Reflection and Refraction

British & IGCSE › Checkpoint 8 › Checkpoint 8 Science — Complete Course

1. Key words

2. How light travels

Light travels in straight lines from a source (the Sun, a lamp) until it meets something. When light hits a surface, three things can happen: it can be reflected (bounce off), absorbed (taken in, which warms the surface), or transmitted (pass through, as with glass or water). Often more than one happens at once — a window lets most light through but you can still see a faint reflection in it.

We show the path of light with rays: straight lines drawn with a ruler, with an arrow showing which way the light is going. An arrow pointing the wrong way loses marks in Checkpoint-style questions.

3. Reflection at a plane mirror

To describe reflection we always draw the normal: a dashed line at 90° to the mirror, at the point where the ray hits it. Both angles are measured from the normal, never from the mirror.

Measuring from the mirror — the classic trap. If a question tells you the angle between the ray and the mirror surface, subtract it from 90° to get the angle of incidence. For example, a ray at 30° to the mirror has i = 90° − 30° = 60°, so r = 60° too.

4. Smooth and rough surfaces

Every surface obeys the law of reflection. What changes is how flat the surface is.

Diffuse reflection is why you can read this page from any angle: light from the page is scattered towards your eyes from wherever you sit.

5. The image in a plane mirror

When you look in a mirror, light from you reflects off the mirror into your eyes. Your brain assumes light travels in straight lines, so it traces the rays back behind the mirror and you "see" an image there.

6. Using reflection: the periscope

A periscope lets you see over a wall or out of a submarine. It has two plane mirrors, parallel to each other and each at 45° to the tube. Light from the object hits the top mirror at an angle of incidence of 45°, so it is reflected at 45° — turned through 90° — straight down the tube. The bottom mirror turns it through 90° again, into your eye.

7. Refraction — light changes speed

Light travels at different speeds in different materials. It is fastest in a vacuum (and almost as fast in air), and slower in water and glass.

Refraction happens at the boundary between two transparent materials. When light crosses the boundary at an angle, the change of speed makes it change direction. The rule depends on whether the light slows down or speeds up:

In the diagram the angle of incidence is 50° and the angle of refraction inside the glass is about 31°. When the light leaves the opposite parallel side it bends back by the same amount, so the emergent ray is parallel to the incident ray — it has only been shifted sideways.

Some light is also reflected at every boundary. That is why you see a faint reflection in a shop window while you can also see through it.

8. Why does a change of speed cause bending? An analogy

An analogy is a model that compares something we cannot see with something familiar (8TWSm.01). Imagine a line of students marching side by side, arm in arm, across a road and onto a muddy field, where everyone must walk more slowly.

• If the line meets the mud at an angle, one end of the line reaches the mud first and slows down while the other end is still on the road, moving fast.
• The fast end catches up, so the whole line swings round and changes direction — towards the normal.
• If the line meets the mud head-on (along the normal), everyone slows at the same moment, so the line slows but does not turn.
• Leaving the mud (speeding up) at an angle, the line swings the other way — away from the normal.

The same idea works for a toy car rolling at an angle from a smooth floor onto a carpet: the wheel that reaches the carpet first slows first, so the car turns. Limitation of the analogy: light is not made of marching people and has no wheels — the analogy shows the effect of a speed change, not what light really is.

9. Refraction in everyday life

Light from an object under water speeds up as it leaves the water, so it bends away from the normal. Your eye traces the ray back in a straight line, so the object seems to be higher than it really is.

• A swimming pool looks shallower than it really is — a safety risk for people who jump in.
• A straight straw or pencil in a glass of water looks bent at the water surface.
• Glass lenses in spectacles, cameras and microscopes use refraction to bend light and form images.
• A fish under water is not quite where it seems — a bird diving for it must allow for refraction.

10. Investigating reflection and refraction

Both laws can be investigated with a ray box (a lamp with a narrow slit), a plane mirror or a rectangular glass block, a protractor, a sharp pencil and paper.

1. Draw a straight line for the mirror (or trace round the glass block). Draw the normal at 90° with the protractor.
2. Draw lines from the normal at the angle of incidence you want: 20°, 30°, 40°, 50°, 60°.
3. Shine the narrow ray along each line. Mark the reflected (or emergent) ray with two small crosses far apart, then join them with a ruler.
4. Measure each angle from the normal with the protractor and record it in a table.
5. Repeat each angle and calculate a mean if the readings differ.

Accuracy: use a narrow ray, sharp pencil, crosses far apart, and read the protractor straight on. Work in a darkened room so the ray is easy to see. Risk: the ray-box lamp gets hot — do not touch the bulb, and switch off when not in use.

Anomalous result: a reading that does not fit the pattern. In a reflection experiment, every reflected angle should be close to the incident angle; a pair such as i = 40°, r = 48° is anomalous and should be measured again.

11. Worked examples

Worked example 1 — A ray of light hits a plane mirror. The angle of incidence is 35°. (a) What is the angle of reflection? [1] (b) What is the angle between the incident ray and the reflected ray? [1]

1. (a) Law of reflection: r = i = 35°.
2. (b) The angle between the two rays is i + r = 35° + 35° = 70°.
3. Common mistake: giving 35° for (b). The angle between the rays is made of both angles together.

Worked example 2 — A ray hits a mirror. The angle between the ray and the mirror surface is 25°. Calculate the angle of reflection. [2]

1. The normal is at 90° to the mirror, so angle of incidence = 90° − 25° = 65° (1).
2. Angle of reflection = angle of incidence = 65° (1).
3. Common mistake: answering 25°. Angles are always measured from the normal.

Worked example 3 — Ali stands 1.2 m in front of a plane mirror. (a) How far is his image from the mirror? [1] (b) He walks 0.5 m towards the mirror. How far is he now from his image? [2]

1. (a) The image is as far behind the mirror as he is in front: 1.2 m.
2. (b) New distance to mirror = 1.2 − 0.5 = 0.7 m (1). Image is 0.7 m behind the mirror, so distance from Ali to image = 0.7 + 0.7 = 1.4 m (1).

Worked example 4 — A ray of light passes from air into a glass block. Its angle of incidence is 40°. Which could be the angle of refraction: 0°, 26°, 40° or 55°? Explain your choice. [2]

1. 26° (1).
2. Light slows down as it enters glass, so it bends towards the normal and the angle of refraction must be smaller than 40° — but not 0°, because only a ray along the normal goes straight through (1).

Worked example 5 — Explain why a ray of light that enters a glass block along the normal does not change direction, but a ray that enters at an angle does. [2]

1. In both cases the light slows down when it enters the glass (1).
2. Along the normal, the whole of the light front reaches the glass at the same moment and slows together, so it does not turn; at an angle, one side slows first, so the ray changes direction (1).
3. Common mistake: saying the ray along the normal "does not refract because its speed does not change". The speed does change — only the direction stays the same.

Worked example 6 — A student shines rays at a mirror and records these results. Identify the anomalous result and suggest what she should do. [2]

1. The anomalous result is at i = 40° (r = 47°): the law says r should be about 40°; the others are within 1° (1).
2. She should repeat that measurement (check she measured from the normal and marked the ray accurately) and leave the anomaly out of any conclusion (1).

Practice

All pages in this course

Start here

• Stage 8 Science: The Tests and How to Prepare
• Thinking and Working Scientifically: Models, Investigations and Analysing Results

Biology

• Joints and Antagonistic Muscles
• Blood and Its Components
• Breathing, Gas Exchange and Diffusion in the Lungs
• A Balanced Diet, Energy Stores and Lifestyle
• Aerobic Respiration
• Ecosystems, Bioaccumulation and Invasive Species

Chemistry

• The Structure of the Atom
• Purity, Concentration and Chromatography
• Word Equations, Exothermic and Endothermic Reactions
• The Reactivity of Metals
• Impure Products and Solubility of Salts

Physics

• Speed and Distance–Time Graphs
• Balanced and Unbalanced Forces, and Moments
• Pressure, Particles and Diffusion
• Reflection and Refraction
• Dispersion and Colour
• Magnetic Fields and Electromagnets

Earth and space

• The Earth's Magnetic Field and Earth's Resources
• Climate, Weather and Climate Change
• Galaxies and Asteroids

Practice tests

• Stage 8 Science — Practice Test 1
• Stage 8 Science — Practice Test 2
• Stage 8 Science — Practice Test 3