Critical Angle

Recap from National 5

When light is incident on the curved side of a semi-circular block, no refraction occurs and the ray passes straight into the glass.

When light is incident on the flat side, refraction may occur. The following diagram gives the three possible outcomes for the incident ray of light:-

Note - The colour of the rays in the above diagram are for ease of labeling, not denoting colour of light.

θ1 = Ray of light is refracted and leaves the block.

θc = Ray of light is refracted along the glass-air boundary (θrefraction = 90o)

θ2 = Ray of light is reflected back into the glass (Total Internal Reflection)

The outcome we are interested in is θc - the critical angle.

For a ray of light:-

1. If the angle of incidence is less than the Critical angle, Refraction occurs.

2. If the angle of incidence is greater than the Critical angle, Total Internal Reflection occurs.

By applying Snell's Law and using the critical angle we find:-

Optical Fibre

An optical fibre is made of two types of glass, with a dense glass core and a less dense outer cladding. When light enters the end of the fibre, it cannot escape and due to Total Internal Reflection, is reflected down the fibre.