Refraction and total internal reflection Edexcel International A Level Physics revision
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In plain words
Light slows down when it enters glass, and that makes it change direction, the way a car slews round if one wheel hits mud first. Going the other way, from glass to air, light at a steep enough angle can't escape at all and is reflected back in, which is how optical fibres carry it for miles.
Three things to know
- Refractive index n = c ÷ v, the speed in a vacuum over the speed in the material. Snell's law: n₁ sin θ₁ = n₂ sin θ₂, with angles measured from the normal.
- The critical angle C is the angle of incidence in the denser medium for which the angle of refraction is 90°. For a boundary with air, sin C = 1 ÷ n.
- Total internal reflection happens when light travelling in the denser medium meets the boundary at more than the critical angle.
Worked example
Light passes from air into glass of refractive index 1.50 at an angle of incidence of 40°. Find the angle of refraction.
- n₁ sin θ₁ = n₂ sin θ₂: 1.00 × sin 40° = 1.50 × sin θ₂.
- sin θ₂ = 0.643 ÷ 1.50 = 0.429.
- θ₂ = 25.4°.
Tips and tricks
- Always measure angles from the normal, not from the surface.
- Total internal reflection needs two things: the light must be in the denser medium, and the angle must exceed the critical angle.
It lands in your notebook with its questions as flashcards.
Refraction and total internal reflection: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
What is the refractive index of a material in which light travels at 2.0 × 10⁸ m/s?
n = c ÷ v.
Light enters a denser medium. Which way does it bend?
It slows down.
What is the critical angle for a medium of refractive index 2.0, in air?
sin C = 1 ÷ 2.0.
At the critical angle, what is the angle of refraction?
The refracted ray runs along the boundary.
Which uses total internal reflection?
Light is trapped inside the glass core.
Quiz
5 questions
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Worksheet
4 questions, 9 marks. Write your answers on paper, then check them.
Refraction and total internal reflection
Edexcel International A Level Physics WPH · 9 marks · papermunch.org
Name ______________________________ Date ______________
Find the speed of light in glass of refractive index 1.50.[2]
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2.00 × 10⁸ m/s.
Find the critical angle for a glass–air boundary, for glass of refractive index 1.50.[2]
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41.8°. sin⁻¹(1 ÷ 1.50).
State the two conditions for total internal reflection.[2]
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The light must be travelling from a medium of higher refractive index towards one of lower refractive index, and the angle of incidence must be greater than the critical angle.
Describe how to measure the refractive index of a glass block.[3]
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Shine a narrow ray of light into the block and mark its path into and out of the block. Draw the normal where the ray enters, and measure the angle of incidence and the angle of refraction. Repeat for several angles of incidence. A graph of sin i against sin r is a straight line through the origin, and its gradient is the refractive index.
Answers: Refraction and total internal reflection
- 1. 2.00 × 10⁸ m/s.
- 2. 41.8°. sin⁻¹(1 ÷ 1.50).
- 3. The light must be travelling from a medium of higher refractive index towards one of lower refractive index, and the angle of incidence must be greater than the critical angle.
- 4. Shine a narrow ray of light into the block and mark its path into and out of the block. Draw the normal where the ray enters, and measure the angle of incidence and the angle of refraction. Repeat for several angles of incidence. A graph of sin i against sin r is a straight line through the origin, and its gradient is the refractive index.