Refraction Of Light

Refraction of light is the change in direction (bending of light rays) when it passes from one optically transparent medium to another.

Terms used in refraction:

Refracted ray is the bent ray as a result of passing from one optical medium to another.

Normal is an imaginary line perpendicular to the interface of media where the refraction occur.

Angle of incidence is the angle between incident ray and the normal.

Angle of refraction is the angle between refracted ray and the normal.

 

Laws of Refraction

First law of refraction states that the incident ray, the refracted ray and the normal to the interface all lie in the same plane.

Second law of refraction states that for two given media, the ratio $\frac{sin \, i}{sin \, r} = \, constant$, where $i$ is the angle of incidence and $r$ is the angle of refraction.

 

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Refractive index (n) of a medium is given by $n = \frac{sin \, i}{sin \, r}$

• The greater the value of the refractive index of a medium, the greater is the “bending” effect of light when it passes from air into that medium.
• A material with a larger value of n is an optically denser medium.

 

In summary, light travelling from:

• denser medium to less dense medium – Bend away from normal
• less dense medium to denser medium – Bend towards normal

However, if the light ray enters another medium perpendicularly to the boundary, there is no deviation of the ray even when there is change in speed of light.

 

Another definition for refractive index: 

Refractive index (n) of a medium is the ratio of the speed of light in vacuum (c) to the speed of light in that medium (v). Hence, n can be calculated using:

$n = \frac{c}{v}$

This implies that the higher the refractive index of a medium, the slower will be the speed of light through it. This means that a medium’s optical density increases as its refractive index increases.

Consequence of refraction of light: Apparent Depth

A pool of water looks shallower than it really is. A straight object placed in water looks bent at the surfaces.

 

Refractive Index & Wavelength of Light

For visible light, the refractive indexes of most transparent medium decrease with increasing wavelength. Hence,

$$1 < n \left( \lambda_{\text{red}} \right) < n \left( \lambda_{\text{green}} \right) < n \left( \lambda_{\text{blue}} \right)$$

This causes white light to split in its constituent colours (rainbow) when it passes through a prism.

Additional notes about speed of light in medium (v): (OPTIONAL)

Light is not actually “slowed” by the medium. The speed of light is ALWAYS c. This constancy of the speed of light plays a major role in the theory of special relativity by Einstein. (In fact it is built into modern Physics theories) The speed of light in medium used in the calculation above is actually the apparent speed of light in that medium. When light passes through a medium, it is constantly being absorbed and re-emitted by the electrons in the air molecules. However, the re-emission of light is not instantaneous as the electrons “play” with the light for a short while before re-emitting it. This causes the apparent slower speed of light.

Note: The absorption and re-emission of light actually contributes to the development of lasers. You will be able to learn more about this if you choose to study Physics in A level.