Reflection and refraction of light
When light meets a boundary it can be reflected, refracted, transmitted or absorbed. This page covers the two most tested: reflection from a surface and refraction as light passes into a new material.
Part 1 of 3: Learn it
In short
- Angles are measured from the normal, a line at 90° to the surface.
- Angle of incidence = angle of reflection.
- Light bends towards the normal when it slows down (air into glass) and away from it when it speeds up.
Where this is in your specification
Spec points: AQA 4.6.1.3 (required practical 9) and 4.6.2.6, Edexcel Topic 5 (core practical), OCR P5.3
| Board | Topic: Lenses, visible light, sound and black bodies |
|---|---|
| AQA 8463 | 4.6.1.3 to 4.6.1.5, 4.6.2.5, 4.6.2.6 and 4.6.3 (8463) |
| Edexcel 1PH0 | Topics 4 and 5 |
| OCR J249 | P5 |
| Eduqas C420 | 6.3 to 6.5, 5.2 |
| Cambridge IGCSE 0625 | 3.2, 3.4 |
| National 5 C857 75 | Waves: refraction of light |
Reflection
Draw the normal where the ray hits the mirror. The incoming ray and the reflected ray make equal angles with the normal. A smooth surface such as a mirror gives specular reflection: parallel rays stay parallel, so you see a clear image. A rough surface such as paper gives diffuse reflection: rays scatter in many directions.
Refraction
Light changes speed when it moves from one material into another. If it meets the boundary at an angle, that change of speed changes its direction. Into a denser material such as glass, it slows and bends towards the normal; back out into air, it speeds up and bends away. A ray along the normal changes speed but not direction.
The frequency stays the same, so as the speed falls the wavelength becomes shorter.
The glass block practical
- Draw round a rectangular glass block on paper and draw a normal on one long side.
- Shine a narrow ray from a ray box at the block where the normal meets it.
- Mark the ray entering and leaving with crosses, remove the block and join them up.
- Measure the angles of incidence and refraction with a protractor. Repeat for several angles.
Work in a darkened room with a thin beam to make the ray easier to mark.
The angle of incidence on a mirror is 28°. What is the angle of reflection?
Show the answer
28°.
Part 2 of 3: See it worked
Worked examples
Example 1
A ray hits a mirror making an angle of 50° with the mirror's surface. What is the angle of reflection?
- Angle from the normal = 90 − 50 = 40°
- Angle of reflection = angle of incidence
Answer: 40°.
Example 2
A ray enters a glass block at 35° to the normal. Will the angle of refraction be more or less than 35°? Explain.
- Light slows down entering glass
- So it bends towards the normal
Answer: Less than 35°: slowing down in the glass turns the ray closer to the normal.
Common mistakes
- Measuring angles from the surface instead of from the normal.
- Saying light bends because it is reflected. Refraction is caused by a change of speed.
- Bending the ray the wrong way when it leaves glass into air.
- Saying a ray along the normal does not refract at all. It still changes speed; only its direction stays the same.
What kind of reflection does a sheet of white paper give?
Show the answer
Diffuse reflection, because its surface is rough.
Part 3 of 3: Test yourself
Check yourself
Answer each one in your head or on paper first, then open it to check.
The angle of incidence on a mirror is 28°. What is the angle of reflection?
28°.
What kind of reflection does a sheet of white paper give?
Diffuse reflection, because its surface is rough.
What happens to the wavelength of light when it slows down in glass?
It gets shorter (the frequency stays the same).
Jobs that use this
Each link opens the job profile on the National Careers Service (England). In the rest of the UK: My World of Work (Scotland), Careers Wales, nidirect careers (Northern Ireland).
Full lessons and marked practice for this course are coming soon to Brainlag Learn. See courses