Two convex lenses with power 2D are kept in contact with each other. The focal length of the combined lens system is

Which of the following statement(s) about electromagnetic waves, sound waves and water waves is/are correct?

1. They exhibit reflection.

2. They carry energy.

3. They exert pressure.

4. They can travel in vacuum.

Thermal capacity of a body depends on the

Which one of the following metal is used in the filaments of photoelectric cells that converts light energy into electric energy?