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CBSE Class 12 Physics 2015 Delhi Set 1 Paper

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Question : 21 of 26
Marks: +1, -0
Answer the following questions:
(a) In a double slit experiment using light of wavelength 600 nm600 \text{ nm}, the angular width of the fringe formed on a distant screen is 0.1∘0.1^{\circ}. Find the spacing between the two slits.
(b) Light of wavelength 5000 A˚5000 \text{ Å} propagating in air gets partly reflected from the surface of water. How will the wavelengths and frequencies of the reflected and refracted light be affected ?
Solution:  
Finding the spacing between two slits Effect on wavelength and frequency of reflected and refracted light
(a) Angular width of fringes
θ=λd,\theta = \frac{\lambda}{d},
where d=d = separation between two slits
Here θ  =0.1∘=0.1×  π180   radian  \theta \; = 0.1^{\circ} = 0.1 \times \; \frac{\pi}{180} \; \text{ radian} \;
∴ d  =  600×10−9×1800.1×π md \; = \; \frac{600 \times 10^{-9} \times 180}{0.1 \times \pi} \text{ m}
  =3.43×10−4 m\; = 3.43 \times 10^{-4} \text{ m}
  =0.34 m\; = 0.34 \text{ m}
(b) For Reflected light: Wavelength remains same Frequency remains same
For Refracted light: Wavelength decreases Frequency remains same
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