Refraction of Light

Key Concepts

• Refraction of Light
• Terms associated with Refraction of Light
• Refraction through a glass slab

Introduction: 

It is known that transparent objects transmit most of the light that falls on them. However, they do not transmit light along the same path through which the light approaches them. In fact, light changes its direction or bends every time it passes from one transparent substance to another. This is called refraction of light. In this section we will look at some observations of refraction in our daily life, refraction through a glass slab and terms associated with it in detail. 

Explanation: 

Refraction of light: 

When a pencil is dipped in a glass of water, the pencil seems to be bent at the surface of water. This is because the light that reflects off the part of the pencil inside water bends before entering our eyes. Here, air and water are the transparent substances involved and are said to be the medium in which light travels. Thus, when light moves from one medium to another, it bends. 

The transparent substances through which light passes are called the mediums. The phenomenon of bending of light when it passes from one medium to another is called the refraction of light. 

Refraction in daily life: 

The lenses, magnifying glasses, and prisms function on the phenomena of the refraction of light. All of them bend the light rays entering them.  

The lens present inside our eyes bends the light coming from the object, to focus its image on the retina. 

Bent appearance of pencil: 

• We see an object when light reflects off its surface and enters our eyes. 
• The light rays reflecting off the part of the pencil outside the water reaches straight to the eyes, as they travel through only one medium, i.e., air. 
• Whereas, the light rays reflecting off the part of the pencil inside the water bends, when they move from water to air, before entering the eyes. 
• Thus, these light rays travel through two mediums, i.e., water and air, and bend at the interface connecting the two mediums.  

• Consider a point X on the immersed part of the pencil. The light rays reflecting off X would travel through water and bend on entering air before reaching the eyes. 
• Now, these light rays do not appear to come from X but another point Y. 
• Thus, this point on the pencil is perceived by our eyes to be at point Y, instead of X. 
• Similarly, if we consider all the points on the immersed part of the pencil, the bent appearance of the pencil is justified. 

Raised appearance of a coin immersed in water: 

Similarly, 

The light rays reflecting off the coin bends when they move from water to air before entering the eyes. 

Therefore, these light rays travel through two mediums, i.e., water and air, and bends at the interface connecting the two mediums. 

Thus, the coin appears to be present at the point where the light rays appear to be emerging from, i.e., at the point I even if it is actually at O. 

Refraction 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. 

Terms related to refraction of light: 

There are some terms that are used universally to discuss the refraction of light through a glass slab. There are two mediums involved here; glass and air. The terms are as follows: 

1. The surface of separation of glass and air is called the interface. 

2. A ray of light that hits the interface from the first medium (air) is called an incident ray. 

3. The incident ray, on entering the second medium (glass) is called the refracted ray. 

4. The refracted ray, when emerges out of the second medium back to the first medium or any other medium is called the emergent ray. 

5. An imaginary line 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. 

6. The angle made by the incident ray with the normal is called the angle of incidence (i). 

7. The angle made by the refracted ray with the normal is called the angle of refraction (r). 

8. The angle made by the emergent ray with the normal is called the angle of emergence (e). 

Refraction of light from a rarer to a denser and a denser to a 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. 

The 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. 

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

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

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

Summary:

• The phenomenon of bending of light rays when they move from one transparent substance to another is called refraction of light. These substances through with light moves are called mediums.
• 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 bends light coming from the object, to focus its image on to the retina.
• A pencil partially immersed in water appears to be bent at the water-air interface because of refraction of light coming from the immersed part of the pencil. A coin at the bottom of a tumbler of water appears to be raised for the same reason.
• A glass slab is a cuboidal (3-dimensional) piece of glass, which can be used to study the refraction of light.
• Glass and water are optically denser than air.
• When a light ray moves from a rarer to a denser medium, it bends towards the normal.
• When a light ray moves from a denser to a rarer medium, it bends away from the normal.