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Electromagnetic Waves

Multiple Choice Questions

31.

In case of linearly polarised light, the magnitude of the electric field vector

does not change with time
varies periodically with time
increases and decreases linearly with time
is parallel to the direction of propagation

32.

Assertion: The true geographic north direction is found by using a compass needle.

Reason: The impurities meridian of the earth is along the axis of rotation of the earth.

If both assertion and reason are true and reason is the correct explanation of assertion
If both assertion and reason are true but reason is not the correct explanation of assertion
If assertion is true but reason is false
If both assertion and reason are false

33.
Assertion: There are very small sporadic changes in the period of rotation of the earth.
Reason: Shi fling of large air masses in the earth's atmosphere produce a change in the moment of inertia of the earth causing its period of rotation to change.

If both assertion and reason are true and reason is the correct explanation of assertion

If both assertion and reason are true but reason is not the correct explanation of assertion

If assertion is true but reason is false

If both assertion and reason are false

If both assertion and reason are true and reason is the correct explanation of assertion

Normally when a body is moving in a straight line, whatever be the internal changes, the centre of mass continuous in its trajectory with the same velocity and a system that is rotating continues to rotate with the same angular velocity and changes occur in such a way that the angular momentum is conserved. Classical physics, taking the whole of the earth as a system does not allow sporadic changes in the speed of the earth.

However, according to the information on the earth's rotation by U.S. Naval Observatory, sporadic ( occurring at irregular interval ), changes of the speed of rotation of the earth have been observed and this is attributed to the influence of the moon on the tides. This makes it necessary to take the earth and the moon ( at least ) as one system.

In the reason given, though normally when taking the earth as a single system, internal changes do not affect the motion, now that one has to take the earth-moon system, the reason that the earth's angular velocity can also change due to the events on earth alone cannot be ruled out. Then the total angular momentum of the earth-moon system will be conserved. The reason is also right, though according to the rules of the game for the 12^t^h class examinations, one can take only the earth as a whole as a system. In that case, both the assertion and reason are wrong.

34.

The maximum distance upto which TV transmission from a TV of height h can be received is proportional to

h^1/2
h
h³
h²

35.

In short wave communication, waves of which of the following frequencies will be reflected back by the ionospheric layer having electron density 10¹¹ per m³ ?

2 MHz
10 MHz
12 MHz
18 MHz

36.

X-ray beam are affected by

electric field
magnetic field
both (a) and (b)
none of these

37.

All components of the electromagnetic spectrum in vacuum have the same

energy
velocity
wavelength
frequency

38.

The difference between soft and hard X-rays is of :

velocity
intensity
frequency
polarization

39.

If v_g, v_x and v_m are the speeds of gamma rays, X rays and microwaves respectively in vacuum then :

v_g > v_m > v_x
v_g > v_x > v_m
v_g = v_m = v_x
none of these

40.

The minimum wavelength of X-rays emitted by X-ray tube is 0.4125 Å. The accelerating voltage is :

30 kV
50 kV
80 kV
60 kV