Waves

Menu• Recap of KS3 Light

• Reflection

• Diffuse & Regular Reflection

• Refraction

• Total Internal Reflection

• Waves

• The Electromagnetic Spectrum

Light Travels in Straight Lines

Which direction is the Sun? Remember

….Light travels in straight lines!

Sun

HOW FAST IS LIGHT?It is the fastest thing in the universe. It travels at

300,000 kilometres per second.

At this speed it can go around the world over 7 times in just one second.

Light Travels Faster Than Sound.

Thunder and lightning start at the same time, but we will see the lightning first.

The light & sound happen at the same time when a firework explodes

We see things because the light is reflected into our eyes

(The sun is different – it produces its own light.)

What is a shadow?

Shadows are places where light is blocked

Summary of Light• Light travels in straight lines• Light travels much faster than

sound• We see things because light is

reflected into our eyes• Shadows are formed when

light is blocked by an object

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Reflection

How is light reflected from a mirror?

Incident Ray

NormalReflected

RayAngle of

IncidenceAngle of

Reflection

Plane Mirror

Is there a law to explain reflection?

Angle of incidence = Angle of reflection

500 500

Regular Reflection

Smooth, shiny surfaces like mirrors give ‘regular’ reflection

Rough, dull surfaces like a table top give ‘diffuse’ reflection. This is where light is scattered in all directions.

Diffuse Reflection

Clear Reflection

Diffuse reflection

What ‘type’ of reflection can you see?

What are mirrors used for?

Periscopes

What are mirrors used for?

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Refraction

RefractionRefraction happens when waves change speed as they travel through a different medium.

The light rays are slowed down by the water and are refracted, causing the pencil to look ‘broken’. What are the 2 mediums in this example?

Why is it so difficult to spear fish?

Diamonds & prisms use the property of refraction.

Air

Glass

Air

Bends towards normal when entering a MORE dense medium

Bends away from normal when entering a LESS dense medium

Displacement

How is light refracted in a glass block?

Air

Air

Glass

What happens if light approaches along ‘NORMAL’?

The light ray isn’t deviated from its position even though it slows down in glass and speeds up in air.

Angle of incidence LESS than critical

Angle of incidence the SAME as critical

Angle of incidence GREATER than critical results in TOTAL INTERNAL REFLECTION

Angle of incidence GREATER than critical angle

Angle of incidence LESS than the critical angle

Total internal reflection can turn a prism into a mirror!

The special property of TOTAL INTERNAL REFLECTION is used in OPTICAL FIBRES

Uses of Optical Fibres include:

• Communication

• Endoscopes (the Magic Eye)

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What is a Wave?

All waves carry energy without transferring matter. Light, infra red, and microwaves all make things warm up (which shows that they can carry energy).

Loud sounds can make things vibrate or move. Even the quietest sound can move your ear drum.

Waves can be….• reflected

• refracted

• diffracted

Waves are split into 2 types:-

LONGITUDINAL (like sound)

TRANSVERSE (like light)

Longitudinal WavesDirection of wave Travel

In Longitudinal waves the VIBRATIONS are along the SAME DIRECTION as the Wave is Travelling

SOUND IS A LONGITUDINAL WAVE

Vibrations

Transverse WavesDirection of Wave Travel

Vibrations are at right Angles to Direction of

Travel

Most Waves are Transverse. LIGHT IS A TRANSVERSE WAVE

This is a Transverse Wave on a ‘Wave Machine’

Describing Waves

Amplitude

Horizontal

Displacement

Vertical

displacement

Amplitude

THE AMPLITUDE shows the displacement of the particles. It is related to energy

Describing WavesVertical

displacement

THE WAVELENGTH is a full cycle of the wave

Horizontal

Displacement

Wavelength

Describing Waves

Time

Vertical

displacement

THE PERIOD is the time taken for 1 complete cycle. (Notice the change in the horizontal axis)

Frequency

The frequency is the number of waves that pass a set point each second

It is measured in hertz (Hz)

The Wave EquationSpeed Frequency & Wavelength are all linked together using the following equation:-

SPEED = FREQUENCY x WAVELENGTH

m/s Hz m

3 waves travel at the same speed but have different frequencies & wavelengths. Complete the Table

Speed(m/s)

Frequency(Hz)

Wavelength

(m)

Wave 1 16 8

Wave 2 32

Wave 3 2

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The Electromagneti

c spectrum

The Electromagnetic SpectrumRadio Waves

Micro-waves

Infra Red

Visible

U.V

X-rays Gamma

rays

• Longest Wavelength

• Lowest Frequency

• Lowest Energy

• Shortest Wavelength

• Highest Frequency

• Highest Energy

The Electromagnetic Spectrum

All E.M. waves have the same properties of light. They can all:

• Travel at the speed of light

• Be Reflected

• Be Refracted

• Travel through a vacuum as a transverse wave

The properties change as the wavelength (or frequency changes) There are 7 basic types:

* Radio waves * Microwaves

* Infra Red * Visible

* Ultra Violet * X-Rays

* Gamma Rays

The E.M Spectrum

Radio Waves

Used mainly for communication

Microwaves Microwaves have 3 main uses:

• for cooking food rapidly

•Satellite Transmission

•Mobile phone communications

Infra Red: •Otherwise known as Heat Radiation. It is given out by all hot objects.

• We feel it on your skin as radiant heat.

•Used for night-vision equipment and in remote controls for TVs.

Some Examples of Infra Red Radiation

Can you think of examples where Infra Red photography is useful to us? Look

at the photographs for clues

Fire Fighters Use Infra Red Cameras to Find People in

Situations Where They Can’t Rely on Their

Eyesight

Visible Light (ROYGBIV)• This is the ONLY light from the E.M. spectrum that our eyes can detect.

• We use colour to send signals.

•They are used in Optical Fibre Digital Communications

The colours of the rainbow:

• Red• Orange• Yellow• Green• Blue• Indigo• Violet

Ultraviolet (U.V.)Used to:•Detect forgeries

•Find hidden security marks

•Make clothes glow ‘whiter than white!’

•Make your teeth appear whiter

•Give us a suntan

•Kill insects

Dangerous – can cause cancer and make you blind!

Ultra Violet Light can cause fluorescence

X-Rays:

We use X-rays to:

•Detect broken bones

•To ‘screen’ bags for bombs, guns etc at airports.

This X Ray is being used to ‘look inside’ the coffin of a mummy

without ‘opening’ it

Gamma Radiation:Used to:

• Kill harmful bacteria in food to keep it fresher for longer.

•Sterilise medical instruments.

•Treat cancer.

Gamma rays are VERY dangerous. In high doses, Gamma rays (along with X-rays and U.V. rays) can kill normal cells.

Complete the Table As Comprehensively As

Possible on the Next Slide

Type of E.M.

Radiation

Source of E.M.

Radiation

Uses of E.M.

Radiation

Hazards of using E.M. Radiation

Radio Waves

Microwaves

Infra Red

Visible

Ultraviolet

X Ray

Gamma Ray

Questions on the E.M. Spectrum

1). Name a type of electromagnetic wave that:

a. Is visible to the naked eye.

b. Is emitted by hot objects

c. Can cause fluorescence.

d. Can pass through dense metals.

2). Give 3 properties common to all electromagnetic waves.

3). Put the following in order of wavelength, starting with the longest: ultraviolet, X-rays, red light, microwaves, infrared.

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