P11.3 Reflection and Refraction

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: reflection, absorption and transmission at a boundary; reflection and refraction of light by a glass block; refraction of water waves; explaining refraction with wavefronts. 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

• Describe what can happen to waves at a boundary: reflection, absorption or transmission
• Describe how a ray of light is reflected and refracted at a glass block, measuring angles from the normal, and why colours spread out
• Describe how plane waves are refracted when they cross from deep to shallow water
• (Higher tier) use wavefront diagrams and changes in speed to explain refraction

Key words

• Wavefront: A line joining points on a wave that are in step, e.g. along the top of one ripple. A ray (direction of travel) is drawn at 90° to the wavefronts.
• Normal: A construction line drawn at 90° to a boundary or barrier where a wave meets it.
• Reflection: A wave bouncing back from a boundary. Speed, wavelength and frequency do not change.
• Refraction: The change in direction of a wave when it crosses a boundary between two media at an angle, caused by a change in speed.
• Transmission: A wave passing through a substance and out the other side.
• Absorption: A substance taking in energy from a wave; the substance warms up and the wave’s amplitude falls.
• Angles of incidence and refraction: The angle between the incoming ray and the normal (i), and between the refracted ray and the normal (r).

Waves at a boundary

When a wave reaches a new substance, three things can happen. Usually some of each happens at once:

• Reflected – it bounces back from the surface (light from a mirror, sound from a cliff as an echo, microwaves from metal). A blue jumper looks blue because it reflects blue light and absorbs the other colours.
• Absorbed – its energy is transferred to the substance, which warms up. As the wave goes deeper its amplitude gets smaller. A microwave oven works because water in food absorbs microwaves.
• Transmitted – it passes through and carries on. Clear window glass transmits most light; microwaves pass straight through a plastic bowl.

Which of these happens depends on the substance (and its surface) and on the wavelength of the wave. For example, a sheet of tinted glass transmits some visible light but absorbs most of it, while a sheet of metal reflects microwaves that would pass through a cardboard box.

Reflection and refraction of light

You can study both effects with a ray box and a rectangular glass block on white paper. Draw round the block, mark the rays with crosses, then join the crosses with a ruler. Draw the normal (a line at 90° to the surface) where the ray enters and leaves. All angles are measured from the normal.

• Reflection: at a mirror or a smooth surface, angle of incidence = angle of reflection.
• Into the glass: light slows down, so it bends towards the normal (angle of refraction smaller than angle of incidence).
• Out of the glass: light speeds up again, so it bends away from the normal. With parallel sides, the ray leaves parallel to the ray that went in.
• Along the normal: a ray that hits the surface at 90° slows down but does not change direction.

Each colour (wavelength) of light travels at a slightly different speed in glass or water, so each is refracted by a slightly different amount. A prism or raindrops therefore spread white light into a spectrum: violet bends most, red least.

Worked example: reading a ray diagram

A ray meets a glass block at 30° to the surface.
The angle of incidence is measured from the normal: 90° − 30° = 60°.
Inside the glass the ray bends towards the normal, so the angle of refraction is less than 60° (about 35° for glass).

Answer: i = 60°, r is smaller than i

Refraction of plane waves

Put a sheet of clear plastic on the bottom of a ripple tank so the water above it is very shallow. Water waves travel more slowly in shallow water. When plane waves cross the boundary at an angle, they change direction: this is refraction.

• speed; What happens decreases
• frequency; What happens stays the same (set by the source)
• wavelength; What happens decreases (λ = v ÷ f, and v is smaller)
• direction (at an angle); What happens bends towards the normal
• direction (at normal incidence); What happens no change – but speed and wavelength still decrease

Going the other way (shallow to deep) everything reverses: the waves speed up, the wavelength increases and they bend away from the normal.

Explaining refraction with wavefronts (Higher tier)

Think of every point on a wavefront moving forwards. When a wavefront meets the boundary at an angle, one end reaches the shallow water first and slows down. The rest of the wavefront is still in deep water and keeps moving fast. So for a short time different parts of the same wavefront move at different speeds, and the wavefront swings round – it changes direction. Once the whole wavefront is in the shallow water, it moves on in a straight line in its new direction.

At normal incidence the whole wavefront reaches the boundary at the same moment and slows down all at once, so its direction does not change.

Worked example: wavelength change at a boundary

A vibrating bar makes waves of frequency 6.0 Hz. They travel at 0.18 m/s in deep water and 0.12 m/s in shallow water.
deep: λ = v ÷ f = 0.18 ÷ 6.0 = 0.030 m = 3.0 cm
shallow: the frequency is still 6.0 Hz, so λ = 0.12 ÷ 6.0 = 0.020 m = 2.0 cm

Answer: the wavelength falls from 3.0 cm to 2.0 cm

Exam tip: Refraction happens because the speed changes. In an exam, link the three ideas: speed changes → wavelength changes (frequency is constant) → direction changes, unless the waves meet the boundary along the normal.

Common mistakes

• Saying the frequency changes when a wave is refracted. Frequency is fixed by the source; the speed and wavelength change.
• Measuring the angles from the boundary instead of from the normal when drawing rays.
• Saying waves always bend when they change speed. At normal incidence they slow down but do not change direction.
• Drawing refracted wavefronts with the same spacing as the incident ones. Slower waves have wavefronts closer together.

Revision questions

Try these, then check your answers below each one.

Question 1. What is refraction? (Choose from: a change in direction of a wave when it crosses a boundary at an angle, because its speed changes, a wave bouncing back from a surface, a wave being absorbed by a substance, a change in the frequency of a wave at a boundary.)

Answer: a change in direction of a wave when it crosses a boundary at an angle, because its speed changes

Working: Refraction is the change of direction caused by a change of speed at a boundary between two media.

Question 2. The diagram shows a ray of light entering a glass block. The dashed line is the normal. Which numbered angle is the angle of refraction? (Choose from: angle 1, angle 2, angle 3, angle 4.)

Answer: angle 3

Working: The angle of refraction is between the refracted ray (inside the glass) and the normal: angle 3. Angle 1 is the angle of incidence.

Question 3. Water waves pass from deep water into shallow water at an angle. Which way do they bend? (Choose from: towards the normal, away from the normal, they do not bend, back the way they came.)

Answer: towards the normal

Working: They slow down in shallow water, so they bend towards the normal and their wavelength gets shorter.

Related lessons

• Previous lesson: P11.2 The Properties of Waves
• Next lesson: P11.4 More about Waves
• In this chapter: P11.1 The Nature of Waves
• In this chapter: P11 RP Required Practical: Waves

Continue in the full course

This lesson is section P11.3 (AGCSC4003) of our free GCSE Combined Science 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 Combined Science course.