Book Store

Download books and chapters from book store.
Currently only available for.
CBSE Gujarat Board Haryana Board

Previous Year Papers

Download the PDF Question Papers Free for off line practice and view the Solutions online.
Currently only available for.
Class 10 Class 12

Electromagnetic Waves

Use the formula λ_m T = 0.29 cm K to obtain the characteristic temperature ranges for different parts of the electromagnetic spectrum. What do the numbers that you obtain tell you?

We know that, every body at a given temperature T, emits radiations of all wavelengths in a certain range.
For a black body, the wavelength corresponding to maximum intensity of radiation at a given temperature T is given by,

According to Wein's law of radiation

λ_{m} T = 0.29 cmK or T = \frac{0.29}{λ_{m}}

For

λ_{m} = 10^{- 6} m = 10^{- 4} cm, T = \frac{0.29}{10^{- 4}} = 2900 K .

In a similar way temperature for other wavelengths can be found.
These numbers tell us the temperature ranges required for obtaining radiations in different parts of the electromagnetic spectrum.

Thus, to obtain visible radiation, say λ_m = 5 × 10^-5 cm, the source should have a temperature

T = \frac{0.29}{5 \times 10^{- 5}} ≃ 6000 K

Note: A body at lower temperature will also produce wavelength of this range, but not with maximum intensity.

172 Views

Write any four characteristics of electromagnetic waves. Give two uses of (i) radio-waves (ii) Microwaves.

Characteristics of electromagnetic waves:

(i) Electromagnetic waves are produced by accelerating or oscillating charge.
(ii) E.M. waves do not require any material medium for their propagation.
(iii) E.M. waves travel in free space with a velocity $C = \frac{1}{\sqrt{μ_{0} ε_{0}}}$ which is equal to the velocity of light (c = 3 × 10⁸ m/s).
(iv) E.M. waves are transverse in nature.

Uses of Radio waves:

(i) They are used in radio and TV communication systems.
(ii) Cellular phones use radio waves to transmit voice communication in the ultrahigh frequency (UHF) band.

Uses of Microwaves:

(i) Microwaves are used in Radar systems for aircraft navigation.
(ii) Microwave ovens are used for cooking purposes.

1994 Views

The oscillating magnetic field in a plane electromagnetic wave is given by
B_y = (8 × 10^-6) sin [2 × 10¹¹ t + 300

π

x] T
(i) Calculate the wavelength of the electromagnetic wave.
(ii) Write down the expression for the oscillating electric field.

Given equation for oscillating magnetic field is,
B_y = (8 × 10^-6) sin [2 × 10¹¹ t + 300

π

x]T .

Comparing the given equation with the equation of magnetic field varying sinusoidally with x and t

B_{y} = B_{0} \sin (\frac{2 π x}{λ} + \frac{2 π t}{T})

we get,

\frac{2 π}{λ} = 300 π

∴

λ = \frac{2}{300} = 0.0067 m

and

B_{0} = 8 \times 10^{- 6} T

(i) Wavelength of the electromagnetic wave,

λ = 0.0067 m

(ii) Magnitude of electric field is calculated by,

E_{0} = {CB}_{0} = 3 \times 10^{8} \times 8 \times 10^{- 6}

= 24 \times 10^{2} = 2400 {Vm}^{- 1}

∴

The required expression for the oscillating electric field is,

E_{z} = E_{0} \sin (\frac{2 π x}{λ} + \frac{2 π t}{T}) = 2400 \sin (300 π x + 2 \times 10^{11} t) V / m .

1190 Views

Distinguish between sky wave and space wave propagation. Give a brief description with the help of suitable diagrams indicating how these waves are propagated.

Sky waves. A radio wave transmitted towards the sky and reflected by the ionosphere towards the desired location of the earth is called a sky wave.
Space waves. A radio wave that travels directly from a high transmitting antenna to the receiving station is called space wave.
Diagram. Various propagation modes of electromagnetic waves (EMW) is shown below.
Sky waves. A radio wave transmitted towards the sky and reflected by

4461 Views

Chapter Chosen

Book Chosen

Subject Chosen

Book Store

Previous Year Papers

Electromagnetic Waves

Distinguish between sky wave and space wave propagation. Give a brief description with the help of suitable diagrams indicating how these waves are propagated.