# 560 - Physics Questions Answers

Physics >> Optics >> Refraction IIT JEE

what is meant by rate of change of transverse magnification

Joshi sir comment

transverse magnification m = v/u so rate of change of trasverse magnification is dm/dt. calculate by differentiation

what is the main use of capacitors

Joshi sir comment

Capacitor :- A capacitor is a device to store energy

Focsl length of lens depends on colour of light?

Joshi sir comment

Yes, f depends on µ and µ depends on colour of light

the ratio of intensities between two coherent sound sources is 4:1. the difference of loudness in decibel between maximum and minimum intensities, when they interfere in space is

(1) 10 log2

(2) 20 log3

(3) 10 log3

(4) 20 log2

Joshi sir comment

I1/I= 4/1 so a1/a2 = 2/1 so amax/ amin = 2+1/2-1 = 3/1 so Imax/ Imin= 9/1

now loudness L = 10 log(I/I0)

so difference of loudness = L1-L2 = 10 log (9I/I) = 10 log9 = 20 log3

two coherent light sources A and B with seperation 2λ are placed the x-axis symmetrically about the origin. they emit light of wavelength λ. obtain the position of maximas on a circle of large radius R lying in the xy plane and with the center at origin

Joshi sir comment

for maxima path diffrence should be integral multiple of λ.

It is given that two coherent light sources A and B with seperation 2λ are placed on the x-axis symmetrically about the origin, and we have to get the points of maxima in the circle so at the point where circle cuts the X axis (path diffrence = 2λ) and where circle cuts the Y axis (path difference = 0) will be the point of maxima. Besides it one point will also be present in the circle in first quadrant where path difference will be λ so it will be a maxima also. it means total 8 points are there

two sound waves having intensities 4 unit and 9 unit coming from two coherent sources arrive at a point along the same line simultaneously  in same phase. resultant intensity at the point will be

Joshi sir comment

Use the formule I = I1 + I2 + 2 √I1 √I2 cosθ

here θ = 0

two sound sources of intensity I and 4I are used in an interference experiment. find the intensity at points where the waves from the two sources are superimposed with a phase difference of

(1) zero

(2) π/2

93)π

Joshi sir comment

Use the formule I = I1 + I2 + 2 √I1 √I2 cosθ

here θ = 0

in a large room, a person receives direct sound wave from a source 120m away from him. he also receives wave from the same source which reaches him after being reflected from 25m high ceiling at apoint midway between them. the two waves interfere constructively for wavelength of

(1) 20, 20/3, 4

(2) 10, 5, 3.33

(3) 10, 20, 30

(4) 15, 25, 35

Joshi sir comment

path difference between two waves one coming directly (path length = 120 m) and other coming after reflection (path length = 130), you can get this by geometry

so path difference = 10 m

for constructive interference path difference should be a multiple of wavelength so (2) is answer

a source of sound and an observer are moving with velocities v = (2i + 4j)m/s and v = (6i - 3j)m/s. if frequency of source is 400 Hz, then frequency observed by observer will be

Joshi sir comment

dot product of the two velocities given = 0, means observer and source are moving perpendicularly, so observer will observe the original frequency.

two waves are represented by y = Acosωt and y = Asin(ωt - π/6), superimpose on each other. the amplitude of the rsultant wave is