Electromagnetic Spectrum

Electromagnetic spectrum

(i) The orderly distribution of electromagnetic waves (on the basis of wavelength or frequency) in the form of distinct groups having widely differing properties is called the electromagnetic spectrum.

The following table gives a complete and detailed picture of electromagnetic spectrum

(i) Various parts of electromagnetic spectrum

Earth's atmosphere and electromagnetic waves

(i) The gaseous envelop surrounding the earth is called earth's atmosphere.

(ii) It mainly consists of nitrogen 78% and oxygen 21% alongwith a little portion of argon, carbon-di-oxide, water vapour, hydrocarbons, sulphur compounds and dust particles.

(iii) The density of atmospheric air goes on decreasing gradually as we go up.

(iv) The earth's atmosphere has no sharp boundary. However, it has been divided into various regions as given below:

(a) Troposphere% It extends upto a height of 12 km from earths surface. The temperature in this region decreases from 298K to 220K and conductivity increases. All climatic changes occur in this region.

(b) Stratosphre% It extends from 12 km to 50 km after troposhpere. At the upper part of this region, approximately 20km thick, most of ozone of atmosphere is concentrated. This layer is called as ozone layer. This layer absorbs very large portion of ultraviolet radiations coming from sun, therefore its temperature increases from 220K to 280K.

(c) Mesosphere% It extends from 50km to 80 km after stratosphere. In this region the temperature decreases from 280K to 180K

(d) Ionosphere% It extends from 80 km to 400 km after mesosphere. The temperature of this region rises from 180K to 700K. In this region ultraviolet radiation coming from sun cause ionisation, therefore this part mostly consists of free electrons and positive ions. The concentration of free electrons is found to be very large in a region beyond 110 km from earth's surface which extends vertically for a few kilometers and is called Kennelly Heaviside layer. Beyond this layer the concentration of free electrons decreases considerably until a height of about 250 km. Beyond it there is another layer of electrons, called Appleton layer.

(v) Greenhouse effect% The atmosphere is transparent to visible radiations, but most ifrared (heat) radiations are not allowed to pass through. The energy from the sun heats the earth which then starts emitting radiations like any other hot body. However, since the earth is much colder than sun, its radiations are mainly in the infra red region. These radiations are unable to cross the lower atmosphere and are reflected back. Low lying clouds also reflect back the infra red radiations. As such, the earth's surface warm at night. This phenomenon is called the Green house effect.

(vi) Propagation of Radio waves%

(a) Low frequency waves-the AM band% Radiowaves having wavelengths of 10m or more (frequency less than 30 Mhz) are said to constitute the AM band. The lower atmosphere is transparent to these waves, but the ionosphere reflects them back. A signal transmitted from a certain point can be received at another point in two possible ways-directly along the surface of the earth (called sky wave) and after reflection from ionosphere (called sky wave). Waves having frequencies upto about 1500kHz (Wavelength above 200m) are mainly transmitted through ground because low frequency sky waves lose their energy very quickly than the sky waves. Therefore, higher frequencies are mainly transmitted through sky. These two regions of the AM band are called medium wave and short wave bands respectively.

(b) High frequency waves-Television transmission% Above a frequency of about 40MHz the ionosphere does not reflect the wave toward the earth. The television signals have frequencies in the range 100-200 MHz. Therefore TV transmission via the sky is not possible-only direct reception via the ground is possible. Therefore, in order to have larger coverage, the transmission has to be done through very tall antennas. The height of transmitting antenna for TV telecast is given by h = $\frac{d^2}{2R_e}$

where d is the radius of the area to be covered for TV telecast and Re is the radius of earth.

Illustration : A T.V. tower has a height of 100m. How much population is covered by T.V. broadcast, if the average population density around the tower is 1000/km2?

(A) 39.5 X 105 (B) 19.5 X 106

(C) 29.5 X 107 (D) 9 X 104

Solution: (A) Radius of the area covered by T.V. telecast

d = $\sqrt{2h{R}_e}$

Total population covered ¾ $\pi$d2 X population density

= 2$\pi$hRe X population density

= 2 X 3.14 X 100 ´ 6.4 X 106 X$\frac{1000}{{10}^6}$

= 39.503 X 105