Total Internal Reflection (Edexcel GCSE Physics)

• As the angle of incidence is increased, the angle of refraction also increases until it gets closer to 90°
• When the angle of refraction is exactly 90° the light is refracted along the boundary
  • At this point, the angle of incidence is known as the critical angle θ_c

• Total internal reflection (TIR) occurs when:

The angle of incidence is greater than the critical angle and the incident material is denser than the second material

• Therefore, the two conditions for total internal reflection are:
  • The angle of incidence > the critical angle (i > θ_c)
  • The incident material is denser than the second material

Critical angle and total internal reflection

Step 1: Draw the reflected angle at the glass-liquid boundary

• • When a light ray is reflected, the angle of incidence = angle of reflection (i = r)
  • Therefore, the angle of incidence (or reflection) is 90° – 25° = 65°

Step 2: Draw the refracted angle at the glass-air boundary

• • At the glass-air boundary, the light ray refracts away from the normal
  • Due to the reflection, the light rays are symmetrical to the other side

Step 3: Calculate the critical angle

• • The question states the ray is “totally internally reflected for the first time” meaning that this is the lowest angle at which TIR occurs
  • Therefore, 65° is the critical angle