JEE Main & Advanced Physics Wave Optics / तरंग प्रकाशिकी Earth's Atmosphere

The gaseous envelope surrounding the earth is called it's atmosphere. The atmosphere contains 78% \[{{N}_{2}},\] 21% \[{{O}_{2}}\], and traces of other gases (like helium, krypton, \[C{{O}_{2}}\] etc.)

(1) Division of earth's atmosphere : Earth atmosphere has been divided into regions as shown.

(i) Troposphere : In this region, the temperature decreases with height from 290 K to 220 K.

(ii) Stratosphere : The temperature of stratosphere varies from 220 K to 200 K.

(iii) Mesosphere : In this region, the temperature falls to 180 K.

(iv) Ionosphere : Ionosphere is partly composed of charged particles, ions and electrons, while the rest of the atmosphere contains neutral molecules.

(v) Ozone layer absorbs most of the ultraviolet rays emitted by the sun.

(vi) Kennelly heaviside layer lies at about 110km from the earth's surface. In this layer concentration of electron is very high.

(vii) The ionosphere plays a vital role in the radio communication.

(2) Green house effect : The warming of earth's atmosphere due to the infrared radiations reflected by low lying clouds and carbon dioxide in the atmosphere of earth is called green house effect.

(3) Role of earth's atmosphere in propagation of radio waves

(i) Radio waves classification

(a) Very low frequency (VLF) \[\to \] 10 KHz to 30 KHz

(b) Low frequency (LF) \[\to \] 30 KHz to 300 KHz

(c) Medium frequency (MF) or medium wave (MW) \[\to \] 300 KHz to 3000 KHz

(d) High frequency (HF) or short wave (SW) \[\to \] 3 MHz to 30 MHz

(e) Very high frequency (VHF) \[\to \] 30 MHz to 300 MHz

(f) Ultra high frequency (UHF) \[\to \] 300 MHz to 3000 MHz

(g) Super high frequency or micro waves \[\to \] 3000 MHz to 300, 000 MHz

(ii) Amplitude modulated transmission : Radio waves having frequency less than or equal to 30 MHz form an amplitude modulation band (or AM band). The signals can be transmitted from one place to another place on earth's surface in two ways

(a) Ground wave propagation : The radio waves following the surface of the earth are called ground waves. (b) Sky wave propagation : The amplitude modulated radio waves which are reflected back by the ionosphere are called sky waves.

(iii) Frequency modulated (FM) transmission : Radio waves having frequencies between 80 MHz and 200 MHz form a frequency modulated bond. T.V. signals  are normally frequency modulated.

(4) T.V. Signals

(i) T.V. signals are normally frequency modulated. So T.V. signals can be transmitted by using tall antennas.

(ii) Distance  covered by the T.V. signals \[d=\sqrt{2hR}\]

(h = Height of the antenna, R = Radius of earth)

(iii) Area covered \[A=\pi {{d}^{2}}=2\pi hR\]

(iv) Population covered = Area \[\times \]Population density.