﻿ Which mode of propagation is used by short wave broadcast services? from Class 12 CBSE Previous Year Board Papers | Physics 2018 Solved Board Papers

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# CBSE Class 12 Physics Solved Question Paper 2018

21.

What do you understand by the statement, "Electromagnetic waves transport momentum"?

When electromagnetic waves hit the body the mass is lost by the momentum is conserved. i.e., transferred from Electromagnetic waves to the body.

22.

Why are infra-red waves often called heat waves? Explain.

Infrared waves produced from hot bodies. Hence, it often called heat waves.

23.

A symmetric biconvex lens of the radius of curvature R and made of glass of refractive index 1.5, is placed on a layer of liquid placed on top of a plane mirror as shown in the figure. An optical needle with its tip on the principal axis of the lens is moved along the axis until its real, inverted image coincides with the needle itself. The distance of the needle from the lens is measured to be x. On removing the liquid layer and repeating the experiment, the distance is found to bey. Obtain the expression for the refractive index of the liquid in terms of x and y.

Let,

f = focal length liquid +lens

f1 = focal length of lens

f2 = focal length of liquid mirror

24.

What kind of fringes do you expect to observe if white light is used instead of monochromatic light?

If the White light is used instead of monochromatic light then we see a sequence in which central fringe will be white and remaining will be coloured in VIBGYOR sequence.

25.

If one of two identical slits producing interference in Young’s experiment is convered with glass, so that the light intensity passing through it is reduced to 50%, find the ratio of the maximum and minimum intensity of the fringe in the interference pattern.

As we know that intensity is directly proportional to the square of the amplitude.

26.

If light of wavelength 412.5 nm is incident on each of the metals given below, which ones will show photoelectric emission and why?
 Metal Work Function (eV) Na 1.92 K 2.15 Ca 3.20 Mo 4.17

A metal will show photoelectric emission in below situation:
E ≥ work function
Now according to the relation below:

The work function of k and Na are less than the incident energy of the photon. Hence, only potassium (k) and Sodium (Na) will show photoelectric emission.

# 27.Which mode of propagation is used by short wave broadcast services?

Skywave is used to propagate shortwave broadcast services.

28.

A carrier wave of peak voltage 15 V is used to transmit a message signal. Find the peak voltage of the modulating signal in order to have a modulation index of 60%.

It is given that peak voltage of carrier wave, Ac = 15V

and, Modulation index, mu = 60% = 60/100

Then, the peak voltage of the modulating signal (Am) will be given by the relation,

29.

Show using a proper diagram how unpolarised light can be linearly polarised by reflection from a transparent glass surface.

a) When an unpolarized light falls on a polaroid, only those electric vectors that are oscillating along a direction perpendicular to the aligned molecules will pass through. Thus, incident light gets linearly polarized.

Electric vectors which are along the direction of the aligned molecules gets absorbed.

Whenever unpolarized light is incident on the boundary between two transparent media, the reflected light gets partially or completely polarized. When reflected light is perpendicular to the refracted light, the reflected light is a completely polarized light.

30.

The figure shows a ray of light falling normally on the face AB of an equilateral glass prism having refractive index 3/2, placed in water of refractive index 4/3. Will this ray suffer total internal reflection on striking the face AC? Justify your answer.

No, it will not suffer total internal reflection

so if i is less than 60o then TIR will not happen