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14.2 Refraction of Light

GCSE Physics · AQA · Chapter 14: Light

A free GCSE Physics lesson for AQA (8463) from OBA School: refraction at a boundary, the refraction rules, partial reflection, (Higher) wavefronts, dispersion by a prism. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Explain that refraction is a change of direction caused by a change of speed at a boundary
  • Describe how a ray bends going from air into glass and from glass into air, and draw ray diagrams to show it
  • Explain why a ray along the normal does not change direction and why the ray leaving a parallel-sided block is parallel to the ray entering it
  • (Higher tier) use wavefronts to explain refraction and use v = f λ to show how the wavelength changes
  • Explain how a prism splits white light into a spectrum

Key words

  • Refraction: The change in direction of a wave when it crosses a boundary between two materials at an angle.
  • Boundary: The surface where two materials meet, e.g. air and glass.
  • Angle of refraction (r): The angle between the refracted ray and the normal.
  • Optically denser: A material in which light travels more slowly (glass is optically denser than air).
  • Partial reflection: When some light is reflected at a boundary while the rest is transmitted.
  • Wavefront: A line joining points on a wave that are in step, e.g. along one crest. (Higher tier)
  • Dispersion: The splitting of white light into its colours because each colour refracts by a different amount.

What refraction is

Light travels at 3.0 × 10⁸ m/s in a vacuum and almost as fast in air. It travels more slowly in water (about 2.3 × 10⁸ m/s) and more slowly still in glass (about 2.0 × 10⁸ m/s). When light crosses a boundary at an angle, this change of speed makes it change direction. This is refraction. All waves can be refracted, including sound and water waves.

  • If the ray meets the boundary along the normal (angle of incidence 0°), its speed changes but its direction does not.
  • At any boundary some light is also reflected (partial reflection). The transmitted wave carries less energy, so it has a smaller amplitude. This is why you see a faint reflection of yourself in a shop window.

Key fact: Refraction is caused by a change of speed. The direction only changes if the light meets the boundary at an angle to the normal.

The refraction rules

Angles of incidence and refraction are both measured from the normal. You can investigate this with a ray box and a rectangular glass block on a sheet of paper.

  • air into glass or water (into an optically denser material); Speed decreases; Direction bends towards the normal; Angles r is smaller than i
  • glass or water into air (into an optically less dense material); Speed increases; Direction bends away from the normal; Angles r is bigger than i
  • along the normal; Speed changes; Direction no change of direction; Angles i = r = 0°

Worked example: a ray through a rectangular block

A ray enters the top face of a glass block with an angle of incidence of 50°. The angle of refraction is 31°.
Inside the block the ray reaches the bottom face. The two faces are parallel, so the angle of incidence there is also 31°.
Going from glass into air, the ray bends away from the normal. The process is simply reversed, so it leaves at 50°.

Answer: the emerging ray is parallel to the incident ray, but shifted sideways

Explaining refraction with wavefronts (Higher tier)

Think of straight wavefronts reaching glass at an angle. One end of each wavefront enters the glass first and slows down, while the rest is still moving quickly in air. The wavefront therefore swings round, and the direction of travel bends towards the normal. Leaving the glass, the end that comes out first speeds up, so the wave bends away from the normal.

The frequency does not change at a boundary: the same number of waves must arrive and leave each second. Because v = f λ, a lower speed with the same frequency means a shorter wavelength.

wave speed = frequency × wavelength v = f λ

v in metres per second (m/s), f in hertz (Hz), λ in metres (m). Rearranged: λ = v ÷ f

Worked example: red light entering glass

Red light has a frequency of 4.5 × 10¹⁴ Hz. Speed in air = 3.0 × 10⁸ m/s; speed in glass = 2.0 × 10⁸ m/s.
In air: λ = v ÷ f = 3.0 × 10⁸ ÷ 4.5 × 10¹⁴ = 6.7 × 10⁻⁷ m
In glass: λ = 2.0 × 10⁸ ÷ 4.5 × 10¹⁴ = 4.4 × 10⁻⁷ m

Answer: frequency unchanged; wavelength falls from 6.7 × 10⁻⁷ m to 4.4 × 10⁻⁷ m

Refraction in everyday life

Light from an object under water bends away from the normal as it leaves the water. Your brain traces the rays back in straight lines, so the object seems to be higher than it really is. Pools look shallower than they are, and a straw in a glass of water looks bent at the surface.

Dispersion by a prism

White light is a mixture of colours. In glass the speed of light depends slightly on its wavelength: violet light slows down the most and red light the least. So when white light enters a triangular prism, each colour is refracted by a different amount and the colours spread out into a spectrum (red, orange, yellow, green, blue, violet). This is called dispersion.

Exam tip: In a prism the two faces are not parallel, so the light does not come out parallel to the incident ray; it is bent twice in the same direction.

Common mistakes

  • Measuring angles from the surface of the block instead of from the normal.
  • Saying light speeds up when it enters glass. It slows down, which is why it bends towards the normal.
  • Saying the frequency changes at a boundary. Frequency stays the same; speed and wavelength change.
  • Bending a ray that meets the boundary along the normal. It goes straight on.
  • Drawing the ray leaving a rectangular block at a different angle to the incident ray. It leaves parallel to it.

Revision questions

Try these, then check your answers below each one.

Question 1. A ray of light passes from air into glass at an angle to the normal. What happens to it? (Choose from: It bends towards the normal, It bends away from the normal, It carries on in a straight line, It is totally reflected.)

Answer: It bends towards the normal

Working: Glass is optically denser than air, so light slows down and bends towards the normal: the angle of refraction is smaller than the angle of incidence.

Question 2. True or false? A ray that meets a glass block along the normal changes direction.

Answer: False

Working: False. The angle of incidence is 0°, so the ray goes straight on (its speed still changes).

Question 3. What causes refraction of light at a boundary? (Choose from: a change in the speed of the light, a change in the frequency of the light, a change in the amplitude of the light, reflection inside the material.)

Answer: a change in the speed of the light

Working: Light travels at different speeds in different materials. The change of speed at a boundary makes the light change direction if it meets the boundary at an angle.

Related lessons

Continue in the full course

This lesson is section 14.2 (AGCPH1402) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.

 

 

 

 

 

 

 

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