Understanding Waves, Reflection and Refraction

Physics Revision

In this lesson you will revisit:

• The basic principles of waves.

• The wave formula.

• Reflection.

• Refraction and Total Internal Reflection.

You need to be able to:

• Describe the parts of a wave.• Know the difference between longitudinal and

transverse waves.• Use the wave equation.• Explain the processes of reflection, refraction and

total internal reflection.• Know how sound waves behave and the uses of

ultrasound.• Describe how seismic waves behave and what they

tell us about Earth’s structure.

Crest / Peak

Time

Trough

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Dis

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Waves : Introduction

a = amplitude = wavelength Frequency is the number of waves that pass a point each second.

Transverse Waves

• Most waves are transverse.

• Transverse waves include light and all other electromagnetic waves, ripples on water, waves on strings and a rope being flicked up and down.

• The vibrations are at 90º to the direction of travel of the wave.

Longitudinal Waves

• The only ones that you need to know are sound waves and earthquake p-waves.

• The vibrations are along the same direction as the wave is travelling.

The Wave Formula

V = x

v

f

x

Reflection of Light

• It allows us to see objects.• From a rough surface, like paper, light

reflects in all directions – DIFFUSE REFLECTION.

• When light reflects from an even surface it’s all reflected at the same angle – CLEAR REFLECTION.

• Angle of incidence = Angle of reflection

Refraction – a property of all waves.

• This is when waves change direction as they enter a different substance.

• It is caused by the change in speed of the waves.

• In water waves slow down in shallower regions.

• Sound waves can also be refracted.

Air to Perspexangle of incidence > angle of refraction

i > r

As the light ray moved from air into perspex it moved towards the normal.

If light rays move from a less dense medium (air) to a more dense medium (perspex) they ‘bend’ towards the normal.

i > r

Perspex to Airangle of incidence < angle of refraction

i < r

As the light ray moved from perspex into air it moved away from the normal.

If light rays move from a more dense medium (perspex) to a less dense medium (air) they ‘bend’ away from the normal.

i < r

Total Internal ReflectionThis angle is called the critical angle [ c]

i < c

Refraction

i = c

Critical case

i > c

Total Internal Reflection

[TIR]

Different materials have different critical angles. Diamond has the lowest at 24º which is why it reflects so much light.

Uses of Total Internal Reflection

• Binoculars.

• Periscopes

• Reflectors

• Optical Fibres

• You will need to know the position of the prisms in binoculars and periscopes.

Which wave below is a longitudinal wave?

A. Ultraviolet

B. Water

C. Light

D. Sound

Which wave below is a longitudinal wave?

What are the units of frequency?

A. Metres

B. Hertz metres

C. Hertz/metres

D. Hertz

A wave has a wavelength of 6m and a frequency of 5Hz. What is the speed of the wave?

A. 1.2 m/s

B. 0.88 m/s

C. 3.0 m/s

D. 30 m/s

Which of the following is not a light source?

A. The Sun

B. A star

C. Traffic lights

D. A book

Which of the following is not seen by reflected light?

A. Your hand

B. Jupiter

C. Light bulb

D. The Moon

Which of the following is the most dense?

A. Air

B. Water

C. Glass

D. Lead

When light changes direction as it moves from one medium to another we call this effect what?

A. Reflection

B. Refraction

C. Diffraction

D. Total internal reflection

What happens to the speed of light as it moves from air into glass?

A. Decreases

B. Increases

C. No effect

D. Decreases and increases

If a ray of light moves from air to glass parallel to the normal what happens?

A. No change in direction

B. It bends away from the normal

C. It bends towards the normal

D. It stops