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NCERT Class XII Chapter
Wave Optics
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Question : 4 of 21
Marks: +1, -0
In a Young’s double-slit experiment, the slits are separated by 0.28 mm and the screen is placed 1.4 m away. The distance between the central bright fringe and the fourth bright fringe is measured to be 1.2 cm. Determine the wavelength of light used in the experiment.
Solution:  
Here d = 0.28 mm, D = 1.4 m
Distance of fourth bright fringe from center = 1.2 cm = 1.2 × 10−210^{-2} m
Linear position of nth bright fringe , yny_n = nDλd\frac{nD\lambda}{d}
Linear position of 4th bright fringe , y4y_4 = 4Dλd\frac{4D\lambda}{d}
1.2 × 10−210^{-2} = 4(1.4)λ0.28×10−3\frac{4(1.4)\lambda}{0.28 \times 10^{-3}} , λ = 600 nm
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