Refraction of Light

 

LIGHT

Light is a form of energy which produces sensation of vision in our eyes. 

Light travels in a straight path in a medium. This is called rectilinear propagation of light.

But there is this cool optical phenomena called refraction.

WHAT IS REFRACTION?

When a ray of light travelling in one transparent medium falls obliquely on the surface of another transparent medium, it travels in the other medium in a direction different from its initial path.

SPEED OF LIGHT IN DIFFERENT MEDIA

Light travels faster in air than in water or glass. The speed of light in air is 3 × 10⁸ m/s, in water it is 2.25 × 10⁸ m/s and in glass it is only 2 × 10⁸ m/s. 

Glass is optically denser than water and water is optically denser than air. Air is optically rarer than both water and glass.

Now what does denser and rarer mean?

A medium is said to be denser if the speed of light in it decreases, while it is said to be rarer if the speed of light in it increases. But in no medium, it can be more than 3 × 10⁸ m/s.

What happens when a ray of light passes from one rarer medium to a denser medium? 

When a pencil is placed into a glass of water, you can observe that the part of the pencil above the surface of the water and the part of the pencil below the surface of the water appears at a different angle. The pencil dipped into water at an angle appears bent (or broken):

The change in direction of a path of light when it passes from one optically transparent medium to another, is called refraction of light.

TERMS RELATED TO REFRACTION

• Incident ray: The ray of light falling on the surface separating the two media.
• Refracted ray: The ray of light travelling in the other medium in the changed direction.
• Normal: The perpendicular drawn on the surface separating the two media, at the point where the incident ray strikes at the point of incidence.
• Angle of incidence: The angle between the incident ray and the normal. Represented as 'i'.
• Angle of refraction: The angle between the refracted ray and the normal. Represented as 'r'.

There are three cases of refraction:

I. When a ray of light passes from a rarer medium to a denser medium, the ray of light moves towards the normal. In this case, ∠i > ∠r.

II. When a ray of light passes from a denser medium to a rarer medium, the ray of light moves away from the normal. In this case, ∠i < ∠r.

III. When the ray of light strikes the surface of the medium perpendicularly, it passes through the medium undeviated. 

When a ray of light passes from one transparent medium to another transparent medium having the same refractive index, it also remains undeviated.

Picture credits: Google