$(a)$ Describe the structure of a windmill.
$(b)$ Give one example to illustrate that energy from a windmill was used in the past to do mechanical work.

What is hydroelectricity? Explain the basic principle involved in the generation of this energy.

Choose the only renewable source of energy from the following:

Write down three advantages of harnessing wind energy.

Ram visited Manikaran Sahib in Himachal Pradesh where he saw an experimental geothermal plant. When he saw a hot spring,he wanted to know more about it. His father explained to him about geothermal energy and hot springs. Now answer the following questions:
$(a)$ What did Ram's father explain to him about hot springs?
$(b)$ Why is geothermal energy not generated on a large scale?
$(c)$ What is the main advantage of using geothermal energy?

State whether the following statement is true or false:
Spherical reflectors are used to obtain high temperatures in solar heating devices.