Questions - Answers
Very Short Answer type Questions
1. What is the average Wavelength of X - Rays?
A: 0.1 A° to 100 A°
2. Give any one use of infrared rays.
A: Uses: 1) In reading the secret writing of ancient walls.
2) In knowing the molecular structure.
3) In taking photographs during night and foggy conditions.
3. If the wave length of electromagnetic radiation is doubled. What happens to the energy of photon?
4. What is the principle of production of electromagnetic waves?
A: An accelerated charge produces electric and magnetic fields which vary both in space & time. The two oscillating fields act as sources of each other and sustain each other. This results in the propagation of electromagnetic waves through space.
5. What is the relation between the amplitudes of the electromagnetic fields in free space for an electromagnetic wave?
A: The amplitude ratio of electric and magnetic fields in free space is
6. What are the applications of Microwaves?
A: 1) For radar system in air - craft navigation.
2) In long distance communication.
3) In microwave ovens.
7. Microwaves are used in radars. Why?
A: Because of their smaller wavelength, microwaves do not bend around the corners of an obstacle and also do not spread. So microwaves are transmitted as a beam signal in particular direction.
8. Give two uses of infrared rays.
A: Uses: 1) Photography through haze and fog and in night.
2) Thermography.
9. The charging current for a capacitor is 0.6 A. What is the displacement current across the plates?
A: According to property of continuity, charging current = displacement current = 0.6 A.
Short Answer type Questions
1. What is green house effect and its contribution towards the surface temperature of earth?
A: Radiations coming from the sun heats up the earth surface. In turn the earth radiates heat radiations (or) IR radiations. Because of its low frequency these IR radiations are reflected back to earth surface by clouds and carbondioxide present in earth atmosphere. Due to these heat radiations the surface temperature of earth increases. This phenomenon is called Green house effect.
Long Answer Type Questions
1. State six characteristics of electromagnetic waves. What is Green house effect?
2) They do not require any material medium (or) vacuum, all electromagnetic waves travel with the same speed equal to speed of light.
Speed of electromagnetic waves in free space is given by the equation.
Where µ0 and ε0 are permeability and permitivity of free space. In material medium, the speed of electromagnetic waves is given by the equation.
Where µ and ε are permeability and permitivity of medium.
3) The electromagnetic waves carry energy as they travel through space and this energy is shared equally between electric and magnetic fields.
5) Electromagnetic waves obey the principle of super position. They show the properties of reflection, refraction, interference, diffraction and polarisation.
6) Electromagnetic waves are not deflected by electric and magnetic fields.
7) Electromagnetic waves transport linear momentum as play travel through space.
U = energy transformed by e.m. waves.
C = Velocity of light
8) The amplitude ratio of electric and magnetic field is constant and is equal to velocity of e.m. waves in free space.
9) The electric field (E) of an electromagnetic wave is responsible for the optical effects. Therefore electric field vector (E) is called light vector.
Green house effect:
The heating of earth's atmosphere due to IR rays reflected by the layer of CO2 in atmosphere is called Green house effect.
2. Give the brief history of discovery of knowledge of electromagnetic waves.
A: Sources of electromagnetic waves: Waves of energy that are caused by the acceleration of charged particles and consists of electric and magnetic fields vibrating transversely and simultaneously at right angles to each other and to the direction of propagation are called electromagnetic waves. These waves are produced in the following physical phenomena.
1) An oscillating charge emits on electromagnetic wave which has the same frequency of that of the oscillating charge.
2) An accelerated charge emits an electromagnetic wave.
3) Electromagnetic waves are produced when fast moving electrons hit a target of high atomic number.
Nature of Electro magnetic waves: Electromagnetic wave is transverse in nature. It can be shown from Maxwell's equations that electric and magnetic fields in on electromagnetic wave are perpendicular to each other and to the direction of propagation. It is as shown in the figure.
The electromagnetic wave propagating along the Z direction. The electric field Ex is along the X-axis and varies sinusoidally with Z at a given time (t). The magnetic field By is along the Y-axis and again varies sinusoidally with Z. The electric field & magnetic field Ex and By are perpendicular to each other and to the direction Z of propagation. We can write Ex and By as follows.
Ex = E0 Sin (kz - ωt)
By = B0 Sin (kz - ωt)
There k is related to the wavelength λ of the wave by the usual equation
The relation ω = ck is the standard one for waves. This relation is often written in terms of frequency ν and wavelength (λ) as
It is also seen from Maxwell's equation that the magnitude of the electric and magnetic field in on electromagnetic wave are related as
Hertz's equation:
Even though Maxwell's theory was advanced in 1865, Heinrich Hertz, 1888 actually demonstrated the production and detection of electromagnetic waves by electrical means.
The above figure shows the experimental arrangement used by Hertz. Two large metal spheres S and S', are connected to two large metal plates P and P' respectively. The spheres are connected to an induction coil.
By causing a discharge between the spheres, the current distribution in the plates in such that an electromagnetic wave (EM) is generated with its electric vector parallel to the gap SS'. The radiation that produced was detected by 'Hertz' by using an open wire loop which acted as a resonator. The EM wave reaching at the gap GG' of the detector has an electric field strong enough to establish a high potential difference between this gap and cause a spark.
When the detector gap GG' is at right angles to the source gap SS'. No electromagnetic radiation is detected. The sensivity is maximum when the gap GG' is parallel to gap SS'. This clearly demonstrates the state of polarisation. Leading to the direction of propagation
Problems
1. A radio can tune into any station in the 7.5 MHz to 12 MHz band. What is the corresponding wavelength band?
2. What physical quantity is the same for X-rays of wavelength 0-10m, red light of wavelength 6800 A° and radiowaves of wavelength 500 m?
Sol: The speed in vacuum is the same for all. This speed is 3 × 108 ms-1.
3. The amplitude of the magnetic field part of a harmonic electromagnetic wave in vacuum is B0 = 510 T. What is the amplitude of the electric field part of the wave?
4. About 5% of the power of a 100 W bulb is converted to visible radiation. What is the average intensity of visible radiation?
a) At a distance of 1 m from the bulb?
b) At a distance of 10 m?
Assume that the radiation is emitted isotroptically and neglect reflection.
Sol: Power converted into visible radiation.
