a) What is a geostationary satellite? b) What are parking orbits?

 a) What is a geostationary satellite? b) What are parking orbits?
Ans.: a) A satellite which appears to be fixed at a place above the earth is called a geostationary satellite. A geostationary satellite has the time period round the earth equal to the rotational period of the earth about its axis ie 1 day.
The orbit of a geostationary satellite round the earth ( at a height of about 36,000 km from the earth's surface) is called the parking orbit of the satellite.
Q.12: Explain the terms (i) Ground wave (ii) Sky wave.
Ans.: i) Ground wave : If a signal transmitted from a certain point on the earth reaches some other point travelling directly along the surface of the earth it is called a ground wave. Radiovvaves having frequencies upto 1500 kHz (wavelength above 200 m) are mainly transmitted through ground because in sky they lose energy very quickly.
Sky wave m If a signal is received at some other point after reflection from the ionosphere, it is called a sky wave. Higher frequency signals are mainly transmitted through the sky because on the ground they are absorbed quickly.


greenhouse effect
The clouds act as 'blankets' of the earth. The lower atmosphere, therefore, is very rich in infrared radiation and all the objects on earth absorb it, gettingheated in the process. This phenomenon is commonly called the greenhouse effect, called so because the water droplets hanging in the atmosphere act as the glass roofs of a green house and are able to trap the radiations. The components of the solar radiation whose wavelength is shorter than those ofviolet light are quite dangerous and can cause damage to living cells. Howevar, nature has given us a protective umbrella called the ozone layer ofthe atmosphere which absorbs these harmful radiations.