Question

Which one of the following materials cannot be used to make a lens?

Raman uses a lens to focus sunlight onto a piece of paper and it starts to burn. What type of lens is he using?

A person got his eyes tested by an optician. The prescription for the spectacle lenses to be made reads : Left eye : + 2.50 D Right eye : + 2.00 D

Hold a concave mirror in your hand and direct its reflecting surface towards the sun. Direct the light reflected by the mirror on to a white card-board held close to the mirror. Move the card-board back and forth gradually until you find a bright, sharp spot of light on the board. This spot of light is the image of the sun on the sheet of paper; which is also termed as "Principal Focus" of the concave mirror.