Light reflection and refraction Worksheet20

With respect to air, the refractive index of ice is 1.31 and that of rock salt is 1.54. Calculate the refractive index of rock salt w.r.t ice.

Draw a labelled ray diagram to locate the image of an object formed by a convex lens of focal length 20 cm when the object is placed 30 cm away from the lens.

Explain with the help of a diagram, why a pencil partly immersed in water appears to be bent at the water surface.

Draw the ray diagrams to represent the nature, position and relative size of the image formed by a convex lens for the object placed (i) at 2F_{1} (ii) between F_{1} and optical centre O of the lens.

What is the minimum number of rays required for locating the image formed by a concave mirror for an object Draw a ray diagram to show the formation of a virtual image by a concave mirror.

Calculate the distance at which an object should be placed in front of a thin convex lens of focal length 10 cm to obtain a virtual image of double its size.

A convex lens of focal length 40 cm is placed in contact with a concave lens of focal length 25 cm. What is the power the combination?

Find the position of an object, which when placed in front of a concave mirror of focal length 20 cm produces a virtual image, which is twice the size of the object.

A concave lens made of a material of refractive index n_{1} is kept in a medium of refractive index n_{2}. A parallel beam of light is incident on the lens. Complete the path of rays of light emerging from the concave lens if (i) n_{1} > n_{2} (ii) n_{1} = n_{2} (iii) n_{1} < n_{2}

Find the position, nature and size of the image formed by a convex lens of focal length 20 cm of an object 4 cm high placed at a distance of 30 cm from it.
Answer:

Here, ^{a}n_{i }= 1.31 and ^{a}n_{r} = 1.54, ^{i}n_{r} = ?
Now, .

Here, v = ?, f = 20 cm, u = –30 cm,
From
v = 60 cm
The labelled ray diagram is shown in

ABC is a pencil immersed partially in water. Rays of light starting from A suffer refraction at the water surface XY, bend away from normal and appear to come from A'. Therefore, the pencil ABC appears as A'BC, bent at the water surface at B.

(i) When the object AB is placed at 2 F_{1}, the image A'B' formed at 2F_{2} is real, inverted and of same size as the object, as shown in the ray diagram.
(ii) When the object AB lies between F_{1} and optical centre O of convex lens, the image A'B' is formed on the same side of the lens as the object. The image is virtual, erect and magnified as shown in the ray diagram.

A minimum of two rays are required for locating the image formed by a concave mirror for an object.
When the object AB is held between pole P and principal focus F of the concave mirror, the image formed, A' B' is virtual, erect and magnified as shown in the figure.

Here, u = ?, f = 10 cm, m = + 2, as image is virtual.
As ∴ v = 2u
As ∴
∴ Object should be placed at a distance of 5 cm from the lens.

f_{1} = 40cm, f_{2} = –25 cm, P = ?
∴
As P = P_{1} + P_{2}
∴ P = 2.5 – 4.0 = –1.5 D

Here, object distance, u = ?
Focal length of concave mirror, f = –20 cm
Linear magnification, m = 2
As ∴ v = –2u
As ∴
or
Hence the object is at 10 cm in front of the concave mirror.

(i) When n_{1} > n_{2}, light goes from rarer to denser medium. Therefore, on passing through a concave lens, it diverges
(ii) When n_{1} = n_{2}, there is no change in medium. Therefore, no bending or refraction occurs
(iii) When n_{1} > n_{2}, light goes from a denser to a rarer medium. Therefore, in passing through a concave lens, it converges.

Here, v = ?, h_{2} = ?, = 20 cm
h_{1 }= 4 cm, u = –30 cm
As
∴
v = 60 cm
As v is positive, the image is real, formed on the right side of the lens.
Also,
∴
h_{2} = –8 cm
Negative sign of h_{2} indicates that the image is inverted.