signal is sent to a remote sensor on the

surface of the Moon from a transmitter on Earth. The sensor immediately sends an acknowledgement back to the transmitter. How long does it take in seconds between the transmitter emitting the signal and receiving the acknowledgement?

5. Mobile phone signals travel as microwaves at the speed of light, 3 x 108m/s. If you are 20km

(20000m) away from the nearest phone mast:

a. How long does it take for your signal to get from your handset to the nearest mast?

b. What implications does this have for mobile-phone communications?

c. How do mobile phone companies get around this problem?

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________________________ Lesson 2: Wave Speed

Amcanion y wers… Progress

By the end of the lesson, you will… Start End Review

…calculate how long it will take for a wave to travel from point A to B using Speed = Distance/Time …measure the speed of a wave along a spring.

Allweddeiriau

Label the Wave This is a diagram of a transverse wave. Label the following: Amplitude, Peak/Crest, Trough, Wavelength and the Direction of Wave.

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Measuring the speed of waves Wave speed is the velocity at which each wave crest moves and is measured in metres per second (m/s).

The wave equation A slinky spring can be used to demonstrate waves. If you move the spring quickly from side to side you can set up a wave where the peaks and troughs do not appear to move along the slinky. If you increase the frequency of moving the spring, setting up another wave, then you can clearly see that the peaks and troughs get closer together – there is a direct link between the frequency and the wavelength of the waves. The wave equation directly links wave speed, frequency and wavelength:

𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤 𝑠𝑠𝑠𝑠𝑤𝑤𝑤𝑤𝑠𝑠 (𝑡𝑡/𝑠𝑠) = 𝑓𝑓𝑓𝑓𝑤𝑤𝑓𝑓𝑓𝑓𝑤𝑤𝑑𝑑𝑑𝑑𝑓𝑓,𝑓𝑓 (𝐻𝐻𝐻𝐻) 𝑥𝑥 𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑑𝑑𝑤𝑤𝑑𝑑ℎ, 𝜆𝜆 (𝑡𝑡)

Electromagnetic waves all travel at the same speed – the speed of light, which is 3 x 108m/s. This special number is given its own symbol, v. As wavelength has the symbol ℷ, and frequency, f, then the wave equation (for electromagnetic waves) becomes:

𝑤𝑤 = 𝜆𝜆 𝑥𝑥 𝑓𝑓

Worked Examples Questions A slinky produces waves with a frequency of 2Hz and a wavelength of 0.75m. What is the speed of the waves on the slinky?

A submarine uses sonar with a frequency of 7500Hz to echolocate objects on the seabed. If the speed of sound in seawater is 1500m/s what is the wavelength of the sonar waves?

Answers 𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤 𝑠𝑠𝑠𝑠𝑤𝑤𝑤𝑤𝑠𝑠 = 𝑓𝑓𝑓𝑓𝑤𝑤𝑓𝑓𝑓𝑓𝑤𝑤𝑑𝑑𝑑𝑑𝑓𝑓 𝑥𝑥 𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑑𝑑𝑤𝑤𝑑𝑑ℎ

𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤 𝑠𝑠𝑠𝑠𝑤𝑤𝑤𝑤𝑠𝑠 = 2 𝐻𝐻𝐻𝐻 𝑥𝑥 0.75 𝑡𝑡

𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤 𝑠𝑠𝑠𝑠𝑤𝑤𝑤𝑤𝑠𝑠 = 1.5𝑡𝑡𝑠𝑠

𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤 𝑠𝑠𝑠𝑠𝑤𝑤𝑤𝑤𝑠𝑠 = 𝑓𝑓𝑓𝑓𝑤𝑤𝑓𝑓𝑓𝑓𝑤𝑤𝑑𝑑𝑑𝑑𝑓𝑓 𝑥𝑥 𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑑𝑑𝑤𝑤𝑑𝑑ℎ

Rearranged

𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑑𝑑𝑤𝑤𝑑𝑑ℎ = 𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤 𝑠𝑠𝑠𝑠𝑤𝑤𝑤𝑤𝑠𝑠𝑓𝑓𝑓𝑓𝑤𝑤𝑓𝑓𝑓𝑓𝑤𝑤𝑑𝑑𝑑𝑑𝑓𝑓

= 1500 𝑡𝑡/𝑠𝑠7500 𝐻𝐻𝐻𝐻

= 0.2 𝑡𝑡

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Test Yourself 1. Whales can communicate across vast oceans, often over thousands of kilometres. They do

this by generating very low-frequency sound waves at high energy. A typical whale song has a frequency of 3 Hz but a wavelength of 500 m. What is the speed of the whale song in seawater?

2. An oscilloscope measures the frequency of an electrical signal to be 50 Hz with a wavelength

of 0.2 m. What is the speed of the signal wave?

3. An oboe makes a musical note with a frequency of 200 Hz and a wavelength of 1.65 m. What

is the speed of the sound?

4. The bright red light produced by a laser pointer has a wavelength of 6 x 10-7 m and a

frequency of 5 x 1014 Hz. What is the speed of the light?

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5. Test-match cricket is transmitted by BBC Radio 4 Longwave with a wavelength of 1500 m. The radio waves travel at the speed of light (3 x 108 m/s). What is the frequency of Radio 4 Longwave?

6. A surfer is watching the surf on a beach. She counts 20 waves hitting the shore in 5 minutes.

She estimates that the waves are travelling at a wave speed of 4.5 m/s. Estimate the wavelength of the waves.

7. Seismic waves can have low frequencies, typically 25 to 40 Hz. The speed of seismic waves

in granite is 5000 m/s but only 3000 m/s in sandstone. During an earthquake in a region of sandstone and granite, what would be the shortest and longest seismic wavelengths recorded?

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Past Paper Questions 1) The data in the table shows how the speed of water waves changes with the depth of water.

a)

i) Use the data in the table below to plot a graph showing the variation of wave speed with depth of water.

[3]

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ii) Describe how the wave speed changes with the depth of water.

[2]

b) Use your graph to answer the following questions. Water waves produced by a wave machine in a swimming pool have a wavelength of 8.1 m where the depth of water is 3.0 m.

i) Use a suitable equation to calculate the frequency of these waves in the pool.

[3]

ii) As the waves travel from A to B in the pool, their frequency remains constant. Explain what happens to their wavelength.

[2]

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Lesson 3: Specified Practical – Speed of water waves

Amcanion y wers…

By the end of the lesson, you will…

…investigate the speed of a water wave.

Allweddeiriau

Test Yourself

1. A surfer takes 10s to travel 50m on the crest of a wave onto a beach. What is her speed?

2. The wavelength of the waves in Question 1 is 40m. What is the frequency of the waves?

3. Calculate the speed of sound waves travelling through wood with a frequency of 5kHz and a

wavelength of 79.2cm.

4. Explain the difference between a transverse and a longitudinal wave.

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Specified Practical Introduction The speed at which water waves travel through water depend on a number of factors. In this experiment, you will be deciding on one possible factor to investigate. This will be your independent variable. The following relationships may help you to decide how you will measure the speed of the waves you are investigating.

𝑠𝑠𝑠𝑠𝑤𝑤𝑤𝑤𝑠𝑠 = 𝑠𝑠𝑑𝑑𝑠𝑠𝑑𝑑𝑤𝑤𝑑𝑑𝑑𝑑𝑤𝑤 𝑑𝑑𝑓𝑓𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑠𝑠 ÷ 𝑑𝑑𝑑𝑑𝑡𝑡𝑤𝑤 𝑑𝑑𝑤𝑤𝑡𝑡𝑤𝑤𝑑𝑑.

𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤 𝑠𝑠𝑠𝑠𝑤𝑤𝑤𝑤𝑠𝑠 = 𝑓𝑓𝑓𝑓𝑤𝑤𝑓𝑓𝑓𝑓𝑤𝑤𝑑𝑑𝑑𝑑𝑓𝑓 × 𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑑𝑑𝑤𝑤𝑑𝑑ℎ.

Equipment

• Tray • Stopwatch • Ruler • Beaker • Something to make waves in the tray • Water

Figure 8. Apparatus for measuring waves

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Method

1. Measure the length of the tray and record the result. 2. Add water to the tray to give a depth of 0.5cm and record the volume used. 3. Lift the end of the tray up a few cm and gently replace on the desk. 4. Start the stopwatch when the wave produced hits the end of the tray. 5. Record how long it takes the waves to travel 3 lengths of the tray. 6. Repeat steps 3-5 four more times. 7. Repeat steps 2-6 increasing the depth each time by 0.5cm up to 3.0cm.

Risk Assessment

One hazard for this investigation has been identified. Complete the risk assessment to clarify what the risk is, and then what can be controlled to ensure that the hazard and risk is limited.

Hazard Risk Control Measure

Wet floors are slippery

Results Table

Below is a suitable results table for this practical, however, there are vital aspects missing. Can you identify these and add them to the table?

Depth of water

Length of tray

Time taken for waves to travel three lengths of the tray (s) Mean Speed Trial 1 Trial 2 Trial 3 Trial 4 Trial 5 Mean

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Analysis

Calculate the mean speed of the waves using:

𝒎𝒎𝒎𝒎𝒎𝒎𝒎𝒎 𝒔𝒔𝒔𝒔𝒎𝒎𝒎𝒎𝒔𝒔 =𝒔𝒔𝒅𝒅𝒔𝒔𝒅𝒅𝒎𝒎𝒎𝒎𝒅𝒅𝒎𝒎𝒎𝒎𝒎𝒎𝒎𝒎𝒎𝒎 𝒅𝒅𝒅𝒅𝒎𝒎𝒎𝒎

Plot a graph of your results. You will need to plot depth against speed. Once you have drawn your graph. Stick it in the space below.

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Questions 1. Why is it important to ensure that your container is on a level surface if you are

investigating the effect of water depth on wave speed?

2. Might using salt water rather than fresh water make any difference to the speed of waves

on water? Give a reason for your answer.

3. Think about your experimental procedure and the measurements you took.

a. What are the main sources of error in your experiment? b. How could you reduce these errors?

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________________________ Lesson 4: Reflection

Amcanion y wers… Progress

By the end of the lesson, you will… Start End Review

…draw a labelled diagram of a reflected light ray. …state the law of reflection. …investigate the reflection of light.

Allweddeiriau

Exam Practice 1) Waves are shown on the grid below.

a) Write down the amplitude of the waves.

[1]

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b) Write down the wavelength of the waves.

[1]

c) If 10 waves are produced in 5 seconds, calculate their frequency.

[2]

d) Use the equation to calculate the speed of the waves and state the unit.

[3]

e) Complete the sentence below. Select the correct statement in the brackets.

If the wave amplitude was doubled, the speed of the waves would (double / stay the same / halve).

[1]

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Reflection of waves Reflection is a fundamental property of all waves. When straight (plane) wave fronts hit a flat barrier, they rebound off, obeying the law of reflection. The image shows this happening in a ripple tank.

Figure 9. Water waves reflecting off a plane barrier in a ripple tank

A diagram showing how this happens is given below.

Figure 10

The imaginary rays, drawn at right angles to the wave-fronts, show the direction of travel of the wave-fronts. The angles between the incident and reflected rays and the normal line (an imaginary line at right angles to the barrier/mirror) are equal, obeying the law of reflection, where:

𝑤𝑤𝑑𝑑𝑤𝑤𝑤𝑤𝑤𝑤 𝑜𝑜𝑓𝑓 𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑠𝑠𝑤𝑤𝑑𝑑𝑑𝑑𝑤𝑤 = 𝑤𝑤𝑑𝑑𝑤𝑤𝑤𝑤𝑤𝑤 𝑜𝑜𝑓𝑓 𝑓𝑓𝑤𝑤𝑓𝑓𝑤𝑤𝑤𝑤𝑑𝑑𝑑𝑑𝑑𝑑𝑜𝑜𝑑𝑑

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Practical - Investigating the reflection of light

Equipment • Protractor • Sheet of plain white paper • Sharp pencil • Raybox • 12V power supply • Plane mirror

Safety Notes • The raybox bulb gets hot – do not touch.

Figure 11. Investigating the reflection of light from a plane mirror

Method 1. Set up the apparatus as shown above. 2. Use a protractor to measure our 100 angles of incidence on the paper – mark these on the

paper using a pencil. 3. Shine the raybox down each angle of incidence. Record the position of each corresponding

angle of reflection. 4. Measure ad record the angle of reflection for each reflected ray.

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Analysing your results • Plot a graph of angle of reflection [y-axis] against angle of incidence [x-axis]. • Use your graph to confirm the law of reflection – the relationship between the angle of

incidence and the angle of reflection. • Stick your graph in the space below.

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________________________ Lesson 5: Refraction

Amcanion y wers… Progress

By the end of the lesson, you will… Start End Review

…investigate refraction. …explain how refraction causes water to appear more shallow than it is.

Allweddeiriau

Mirror Maze Can you work out the reflections and show the path of the laser light to the detector. Not all mirrors are used. You will need to write the angles of incidence and reflection if the mirror is used.

Mirror

Detector

Mirror

45°

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Refraction of Waves When water waves travel from deep water into shallow water, they slow down and the wave-fronts get closer together, decreasing their wavelengths. This effect is called refraction when the wave-fronts hit the boundary between the deeper water and the shallow water at an angle they appear to change direction. This is shown in the image.

Refraction is also a general property of waves, and occurs when any waves travel across the boundary from one medium where they travel faster to another medium where they travel slower (or vice versa). The refraction of light through a glass block shows the rays of light changing direction as they go from the air into the glass, and then back again.

Figure 13. The refraction of light through a glass block

The image below is a diagrammatic version with the angles labelled.

Figure 14. The angle of incidence and the angle of refraction

Figure 12. The refraction of water waves in a ripple tank

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Practical - Investigating Refraction

Equipment

• Raybox • 12v power supply • Glass or Perspex rectangular block • Protractor • Sheet of plain white paper • Sharp pencil

Safety Notes

• The raybox will get hot – do not touch.

Method

1. Set up the apparatus as shown in the diagram. 2. Draw around the block. 3. Use a protractor to measure our 100 angles of incidence on the paper – mark these on the

paper using a pencil. 4. Shine the ray from the raybox down each angle of incidence. Record the position of each

corresponding exit ray. 5. Connect each exit ray to the point where the incident ray enters the block. 6. Measure each corresponding angle of refraction.

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Analysing your results

• Plot a graph of angle of refraction [y-axis] against angle of incidence [x-axis]. • There is no simple relationship between the angle of refraction and the angle of incidence as

there is with reflection – describe the pattern shown on the graph. • Stick your graph in the space below.

As a general rule, if waves such as light waves travel from a material in which they are travelling fast to a materials where they are travelling slower, they will bend towards the normal line (and vice versa).

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________________________ Lesson 6: The Electromagnetic Spectrum

Amcanion y wers… Progress

By the end of the lesson, you will… Start End Review

…list the members of the electromagnetic spectrum. …describe the properties of all electromagnetic waves. …give uses and dangers for radiowaves, IR and visible light.

Allweddeiriau

Exam Practice 1. Small water waves are created in a ripple tank by a wooden bar. The wooden bar vibrates up

and down hitting the surface of the water. Figure 9 shows a cross-section of the ripple tank and water.

(a) Which letter shows the amplitude of a water wave? Circle one letter.

J K L

[1] (b) Describe how the wavelength of the water waves in a ripple tank can be measured accurately.

[2]

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The electromagnetic spectrum The photos shown below are all of exactly the same thing – the Sun.

Figure 15. The visible and hidden Sun - what we can see and what we can't

The pictures have been taken with a range of different terrestrial and space-based telescopes and cameras, using different parts of the electromagnetic (em) spectrum. We cannot see most of the electromagnetic spectrum – our eyes can only detect visible light, and our skin can detect some infra-red. The photos show the intensities of the different parts of the spectrum in false colour. Higher-energy parts tend to be brighter colours than lower-energy parts.

The electromagnetic spectrum is a family of waves with several things in common. All electromagnetic waves:

• Travel at the same speed in the vacuum of space (the speed of light, 3 x 108 m/s) • Transfer energy from place to place • Can transmit information • Raise the temperature of the material that absorbs them • Can be reflected and refracted.

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The vast reaches of the Universe are continually bathed in all parts of the spectrum. Very hot, super-massive objects like stars, black holes, neutron stars and galactic centres, all produce huge amounts of waves from all the different parts of the spectrum. In fact, the hotter and more energetic the object is, the higher the energy of the electromagnetic waves that can be emitted, colder lower-energy objects, like planets, nebulae (gas clouds) and the background space of the Universe, only emit lower-energy electromagnetic waves like radio waves, microwaves and infra-red. The complete electromagnetic spectrum is shown below.

Figure 16. The electromagnetic spectrum

Radio and TV Waves Radio waves have the longest wavelengths, lowest frequencies and lowest energies. They are emitted from a wide range of objects in space. Stars, nebulae, comets, planets and galaxies all emit radio waves. Radio signals from space are particularly useful when astronomers are looking at relatively low-energy and low-temperature objects. They are particularly good for studying the structure of nebulae produced by exploding supernovae – it’s in these huge gas clouds that new stars are forming. Radio waves can be used on Earth to

transmit communications signals. TV and radio signals are produced by aerial transmitters and picked up by aerial receivers. They can be relayed across the planet by geostationary satellites, allowing a global communications network.

Figure 18. The Eagle Nebula

Figure 17. These appliances rely on Radio and TV signals

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Microwaves The Universe was formed about 13.5 billion years ago, as the result of a huge explosion known as the Big Bang. The explosion produced high-energy gamma rays that filled the Universe. Over billions of years, the Universe has expanded and cooled, and the gamma rays produced at the time of The Big Bang have also ‘cooled’ (lost energy). As they have lost energy, the gamma rays have turned progressively into X-rays, then ultraviolet, then visible light, infra-red and finally microwaves. When microwave telescopes study the background radiation of the Universe, they find huge amounts of microwaves, left over from The Big Bang – the Universe’s equivalent of a smoking gun!

Microwaves are also used in microwave ovens. In a microwave cooker, the waves come in from the top. They are reflected off the metal sides onto the food to be cooked. The glass door has a metal

mesh in it. This stops the waves from escaping, which could be harmful. The frequency is chosen so that the microwaves penetrate the food and energy is transferred (mostly) to the water molecules in it. As a result the food is cooked quickly and evenly from the inside. Normal cooking heats from the outside and it takes some time for the heat to travel to the centre of the food. The radio and TV waves used to communicate with satellites are microwaves with wavelengths slightly shorter than radio waves. Mobile phones use microwaves too, and just like TV transmitters, mobile-phone signals depend on a good line of sight.

Figure 19. NASA's Cosmic Background Explorer searches for microwave radiation from outer space

Figure 20. A microwave oven

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Infra-red waves Our atmosphere only lets a small amount of infra-red through it. Infra-red telescopes on Earth are, therefore, quite limited. In order to get better infra-red signals, infra-red detectors have to be mounted onto telescopes in low-Earth orbit, to get above the influence of our atmosphere. One such example is the Herschel Space Observatory. Infra-red detectors have to be cooled to extremely low temperatures and shielded from the infra-red radiation produced by the Sun. Infra-red can pass through thick dust clouds (nebulae) in space, so infra-red telescopes are particularly good at observing star-forming regions and looking into the centre of our galaxy. Cool stars and cold interstellar nebulae, which are invisible in optical light, are also imaged in infra-red.

Figure 22. Infra-red image of the Helix nebula

Figure 23. Firefighters use infra-red cameras to search for people in smoke-filled buildings

We know and feel infra-red radiation as heat radiation, particularly in very hot objects like fires or from the Sun. Everything that is above a temperature of absolute zero (-2730C) emits infra-red radiation. Infra-red radiation, in itself, is not dangerous, so long as you do not get too much of it. If you stand in front of a bonfire for too long, the radiated energy will not only warm you up, it could burn you. Infra-red cameras detect heat. They are used by the fire service to find people in smoke-filled buildings and by police helicopters to locate suspects at night. Infra-red cameras show which houses are well insulated and which are not.

Figure 21. The Herschel Space Observatory

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Visible Light Our Sun produces vast quantities of visible light from its visible surface called the photosphere. The warm, yellow-white light that we see is actually a complete spectrum of colours, first studied by Isaac Newton in 1704. Other stars appear to be different colours due to their temperatures. The largest, hottest stars are massive blue supergiants like Rigel in the constellation of Orion. In the same constellation is Betelgeuse – a huge red supergiant star. The Sun is our main source of both light and heat. Its energy keeps us warm and is essential to life. Plants use visible light for photosynthesis to make their own food and oxygen it is the only art of the electromagnetic spectrum that we can detect with our eyes.

Figure 25. The Sun's energy enters our food chain via plants

Figure 24. A prism is used to show the colours that make up visible light

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1) Infra-red, radio waves and microwaves are types of electromagnetic radiation used in long-distance communication.

a) Complete the table below by selecting from infra-red, radio waves or microwaves.

[3]

b) Which of the three types of radiation given above had the longest wavelength?

[1]

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________________________ Lesson 7: The Electromagnetic Spectrum Continued

Amcanion y wers… Progress

By the end of the lesson, you will… Start End Review

…give uses of UV, X-Rays and Gamma Rays. …describe the dangers of these EM waves.

Allweddeiriau

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Ultraviolet radiation Ultraviolet (UV) radiation is produced by hot, highly energetic objects such as:

• Very massive, bright young stars, like the Pleiades star cluster in the constellation of Taurus

• Super-hot white dwarf stars such as Sirius, the Dog Star, in the constellation of Canis Major.

• Active galaxies like Centaurus A.

Most UV radiation coming from space is absorbed by our atmosphere, so UV astronomers need to put UV telescopes aboard satellites in Earth orbit, such as the Extreme Ultraviolet Explorer (EUVE), which operated from 1992 to 2001.

Figure 27. Ultraviolet images of a) the Pleiades cluster and b) the Centaurus A galaxy

At the end of the electromagnetic spectrum, the waves become increasingly dangerous. As the wavelengths gets shorter, the frequency increases. As the frequency increases, so does the energy in the radiation. The ultraviolet radiation that does get through our atmosphere damages the skin because the radiation has enough energy to ionise atoms in the skin cells. A sun tan shows that your skin has already been damaged. Sometimes ionising radiation can cause cells to mutate. This can lead to cancer.

Figure 28. Skin cancer is usually triggered by damaging ultraviolet radiation

Figure 26. The Extreme Ultraviolet Explorer

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X-rays X-rays are produced by the most energetic and hottest objects in the Universe. Black holes, neutron stars and the huge explosions of dying super-massive stars (supernovae) are all emitters of X-rays. The X-rays are often produced by material moving at extremely high speed. Black holes produce lots of X-rays, as the matter surrounding the black hole is sucked inwards by the huge force of gravity. As the matter accelerates into the black hole, it emits high energy X-rays in a beam that can be used to detect the presence of the black hole. X-rays are absorbed by our atmosphere, so any X-ray astronomy needs to be carried out on-board orbiting space telescopes such as the Chandra X-ray Observatory, launched by the space shuttle Columbia in 1999.

X-rays are also ionising (like UV) and overexposure to them can cause cancer. However, they are widely used in medicine where the benefits of the X-ray greatly outweigh the dangers from it. They can be used in carefully controlled conditions to cure cancers. Very powerful X-rays are also used to detect flaws and fractures in metals.

Figure 30. The Chandra X-ray observatory in orbit

Figure 29. An angiography machine uses X-rays to diagnose heart conditions in patients

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Gamma rays On Thursday, 23 April 2009, the Swift Gamma-Ray Burst Telescope, in orbit around the Earth, detected by the most distant object ever observed. Gamma-Ray Burst (GRB) 090423, a 10-second burst of high-energy gamma rays, was imaged and confirmed by other telescopes to be over 13 billion light years away. In fact, the explosion that produced this burst of gamma rays occurred only 600 million years after the Big Bang – about 5% of the age of the Universe! Astronomers thinking that GRB090423 was a huge star exploding as a supernova, producing a supermassive black hole.

Find out more about the Swift Gamma-Ray Burst Mission at; http://swift.gsfc.nasa.gov/index.html

Gamma radiation also comes from the nuclei of radioactive materials such as uranium. Gamma rays, like X-rays and UV, are ionising, causing atoms to become charged and, so are very dangerous to all living things. They can cause cancers or kill cells directly. Like X-rays, they are used to detect flaws in metals. They can also be used to image and treat cancer, to sterilise medical instruments and to check people and vehicles at ports for illegal imports of radioactive materials. The word ‘radiation’ can be applied equally to electromagnetic waves or to the energy given out by radioactive materials.

Figure 33. A medical gamma ray camera Figure 34. Image from a medical gamma ray camera

Figure 31. Artist's impression of GRB090423 showing the gamma ray emissions in orange/yellow (centre) and two foreground stars (top right and bottom left)

Figure 32. Artist's impression of the Swift GR telescope

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Test Yourself 1. Which part of the electromagnetic spectrum has:

a. The longest wavelength?

b. The highest frequency?

c. The least energy?

2. Which parts of the electromagnetic spectrum are missing from this list?

Gamma UV Visible Radio

3. State and explain which parts of the electromagnetic spectrum are used in hospitals.

4. The sun emits all parts of the electromagnetic spectrum. What does this tell you about the

temperature of the sun?

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5. Why are some telescopes put into orbit?

6. What information does the electromagnetic spectrum give us about astronomical objects?

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1) a) Complete the diagram below. Draw a line from each type of wave on the left to show its

correct position in the electromagnetic (EM) spectrum. Draw four lines only. One has been done for you.

[3]

b)

i) Microwave radiation is an EM wave in the wavelength range of 0.1 cm to 30 cm. State one possible wavelength for a radio wave.

[1]

ii) State one property that is the same for radio waves and microwaves.

[1]

44

________________________ Lesson 8: Using Microwaves for Communication

Amcanion y wers… Progress

By the end of the lesson, you will… Start End Review

…describe how microwaves are used for satellite communication. …compare the use of radiowaves and microwaves for communication.

Allweddeiriau

Remember the EM Spectrum Create a pneumonic to help you remember the order of the Electromagnetic Spectrum. There is an example included below:

Rabid Monkeys

In Velvet

Underpants eXcrete

Gummy Bears

Radiowaves Microwaves Infra-red

Visible Light Ultra Violet

X-Ray Gamma Ray

45

Communicating using microwaves Mobile phones use microwaves. Microwaves are wireless signals – you don’t need a copper cable or an optical fibre. One disadvantage if using microwaves is that there must be a clear path between the transmitter and the receiver, which might be your television aerial or mobile phone. To cover the largest area, television and mobile phone transmitters are tall and sited on hills. The curvature of the Earth means that repeater stations have to relay the microwave signal to distant trasmitters. Satellites must be used for long-distance communications around the world. Theoretically, only three satellites are needed to transmit signals around the world. In practice, more are used.

The satellites are placed in orbit at a height of 36 000 km. They orbit the Earth, above the equator, exactly in time with the Earth’s rotation. This is called a geosynchronous (geostationary) orbit. Here in the UK, TV, phone, fax and data signs are sent to satellites from one of three BT stations. The Madley Communications Centre near Hereford is the largest Earth station in the world, and most of the UK’s satellite communications pass through it.

Figure 36. Satellite dishes at the Madley Communications Centre

Figure 35. The Earth from above the North Pole; three geosynchronous satellites could send signals to most of the Earth

46

Test Yourself 1. What type of electromagnetic radiation is used by mobile phones for communications?

2. Communications satellites are normally put into geosynchronous orbit. Explain what this

means with the help of a diagram.

3. Explain why repeater stations are needed for long-distance communication by microwaves.

4. Satellites must be used for long-distance microwave communication around the world. Draw

a simple diagram to show how this is possible.

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1) In an answer to a recent exam question, a candidate wrote: ‘A SINGLE geostationary satellite stays in the same place and is the only way of relaying all electromagnetic waves around the world.’ Explain, in detail, what is wrong with the above statement.

[6]

48

2) A geosynchronous (geostationary) satellite orbits high above the Earth. Geosynchronous satellites are used for relaying television programmes to our homes. It takes 0.24s for a signal to get to the satellite from Earth.

a) Which three statements below are correct about this satellite.

(a) It stays above the same point on the Earth at all times. � (b) It relays radio waves. � (c) It orbits the Earth once in 365 days. � (d) It orbits the Sun once in 1 day. � (e) It relays microwave signals. � (f) It orbits above the equator. �

[3]

b) State why a signal sent from the television studio by satellite takes 0.48s to reach your house.

[1]

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3) A mobile phone network uses microwaves to transmit signals between mobile phones and masts. The microwaves have a frequency of 1.5 GHz and travel at a speed of 3 x 108 m/s. the maximum distance that a phone can be from a mast and still receive a signal is 35 km.

a) Select and use a suitable equation to calculate the wavelength of the microwaves.

[3]

b) Select and use a suitable equation to calculate the maximum time for a signal to travel from a phone mast 35 km away.

[3]

50

4)

a) A table of the electromagnetic (EM) spectrum is shown below.

i) Complete the first column of the table to show the missing ionising regions in order of decreasing frequency.

[2]

ii) Typical wavelength ranges for each region of the EM spectrum in metres are listed below in a random order.

Use these values to complete the wavelength column in the table. [2]

b) One ionising region of the EM spectrum has wavelengths in the range of 4 x 10-7 to 1 x 10-9

m. select and use a suitable equation to calculate the maximum frequency of this region of the EM spectrum. The wave speed of EM waves is 3 x 108 m/s.

[3]

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Glossary of terms Amplitude – The amplitude of a wave is measured from the wave axis to the top of a crest or the bottom of a trough.

Frequency – The frequency of a wave is the number of complete waves.

Wavelength – The wavelength of a wave is the distance that a wave takes to repeat itself over one cycle.

Transverse – Transverse waves are waves where the direction of vibration is at right angles to its direction of travel.

Longitudinal – Longitudinal waves are waves where the direction of vibration of the wave is the same direction as its direction of travel.

Black Hole – A black hole is the remnant of a massive supernova. The density of the remnant is so huge that the gravitational field prevents light from escaping.

Nebula – A nebula (plural Nebulae) is an interstellar (that is located between stars) cloud of gas and dust.

Supernova – A supernova (plural Supernovae) is the huge explosion that happens when a red supergiant collapses under its own weight as it runs out of nuclear fuel.