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Physics: Post your doubts here!

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A vibrating tuning fork is held above a glass cylinder filled to the top with water. The water level is
steadily lowered. A loud sound is first heard when the water level is 83.5 cm above the bench.
The next loud sound is heard when the water level is 17.1 cm above the bench.

The speed of sound in air is 340 m s–1.
What is the frequency of the tuning fork?
A 128 Hz B 256 Hz C 384 Hz D 512 Hz

please explain this question.
 
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A vibrating tuning fork is held above a glass cylinder filled to the top with water. The water level is
steadily lowered. A loud sound is first heard when the water level is 83.5 cm above the bench.
The next loud sound is heard when the water level is 17.1 cm above the bench.

The speed of sound in air is 340 m s–1.
What is the frequency of the tuning fork?
A 128 Hz B 256 Hz C 384 Hz D 512 Hz

please explain this question.
Wavelength= 83.5-17.1*(2)
Wavelength=1.33m
V=f*wavelength
F=V/wavelength
F=340/1.33
F=256Hz
 
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For destructive interference the path difference is 0.5λ, 1.5λ, 2.5λ, etc. For the first dark fringe i.e. n=1, it is 0.5λ so if u put the value of n in the C option so you will find that the p.d is 0.5λ, hence C is correct
But shouldnt it be D as it asks only for positive values of n
 
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But shouldnt it be D as it asks only for positive values of n
1 is a positive value of n...but if we use D for the 1st dark fringe, the path diff would be 1.5λ which is not correct...it should be 0.5λ
 
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