Refraction of Light – Definition, Law of Refraction

Refraction of Light 

Introduction

Transparent objects let light pass through them. However, when an object is viewed through them, it looks different from what it actually is. This is because light rays bend when they enter a new medium. This phenomenon of bending of light rays is called refraction of light

. In this session we will be looking at various concepts related to the refraction of light. 

Explanation

Refraction of light: 

The bending of the light rays upon entering a new medium is called refraction of light. This bending of light makes the lenses, magnifying glasses, and prisms functional, as they all bend light rays entering them to accomplish their tasks. The lens present inside our eyes bend light coming from an object, to focus its image onto the retina. 

Refraction of light through a glass slab: 

A glass slab is a cuboidal (3-dimensional) piece of glass, which can be used to study the refraction of light. 

When a beam of light enters a glass slab, it bends slightly from its original path. 

It bends again on emerging out of the glass slab and becomes parallel to the initial beam. 

Refraction takes place because of the change in the speed of light when it enters a new medium. 

Terms related to the refraction of light: 

• There are two mediums involved here; glass and air. 

• The surface of separation of glass and air is called the interface of refraction. 
• A ray of light that hits the interface from the first medium (air) is called an incident ray. 
• The incident ray, on entering the second medium (glass) is called the refracted ray. 
• The refracted ray, when emerges out of the second medium back to the first medium or any other medium is called the emergent ray.  
• The emergent ray emerges parallel to the incident ray. 
• This is because the extent of bending of the ray of light at the opposite parallel faces AB and CD of the glass slab is equal and opposite. 
• The light ray undergoes a lateral shift when it emerges out. 
• An imaginary line is always drawn perpendicular to the interface at the point of incidence is called the normal. A normal line can also be drawn at the point of emergence. 
• The angle made by the incident ray with the normal is called the angle of incidence (i). 
• The angle made by the refracted ray with the normal is called the angle of refraction (r). 
• The angle made by the emergent ray with the normal is called the angle of emergence (e). 

Refraction patterns: 

From denser to rarer medium: 

Air is optically less dense as compared to glass. Thus, it is said to be the rarer medium and glass is said to be the denser medium. 

Mediums like water, glass and various oils are denser than air.  

When a light ray moves from a rarer to a denser medium, it bends towards the normal. 

Therefore, the angle of incidence is greater than the angle of refraction. 

Thus, when a light moves from air to glass, it bends towards the normal. 

From rarer to denser medium: 

When a light ray moves from a denser to a rarer medium, it bends away from the normal. 

Thus, when a light moves from air to glass, it bends away from the normal. 

Here, the angle of incidence is less than the angle of refraction. 

Normal incidence: 

Normal incidence is when the incident light ray goes along the normal to the refracting interface. 

The light ray does not bend at all while passing through the medium in this case. It goes straight through the medium. 

The angle of incidence is equal to the angle of refraction, i.e., 0 degrees. 

Laws of refraction: 

The refraction of light follows two rules while refracting from a medium. These are called the laws of refraction, which are as follows: 

1. The incident ray, the reflected ray, and the normal to the interface of two transparent mediums at the point of incidence, all lie on the same plane. 

2. Snell’s law – The ratio of the sine of angle of incidence to the sine of angle of refraction is a constant, for the light of a given color and for the given pair of mediums. This law is true for 0^o < i < 90^o. 

If ‘i’ is the angle of incidence and ‘r’ is the angle of refraction, then,

This constant is called the refractive index of the second medium w.r.t the first medium. 

Summary

1. The bending of the light rays upon entering a new medium is called refraction of light
2. The emergent ray emerges parallel to the incident ray. The light ray however
   undergoes a lateral shift when it emerges out.
3. When a light ray moves from a rarer to a denser medium, it bends towards the normal.
   For such cases, ∠i > ∠r.
4. When a light ray moves from a denser to a rarer medium, it bends away from the
   normal. For such cases, ∠i < ∠r.
5. During normal incidence, the light ray goes through the medium without bending. For
   such cases ∠i = ∠r=0⁰.
6. The first law of refraction states that, “the incident ray, the reflected ray, and the
   normal to the interface of two transparent mediums at the point of incidence, all lie on
   the same plane”.
7. The second law of refraction states that, “the ratio of the sine of angle of incidence to the sine of angle of refraction is a constant, for the light of a given color and for the
   sin i given pair of mediums. This law is true for 0°<i<90⁰“.i.e., sin i/ sin r=constant. This is also known as Snell’s law.