A concave lens of 20cm focal length forms an image 15cm from the lens. Compute the object distance.

State where an object must be placed so that the image formed by a concave mirror is:

1. Erect and virtual.
2. At infinity.
3. The same size as the object.

(i) Explain why the refractive index of any material with respect to air is always greater 1 .
(ii) In the figure below a light ray travels from air into the semi-circular plastic block. Give a reason why the ray does not deviate at the semi-circular boundary of the plastic block.

(iii) Complete the ray diagram of the above scenario when the light ray comes out of the plastic block from the top flat end.

A diverging mirror of radius of curvature 40cm forms an image which is half the height of the object. Find the object and image positions.

1. Draw a diagram to show how a converging lens focusses parallel rays of light?
2. How would you alter the above diagram to show how a converging lens can produce a beam of parallel rays of light.

A man standing in front of a special mirror finds his image having a very small head, a fat body and legs of normal size. What is the shape of:

1. Top part of the mirror?
2. Middle part of the mirror?
3. Bottom part of the mirror?

Give reasons for your choice.

The magnification of an image formed by a lens is –1. If the distance between the object and its image is 60cm, what is the distance of the object from the optical centre of the lens? Find the nature and focal length of the lens. If the object is displaced 20cm towards the optical centre of the lens, where would the image be formed and what would be its nature? Draw a ray diagram to justify your answer.

A convex lens of focal length 20cm can produce a magnified virtual as well as real image. Is this a correct statement? If yes, where shall the object be placed in each case for obtaining these images?

Distinguish between a real and a virtual image of an object. What type of image is formed?

1. By a plane mirror.
2. On a cinema screen?