Light-Reflection And Refraction

Refractive index What is the first statement of the video clip?

• Hi kids, today we will learn about refraction

• So let's start, we learnt what is reflection and it's types already

• When light exists in a single medium air or water or glass

• It moves in a single straight line and at a constant speed until it is obstructed by some object

• When it is obstructed by some object it reflects at the angle which is the same as the angle at which it hits the object

The velocity of light in water is 3/4times the velocity of light in vacuum. Find

the refractive index of water.

Answer explanation

Let velocity of light in vacuum = v

⇒ Velocity of light in water

Now,

Refractive index of a medium (water) =

⇒ Refractive index =

Hence Refractive index of water = 1.33.

What is the distance between the image and the plane mirror, if the object is

at 15 m?

Answer explanation

In plane mirror the distance between object and mirror is same as the distance

between the image and mirror.

⇒ Image Distance = Object Distance.

∴ Distance between image and plane mirror is 15 cm.

How can we find the (rough) focal length of a concave mirror?

Answer explanation

Method We can put the object far away from the concave mirror and place the

screen where the image is formed. The place where the image formed is sharpest is the

focus of the mirror, measure the distance between the focus and mirror to get the focal

length.

Explanation when an object is kept at the infinity than the image is formed at the focus

of the concave mirror

. A concave mirror of focal length 1.5m forms an image of an object placed at

a distance of 40cm. Find the position and nature of the image

Answer explanation

According to the question;

Focal length (f) = -1.5m;

Object distance (u) = -40cm = 0.40m; (1m = 100 cm)

By mirror formula;

⇒ v = 0.5454m = 54.54cm (1m = 100 cm).

Since v = 54.54cm which is positive hence image is behind the mirror.

The image is formed at the distance of 54.54 cm behind the mirror and is virtual and

erect.

A person wants to see the full length image of tall building in a small

mirror. Which type of mirror is used by him?

Answer explanation

Person will use the convex mirror because it always forms virtual and erect

image and has the wider field of view which will help in capturing the whole building

Why do we not see the actual depth of a lake?

Answer explanation

This is due to the refraction of light; light rays bend as they move from water to

the air. When the light ray from water reaches our eye, the eye traces them back as

straight lines (shown as dashed lines) which intersect at a higher position than where

the actual rays originated. This cause the depth to appear higher and we cannot see the

actual depth of the lake.

The refractive indices of water and glass are and respectively. Write the

relation and find the value of refractive index of water with respect to glass and

glass with respect to water.

Answer explanation

Refractive index of glass w.r.t water = 9/8.

A concave mirror and a convex lens are held separately in water. What

changes, if any, do you expect in the focal length of either?

Answer explanation

The focal length of a mirror does not depend upon the nature of the medium in

which it is placed whereas the focal length of a lens depends upon the medium in which

it is placed. Thus, there will be no change in the focal length of the concave mirror

whereas the focal length of the convex lens will change. If the refractive index of the

material of the convex lens is greater than water it will increase, if it is equal to water it

will become infinity and if it is less than water it will be negative.

. Redraw the following diagrams given below in your answer book and show

the direction of the light ray after refraction from the lens.

Answer explanation

The lens is concave lens, hence it diverges the ray of the light coming from infinity after

refraction;

(b)

The above lens is the convex lens; hence the ray of light passing through focus after

refraction will become parallel to the principal axis after refraction.