Optics 6 Question 22

24. In a Young’s double slit experiment, 12 fringes are observed to be formed in a certain segment of the screen when light of wavelength 600nm is used. If the wavelength of light is changed to 400nm, number of fringes observed in the same segment of the screen is given by

(2001, 2M)

(a) 12

(b) 18

(c) 24

(d) 30

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Solution:

  1. Fringe width, ω=λDdλ

When the wavelength is decreased from 600nm to 400nm, fringe width will also decrease by a factor of 46 or 23 or the number of fringes in the same segment will increase by a factor of 3/2.

Therefore, number of fringes observed in the same segment

=12×32=18

NOTE Since ωλ, therefore, if YDSE apparatus is immersed in a liquid of refractive index μ, the wavelength λ and thus the fringe width will decrease μ times.



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