A beam of light is incident through the holes on one side of a box and emerges out through the holes on its opposite side as shown in the following figure:

The box contains:

When sunlight is concentrated on a piece of paper by a spherical mirror or lens, then a hole can be burnt in it. For doing this, the paper must be placed at the focus of:

A beam of parallel light rays is incident through the holes on one side of a box and emerges out through the holes on its opposite side as shown in the diagram below:

Which of the following could be inside the box?

Which of the following can form a virtual image which is always smaller than the object?

When an object is placed 20 cm in front of lens A, the image is real, inverted, magnified and formed at a great distance. When the same object is placed 20 cm in front of lens B, the image formed is real, inverted and same size as the object.

(a) What is the focal length of lens A?

(b) What is the focal length of lens B?

(c) What is the nature of lens A?

(d) What is the nature of lens B?