परछाइयाँ तब बनती हैं जब प्रकाश के पथ में __________ वस्तुएँ आ जाती हैं।

Objects like the sun are called luminous objects. Which of the following is the correct reason for calling them so?

Shadows are formed when an _______ object comes in the path of light.

If you are asked to project the real image of a well-lit painting on a wall, which one of the following optical devices would you select for this purpose?