• As the angle of incidence is increased, the angle of refraction also increases until it gets 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?C

   

• This angle can be found using the formula:

• This can easily be derived from Snell’s law where:
  • θ1?=?C
  • θ2?= 90°
  • n1??=?n
  • n2?= 1 (air)

   

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

• • When a light ray is reflected, the angle of incidence = angle of reflection
  • Therefore, the angle of incidence (and 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