Reflection, refraction and total internal reflection Cambridge IGCSE Physics revision
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In plain words
Light bounces off a mirror at the same angle it arrived. When light goes from air into glass it slows down and bends towards the normal (an imaginary line at right angles to the surface); coming out, it speeds up and bends away. If light inside glass meets the edge at a big enough angle, it can't get out at all and reflects back in: total internal reflection.
Three things to know
- Law of reflection: angle of incidence = angle of reflection, both measured from the normal. A plane mirror image is the same size, the same distance behind the mirror, virtual and laterally inverted.
- Light slows down and bends towards the normal on entering glass or water, and bends away from it on leaving. Refractive index n = sin i ÷ sin r.
- Total internal reflection happens when light in the denser material hits the boundary at more than the critical angle. It is used in optical fibres.
Worked example
Light enters a glass block at an angle of incidence of 40° and is refracted at 25°. Find the refractive index.
- n = sin i ÷ sin r.
- = sin 40° ÷ sin 25° = 0.643 ÷ 0.423.
- = 1.5.
Tips and tricks
- Every angle is measured from the normal, never from the surface. Draw the normal first, as a dashed line.
- Check your calculator is in degrees.
It lands in your notebook with its questions as flashcards.
Reflection, refraction and total internal reflection: 6 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
A ray strikes a mirror at an angle of incidence of 40°. What is the angle between the incident ray and the reflected ray?
The angle of reflection is also 40°, so the rays are 80° apart.
What happens to light as it passes from air into glass?
Glass is optically denser, so light travels more slowly in it.
Which describes the image in a plane mirror?
The image cannot be caught on a screen, and it is the same size as the object.
When does total internal reflection happen?
It needs the denser medium and an angle above the critical angle.
What are angles of incidence and reflection measured from?
The normal is the line at right angles to the surface.
Which device relies on total internal reflection?
Light is trapped inside the fibre by repeated total internal reflection.
Quiz
6 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
4 questions, 8 marks. Write your answers on paper, then check them.
Reflection, refraction and total internal reflection
Cambridge IGCSE Physics 0625 · 8 marks · papermunch.org
Name ______________________________ Date ______________
A ray hits a plane mirror with an angle of incidence of 35°. State the angle of reflection.[1]
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35°.
Light enters glass from air at an angle of incidence of 30° and is refracted at 19°. Calculate the refractive index of the glass.[3]
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1.5 (1.54). n = sin 30° ÷ sin 19° = 0.500 ÷ 0.326.
State the two conditions needed for total internal reflection.[2]
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The light must be travelling from a more dense to a less dense material, and the angle of incidence must be greater than the critical angle.
Describe how light travels along an optical fibre.[2]
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It is totally internally reflected each time it meets the edge of the glass, so it stays inside the fibre.
Answers: Reflection, refraction and total internal reflection
- 1. 35°.
- 2. 1.5 (1.54). n = sin 30° ÷ sin 19° = 0.500 ÷ 0.326.
- 3. The light must be travelling from a more dense to a less dense material, and the angle of incidence must be greater than the critical angle.
- 4. It is totally internally reflected each time it meets the edge of the glass, so it stays inside the fibre.