Refraction Worksheet

GCSE Physics — Waves

SimpliPhys
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Part 1 — Investigating Snell's Law

Aim

To investigate the relationship between the angle of incidence and the angle of refraction when light passes from one medium to another, and to verify Snell's law.

Key Equation

n₁ sin θ₁  =  n₂ sin θ₂

Method

  1. Open the refraction simulation. Set the top medium to Air (n₁ = 1.00) and the bottom medium to Glass (BK7).
  2. Set the angle of incidence to 10°. Record the angle of refraction shown in the readout.
  3. Increase the angle of incidence in 10° steps up to 80°. Record each angle of refraction.
  4. Repeat the experiment with the bottom medium set to Water.

Results — Air → Glass (BK7)

θ₁ (°) θ₂ (°) sin θ₁ sin θ₂
10
20
30
40
50
60
70
80

Results — Air → Water

θ₁ (°) θ₂ (°) sin θ₁ sin θ₂
10
20
30
40
50
60
70
80

Analysis

Hint: Plot a graph of sin θ₁ (x-axis) against sin θ₂ (y-axis) for each medium on the same axes. Draw lines of best fit.

1. Describe the shape of your sin θ₁ vs sin θ₂ graphs.

2. Calculate the gradient of each line. What does the gradient represent?

3. Using your gradient, calculate the refractive index of glass and water. How do your values compare to the accepted values (glass ≈ 1.52, water ≈ 1.33)?

4. When light enters glass from air, does it bend towards or away from the normal? Explain why using the idea of wave speed.

5. Switch the simulation to wave view. Describe what happens to the spacing of the wavefronts as light passes from air into glass.

Part 2 — Finding the Critical Angle

Aim

To find the critical angle for different materials and investigate total internal reflection (TIR).

Key Equations

sin c  =  n₂ / n₁     (where n₁ > n₂)

Method

  1. Set the top medium to Glass (BK7) and the bottom medium to Air. (Light must travel from the denser medium into the less dense medium.)
  2. Starting at 10°, slowly increase the angle of incidence.
  3. Find the angle at which the refracted ray disappears and total internal reflection begins. This is the critical angle. Record it.
  4. Repeat for the combinations listed in the table below.

Results

Top medium (n₁) Bottom medium (n₂) n₁ value n₂ value Critical angle (measured) Critical angle (calculated)
Glass (BK7) Air
Water Air
Perspex Air
Diamond Air
Glass (BK7) Water

Analysis

Hint: Use sin c = n₂ / n₁ to calculate the expected critical angle for each pair. Compare with your measured values.

1. Show your working for the calculated critical angle of Glass → Air.

2. Which material has the smallest critical angle? Explain why, using the idea of refractive index.

3. What three conditions must be met for total internal reflection to occur?

4. Describe what you observe happening to the refracted ray as you approach the critical angle.

5. Optical fibres use total internal reflection to transmit data. Using your results, explain why glass is a suitable material for optical fibres. What would happen if you used a material with a very low refractive index?

6. Try setting both media to Glass → Water. Explain why total internal reflection can still occur even though neither medium is air.