Electricity — Science STD 10 — Question

Why does the sky appear dark (or black) to an astronaut instead of blue?

A person can read a book clearly only if he holds it at an arm's length from him. Name the defect of vision:

1. If the person is an old man.
2. If the person is a young man.

Name the material which is the best conductor of electricity.

What will happen if another similar glass prism is placed upside down behind the first prism?

How much energy is transferred by a 12V power supply to each coulomb of charge which it moves around a circuit?

Explain why, electric switches should not be operated with wet hands.

What normally drives the alternators in a Thermal Power Station? What fuels can be used to heat water in the boiler?

Identify the device used as a spherical mirror or lens in following cases, when the image formed is virtual and erect in each case.
Object is placed between device and its focus, image formed is enlarged and behind it.

Briefly describe how you would find the focal length of a concave mirror quickly but approximately.

A 4 cm tall object is placed on the principal axis of a convex lens. The distance of the object from the optical centre of the lens is 12cm and its sharp image is formed at a distance of 24cm from it on a screen on the other side of the lens. If the object is now moved a little away from the lens, in which way (towards the lens or away from the lens) will he have to move the screen to get a sharp image of the object on it again? How will the magnification of the image be affected?