Physics MCQs for NEET — Practice Questions with Answers

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A star emitting light of wavelength 5896 Å is moving away from the earth with a speed of 3600 km/sec. The wavelength of light observed on earth will 

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Explanation

 

ν'=ν1-vc1+vcλ=λ1+vc1-vc=λ1+3600c1-3600c=λ1.0120.988=1.012λ=70.75 Ao

A heavenly body is receding away from the earth such that the fractional change in λ is 1, then its velocity is :

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Explanation

When a planet recedes away from the earth and f is the frequency of vibration: f=cλIf v=velocity of moving away from the earth and λ'=apparent wavelength to an obeserver on the earth; λ'=c+vfλ'=c+vcλFractional change in wavelenth=λ'-λλ=c+vcλ-λλ=vc=1v=c 

A slit of width a is illuminated by white light. For red light (λ = 6500 Å), the first minima is obtained at θ = 30°. Then the value of a will be 

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Explanation

For first minima θ=λa or a=λθ

a=6500×108×6π (As 30o = π6 radian)

=1.24×104cm=1.24 microns

The radius of central zone of the circular zone plate is 2.3 mm. The wavelength of incident light is 5893  Å. Source is at a distance of 6m. Then the distance of the first image will be 

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Explanation

f1=r2λ=(2.3×103)25893×1010=9m.

What will be the angular width of central maxima in Fraunhoffer diffraction when light of wavelength 6000Å is used and slit width is 12×10–5 cm 

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Explanation

Angular width =2λd=2×6000×101012×105×102=1rad.

Direction of the first secondary maximum in the Fraunhofer diffraction pattern at a single slit is given by (a is the width of the slit) 

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Explanation

For nth secondary maxima path difference

dsinθ=(2n+1)λ2asinθ=3λ2

A parallel beam of monochromatic light of wavelength 5000 Å is incident normally on a single narrow slit of width 0.001 mm. The light is focused by a convex lens on a screen placed on the focal plane. The first minimum will be formed for the angle of diffraction equal to 

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Explanation

For the first minima dsinθ=λ

sinθ=λd   θ=sin15000×10100.001×103=30o

In the far field diffraction pattern of a single slit under polychromatic illumination, the first minimum with the wavelength λ1 is found to be coincident with the third maximum at λ2. So

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Explanation

Position of first minima = position of third maxima i.e., 1×λ1Dd=(2×3+1)2λ2Dd  λ1=3.5λ2

The angle of polarisation for any medium is 60o, what will be critical angle for this

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Explanation

By using μ=tanθpμ=tan60=3,

also C=sin11μC=sin113

In the propagation of electromagnetic waves, the angle between the direction of propagation and plane of polarisation is 

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Explanation

Plane of polarization is a confinement of the electric/magnetic field vector to a given plane along the direction of propagation. Therefore the angle between them is 00

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