1 of 38

Lesson 5 - Reflection and Refraction

(Higher Tier only)

Wavefront, reflection, refraction, transmitted, absorbed

​

2 of 38

Wavefront, reflection, refraction, transmitted, absorbed

​

3 of 38

Wavefront, reflection, refraction, transmitted, absorbed

​

4 of 38

Paste specification here

SEPS classes will cover this in much greater detail during P14

Wavefront, reflection, refraction, transmitted, absorbed

​

5 of 38

  • Print slides 2 & 3
  • Start
    • Recall questions
  • Main
    • Introduce wavefronts and what can happen to a wave when it meets a boundary.
    • Go through principles of reflection
    • Optional reflection demo (probably not needed with tops sets)
    • Learning check Qs
    • Discuss refraction and explain in terms of wavefronts (video)
    • Optional refraction demo (probably not needed with tops sets)
    • Copy and complete paragraph
    • Explain why wavelength changes during refraction.
  • Plenary
    • Exam question
    • Check notes

Wavefront, reflection, refraction, transmitted, absorbed

​

6 of 38

Optional Demo - Reflection

Safety

  • Ray box and accessories, power supply, mirror, wooden blocks to support the mirror, A4 laminated protractor
  • Bulb in the ray box will become very hot.

Aim

  • To show the principles of reflection

Top tips

  • Darken the room.
  • Have the power supply set to 12V
  • Ensure you put the metal blanks on the sides of the ray box.
  • Ensure the mirror is straight.
  • Use the visualiser to demo to the class

​

​

​

Method

  • Place the mirror (supported by wooden blocks) on the A4 laminated protractor along the “mirror” line at a right angle to the normal line.
  • Remind students that light travels as a wave.
  • Show students that when light waves hit the mirror at an angle the are reflected back off at an angle ( You could point out that the angle of incidence = angle of reflection but this is not required knowledge for trilogy groups)
  • Show that when light waves hit the mirror at a zero angle the light is reflected straight back along the same path.

Wavefront, reflection, refraction, transmitted, absorbed

​

7 of 38

Optional Demo - Refraction

Safety

  • Ray box, accessories and power supply or laser pointer. rectangular perspex block.
  • Bulb in the ray box will become very hot.
  • If using the laser do not darken the room so small pupils (eyes, not students!) are maintained!
  • Do not shine the laser into eyes, and be aware of stray reflections..

Aim

  • To show the principles of refraction

Top tips

  • Use a perspex block with one flat face painted white if available.

​

​

Method

  • Discuss what will happen to the speed of the light as it enters the block (slows down)
  • Place the perspex block on a piece of white paper and draw around it.
  • Draw a normal line on one of the long sides.
  • Use the ray box or laser to send light through the block at the point where the normal meets the block (see diagram)
  • Mark the light rays entering and leaving the block with two dots.
  • Remove the block and use the dots to draw in the incident rays and refracted rays.
  • Add a normal line to the point where the refracted ray leaves the block.
  • Draw a line joining the incident ray and refracted ray through the block.
  • Point out that as the light enters the block it slows down and bends towards the normal, and as it leaves the block it speeds up and bends away from the normal line.

Wavefront, reflection, refraction, transmitted, absorbed

​

8 of 38

P12, Lesson 5 - Reflection and Refraction

Answer the following:

  1. Define:
    1. Wavelength
    2. Amplitude
    3. Frequency
  2. Give the units for
    • Wavelength
    • Amplitude
    • Frequency
  3. Give 2 ways frequency can be calculated.

Wavefront, reflection, refraction, transmitted, absorbed

​

9 of 38

P12, Lesson 5 - Reflection and Refraction

Check your answers:

  • Define:
    • Wavelength - The distance from a point on one wave to the same point on the next wave
    • Amplitude - The height of the wave from the rest position to a peak or trough
    • Frequency - The number of waves passing a point every second
  • Give the units for
    • Wavelength metres
    • Amplitude metres
    • Frequency hertz (Hz)

​

Wavefront, reflection, refraction, transmitted, absorbed

​

10 of 38

P12, Lesson 5 - Reflection and Refraction

Check your answers:

3. Give 2 ways frequency can be calculated.

Frequency = 1 ÷ period

Frequency = wave speed ÷ wavelength

Wavefront, reflection, refraction, transmitted, absorbed

​

11 of 38

P12, Lesson 5 - Reflection and Refraction

Learning objectives:

  • To explain how wavefronts are reflected at different angles.
  • To explain how and why waves are refracted in terms of wavefronts.
  • To explain what can happen to a wave when it meets a different medium.

Wavefront, reflection, refraction, transmitted, absorbed

​

12 of 38

What is a wavefront?

Wavefront, reflection, refraction, transmitted, absorbed

​

13 of 38

When waves travel across water they look like this

Wavefront, reflection, refraction, transmitted, absorbed

​

14 of 38

We see them like this

Wavefront, reflection, refraction, transmitted, absorbed

​

15 of 38

When waves move to a different medium

  • What happens to a wave when it meets a material depends on the material itself and the wavelength of the wave.

​

  • Some, all or none of the wave can be:
    • Reflected
    • Refracted
    • Absorbed
    • Transmitted

Wavefront, reflection, refraction, transmitted, absorbed

​

16 of 38

When waves move to a different medium

Some, all or none of the wave can be:

    • Reflected
    • Refracted
    • Absorbed
    • Transmitted
  • Define the words above

Wavefront, reflection, refraction, transmitted, absorbed

​

17 of 38

What is reflection?

What is a ‘normal’ line?

Wavefront, reflection, refraction, transmitted, absorbed

​

18 of 38

Think back to year 8...

Using a ray box and a mirror we can investigate the relationship between the angle of incidence and the angle of reflection.

Can you remember the relationship?

Wavefront, reflection, refraction, transmitted, absorbed

​

19 of 38

Waves at a non-zero angle to the barrier

Wavefront, reflection, refraction, transmitted, absorbed

​

20 of 38

Reflection ray diagram

Angle of incidence = Angle of reflection

Wavefront, reflection, refraction, transmitted, absorbed

​

21 of 38

What happens when waves hit a barrier straight on?

If they are at a zero angle to the barrier they will be reflected straight back

Direction of travel

Incident waves

Wavefront, reflection, refraction, transmitted, absorbed

​

22 of 38

When waves hit a barrier straight on

If they are at a zero angle to the barrier they will be reflected straight back

Direction of travel

Reflected waves

Incident waves

Wavefront, reflection, refraction, transmitted, absorbed

​

23 of 38

Reflected Waves

Wavefront, reflection, refraction, transmitted, absorbed

​

24 of 38

Learning Check:

  1. Sketch a ray diagram to show reflection (use a ruler!) Include: the normal line and labels for the angle of incidence and the angle of reflection.
  2. Explain what happens to waves when they meet a zero angle barrier (straight on).

​

  1. Extension: What are the 4 things that can happen to waves when they meet a different medium?

​

Wavefront, reflection, refraction, transmitted, absorbed

​

25 of 38

Check

1.

​

​

​

​

2. They are reflected straight back along the same path.

3. Some, all or none of the wave can be reflected, refracted, absorbed or transmitted

​

Wavefront, reflection, refraction, transmitted, absorbed

​

26 of 38

What is refraction?

Wavefront, reflection, refraction, transmitted, absorbed

​

27 of 38

Refraction

Angle of Incidence

Angle of Refraction

As light travels into a more dense medium (from air to glass) the light ray bends towards the normal line.

Wavefront, reflection, refraction, transmitted, absorbed

​

28 of 38

Why do waves bend?

Wavefront, reflection, refraction, transmitted, absorbed

​

29 of 38

Waves bend because they change speed

Wavefront, reflection, refraction, transmitted, absorbed

​

30 of 38

Refraction in water (ripple tank)

Water waves slow down as they move from deep water to shallow water.

Wavefront, reflection, refraction, transmitted, absorbed

​

31 of 38

What happens to the wavefronts during refraction?

  • When wavefronts cross a boundary at an angle they change speed and direction.

​

  • The wavefronts slow down when they cross the barrier so the refracted wavefronts are closer together and at a smaller angle to the boundary than the incident wavefronts.

Wavefront, reflection, refraction, transmitted, absorbed

​

32 of 38

Refraction- Copy and complete

  • Waves are refracted when they _____ _____.
  • When waves ____ ____ they bend towards the normal line. This happens when light travels from air to glass or when water waves move from _______ water to _______ water.

​

  • When waves ______ _____ they bend away from the normal line. This happens when light travels from glass to air or when water waves move from _______ water to _______ water.

​

​

Wavefront, reflection, refraction, transmitted, absorbed

​

33 of 38

Refraction- Check

  • Waves are refracted when they change speed.

​

  • When waves slow down they bend towards the normal line. This happens when light travels from air to glass or when water waves move from deep water to shallow water.

​

  • When waves speed up they bend away from the normal line. This happens when light travels from glass to air or when water waves move from shallow water to deep water.

​

Wavefront, reflection, refraction, transmitted, absorbed

​

34 of 38

Wavelength changes during refraction

​

  • Refracted waves and incident waves have the same frequency but they travel at different speeds so they have different wavelengths.

​

  • Remember the equation?

Wavefront, reflection, refraction, transmitted, absorbed

​

35 of 38

Why does wavelength change?

Wavelength = speed ÷ frequency

​

Waves with a frequency of 5 Hz moving at 20m/s in deep water.

Wavelength = speed ÷ frequency

Wavelength = 20 ÷ 5

Wavelength = 4m

​

As the waves move into shallow water the speed slows to 10m/s but the frequency (set at the source of the waves) remains unchanged.

Because the speed has changed, the wavelength also changes.

Wavelength = speed ÷ frequency

Wavelength = 10 ÷ 5

Wavelength = 2m

Wavefront, reflection, refraction, transmitted, absorbed

​

36 of 38

Exam question

​

​

Wavefront, reflection, refraction, transmitted, absorbed

​

37 of 38

Check your notes

Wavefront, reflection, refraction, transmitted, absorbed

​

38 of 38

Logos / symbols

Wavefront, reflection, refraction, transmitted, absorbed

​