Refraction of Light Revision Notes

In these revision notes for Refraction of Light, we cover the following key points:

• What is refraction of waves?
• How refraction is applied in light waves?
• What do the Laws of Refraction say about light rays travelling through various media?
• What is the critical angle? When it is observed?
• What happens when the angle of incidence is greater than the critical angle?
• What is the index of refraction? How it is calculated?
• What does the Snell's Law says about the angles during refraction?
• How is the total internal reflection applied in technology?

Refraction of Light Revision Notes

Refraction is the change in direction of a wave passing from one medium to another or from a gradual change in the medium.

The two Laws of Refraction are:

1. Incident ray, refracted ray and the normal line are all at the same plane.
2. When the light passes from a less dense medium to a denser one, it approaches the normal line, while when light passes from a denser medium to a less dense one, it moves away from the normal line.

If we denote the angle of incidence by î, the angle of refraction by ȓ, the density of the first medium by p₁ and that of the second medium by p₂, we can write the 2nd Law of Refraction as:

Only in one case the light rays do not bend when they change medium. This occurs when light is incident at 00 to the normal line (or at 900 to the interface of two media). This is because the angle of refraction cannot be smaller than 0. The only thing we can observe in such cases is that objects look in a different distance from the surface than they actually are.

Critical angle is defined as the angle of incidence that provides an angle of refraction of 90-degrees.

Critical angle exists only when the incident medium is denser than the refractive medium as only in this case the refracted ray moves away from the normal line and so, the refraction angle becomes 900 before the incident angle.

The index of refraction n is calculated by the formula

There is a relationship between the refractive indexes and the sines of the incident and refraction angles given by an equation known as Snell's Law. The formula used to express mathematically the Snell's Law is

where

• n₁ is the refractive index in the incident medium,
• n₁ is the refractive index in the refraction medium,
• θ₁ is the angle of incidence, and
• θ₂ is the angle of refraction

We can use Snell's Law to find the values of critical angles in various media. We just have to know the refractive indexes (or the light's speed in those media) and then apply the Snell's Law for θ₂ = 900 (sin 900 = 1).

When the angle of incidence is greater than the critical angle, the light ray is reflected back into the first medium after touching the interface between the two media. This means the interface acts like a plane mirror. As a result, a phenomenon known as total internal reflection does occur.

Submarine is a practical application of total internal reflection. It cannot be detected from above the water as it emits light at angles that are greater than the critical angle of water-to-air (41.80).