Select Board & Class

Refraction of Light

Refraction of light and speed of light

Ankit went to an optician and noticed different types of spectacles there. He observed that while the glasses of some spectacles were relatively thicker in the middle, other glasses were thicker on the edge. The glasses of these spectacles are examples of lenses.

A lens is a transparent material bound by two curved surfaces. Lenses are broadly classified into two categories depending on their surfaces.

However, we will discuss only double spherical lenses here.

Convex lens

A convex lens is made by joining two spherical surfaces in such a way that it is thicker at the centre. Its thickness gradually reduces as we move towards the edge.

A convex lens has the ability to converge the light rays to a point that are incident on it. Thus, it is called a converging lens.

Concave lens

A concave lens is made by joining two curved surfaces in such a way that it is thinner at the centre. Its thickness gradually increases as we move towards the edge.

A concave lens has the ability to diverge a beam of light rays incident on it. Thus, it is called a diverging lens.

Differences between a spherical mirror and a lens

The following table lists some common differences between spherical mirrors and lenses

Spherical mirror

Spherical lens

Image is formed by reflection of light.

Image is formed by refraction of light.

A spherical mirror has only one focus.

A spherical lens has two foci.

The centre of the spherical mirror is termed as its pole.

The centre of the spherical lens is termed as its optical centre.

The second difference arises due to the fact that a lens has two spherical surfaces (i.e. it can be made from the arc of two spheres of equal radius).Therefore, light is refracted twice before it comes out of the lens.

Terms Associated with Lenses:

Optical centre

Optical centre is a point at the centre of the lens. It always lies inside the lens and not on the surface. It is denoted by ‘O’.

Centre of curvature

It is the centre point of arcs of the two spheres from which the given spherical lens (concave or convex) is made. Since a lens constitutes two spherical surfaces, it has two centers of curvature.

The distance of the optical centre from either of the centre of curvatures is termed as the radius of curvature.

Principal axis

The imaginary straight line joining the two centers of curvature and the optical centre (O) is called the principal axis of the lens.

Hold a convex lens and direct it against the sunlight. You will find a bright spot appear on the wall. Can you explain the formation of this bright spot? Light, after refracting through the lens, converges at a very sharp point. Try to obtain the brightest possible spot. Now, place a paper on the wall and observe what happens in the next few minutes.

Focus

The focus (F) is the point on the principal axis of a lens where all incident parallel rays, after refraction from the lens meet or appear to diverge from. For lenses there are two foci (F1 and F2) depending on the direction of incident rays.

The distance between the focus (F1 or F2) and the optical centre (O) is known as the focal length of the lens.

Refraction by Spherical Lenses

Refraction by a spherical lens can be categorized into three cases.

Case I. When the incident light ray is parallel to the principal axis

In this case, the refracted ray will pass through the second focus F2 for a convex lens, and appear to diverge from the first focus F1 for a concave lens.

Case II. When the incident light ray emerges from the first focus F1 of a convex lens, or appears to emerge from the second focus F2 of a concave lens

In this case, light after refraction from both the lenses will move parallel to the principal axis.

Case III. When the light ray passes through the optical centre (O) of a lens

In this case, the light ray will pass through both the lenses without suffering any deviation.

Ben was surprised to see a lemon appear larger than its size when he placed it in a glass filled with water.

What is the reason behind this?

When a light ray travels from one transparent medium to another, it bends at the surface that separates the two media. Hence, the lemon appears larger than its actual size. This happens because different media have different optical densities.

The phenomenon of bending of light as it travels from one medium to another is known as refraction of light.

The phenomenon of refraction shows that the speed of light is different in different media.

As a ray of light moves from an optically rarer medium to an optically denser medium, it bends towards the normal at the point of incidence. Therefore, the angle of incidence (i) is greater than the angle of refraction (r). Hence, i > r.

As a ray of light moves from an optically denser medium to an optically rarer medium, it bends away from the normal. Therefore, the angle of incidence (i) is less than the angle of refraction (r).

Hence, i < r.

Partial reflection and refraction of light at the boundary of two medium Case I: Refraction from rarer to denser medium When light travels from rarer to denser medium, suppose from air to glass, the partially refracted light bends towards the normal and partially reflected light returns back into the same medium.  Case II: Refraction from denser to rarer medium

When light travels from denser to rarer medium, suppose from glass to air, the partially refracted light bends away from the normal and partially reflected light returns back into the same medium.

Effect on various characteristics of light on reflection and refraction:   Characteristics Partially reflected light Partially refracted light               Rarer to denser                      Denser to rarer Speed of light No change Decreases  Increases Frequency of light (f) No change No change No change Wavelength of light (λ=vf) No change Decreases Increases

Speed of Light Light changes its speed when it enters one medium from another. The velocities of light in various media are given in the following table.

Medium

Velocity

Air

3 × 108 m/s

Water

2.25 × 108 m/s

Glass

1.8 × 108 m/s

From the table, we can easily see that light travels with lesser speed through glass an…

To view the complete topic, please

What are you looking for?

Syllabus