Two waves of equal frequencies, have their amplitudes in the ratio of 4:5. They are superimposed on each other. Calculate the ratio of maximum and minimum intensity of the resultant wave.
A
3:1
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B
9:1
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C
27:1
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D
81:1
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Solution
The correct option is D81:1 Let the amplitudes of the waves are A1 and A2, then : A1A2=45 ....(i)
The relation between intesity (I) and amplitude (A) is given by : I∝A2 ⇒I1I2=A21A22=1625
Now, ImaxImin=(√I1+√I2√I1−√I2)2
=⎛⎜
⎜
⎜
⎜
⎜
⎜⎝√I1I2+1√I1I2−1⎞⎟
⎟
⎟
⎟
⎟
⎟⎠2
As, I1I2=1625
⇒√I1I2=45
∴ImaxImin=⎛⎜
⎜
⎜⎝45+145−1⎞⎟
⎟
⎟⎠2=(9−1)2
⇒ImaxImin=81:1
Hence, (D) is the correct answer
Why this Question ?Concept : The intensity of the wave is directly proportional to the square of the amplitude. i.e. (I∝A2)