Refraction Of Light
- Aug 16
- 3 min read
Definition
Refraction of light is the bending of light rays when they enter one medium from another medium with different optical properties.
The bending occurs because the speed of light changes when it travels from one medium to another.
Examples of Refraction
A pencil placed in water appears bent.
A coin at the bottom of a container of water appears slightly raised.
Writing viewed through a glass block appears to be raised slightly.
Why does a pencil appear bent in water?
Light rays from the part of the pencil inside water travel through water and then air before reaching the eye.
When light enters air from water, its direction changes.
Light rays from the part of the pencil above the water travel only through air, so their direction does not change.
Therefore, the pencil appears bent.
Apparent Depth
A coin at the bottom of a container of water appears slightly raised above its actual position.
When the coin is in air, light rays reach the eye straight.
When the coin is in water, the rays bend away from the normal when they enter air from water.
Therefore, the coin appears to be at a position slightly above its actual position.
This is called apparent depth.
Refraction and Speed of Light
A light ray bends when it enters a second medium if it does not enter at an angle of 90° to the surface.
The reason for refraction is the difference in the speed of light from one medium to another.
Speed of light in vacuum = 3 × 10⁸ m s⁻¹.
When light enters another medium from a vacuum, its speed decreases.
Dense and Rare Mediums
A medium in which light travels at a lower speed compared with another medium is called a dense medium.
A medium in which light travels at a higher speed compared with another medium is called a rare medium.
Refraction through a Glass Block
Important Terms
Incident ray (AB) :
The ray travelling towards and entering the glass block is called the incident ray.
Normal (XY) :
The normal is the line drawn perpendicular to the glass surface at the pointof incidence.
Angle of incidence (i) :
The angle between the incident ray and the normal is called the angle of incidence.
Refracted ray (BC) :
The ray travelling inside the glass after entering from air is called the refracted ray.
Angle of refraction (r) :
The angle between the refracted ray and the normal is called the angle of refraction.
Emergent ray (CD) :
The ray that comes out from the glass into air is called the emergent ray.
Angle of emergence (e) :
The angle between the emergent ray and the normal drawn at the point of emergence is called the angle of emergence.
Direction of Bending
Rare medium → Dense medium
When light enters from air (rare medium) to glass (dense medium), the light ray bends towards the normal.
Dense medium → Rare medium
When light enters from glass (dense medium) to air (rare medium), the light ray bends away from the normal.
General Rule
If a light ray bends towards the normal, the second medium is denser than the first medium.
If a light ray bends away from the normal, the second medium is rarer than the first medium.
Laws Of Refraction
There are two laws of refraction.
First Law
The incident ray, refracted ray and the normal to the surface drawn at the point of incidence lie on the same plane.
Second Law
When light refracts from one medium to another, the ratio of the sine of the incident angle to the sine of the refracted angle is a constant that depends only on the two media.
This constant is called the refractive index of the second medium with respect to the first medium.
The second law is also called Snell's law.
Formula
n = sin i / sin r
Where,
n = refractive index
i = angle of incidence
r = angle of refraction
Refractive Index
Refractive index of water relative to air = 1.33
Refractive index of glass relative to air = 1.5
Refractive index has no units.
Refractive index depends on the two media when one medium is considered relative to another.
When vacuum is used instead of the first medium, the refractive index depends only on the second medium.
Since it is practically difficult to measure refractive index relative to a vacuum, the refractive index of a medium relative to air is often used as the refractive index of that medium.
