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In Young’s double slit experiment as shown in figure, interference of light waves were observed on the screen. Thomas Young made two pinholes S₁ and S₂ (very close to each other) on an opaque screen in front of parent source S

S₁ and S₂ behaved as coherent sources, and produced interference patten on the screen which has alternate bright fringe and dark fringes. This experiment proved Huygen’s wave theory of Light.

In YDSE 20th bright fringe is obtained 20 mm distance from central bright point. The distance between the 5th bright fringe and 3rd dark fringe is

2.5 mm

It is given that, 20^th bright fringe is 20 mm from centre

∴ \( y_{20} = 20(\frac{λD}{d}) = 20 mm\)

Hence \(\frac{λD}{d}\) = 1 mm

Now, \( \underset{(Fifth bright)}{y_5} - \underset{(Third dark)}{y'_3} = \frac{5λD}{d}-\frac{5λD}{2d}\)

= \(\frac{5}{2}\frac{λD}{d} = \frac{5}{2}\) (1mm) = 2.5mm