Question

You have an optical kit that contains different lasers of wavelength 400nm, 500 nm & 700 nm, double slits with slit seperation 0.25 mm and 0.5 mm, a screen and an ordinary ruler [scale that you use in school]. Find minimum distance in meters that you can place the screen from the double slit to observe YDSE pattern on screen accurately .

Answer 1

Least count of ruler is 0.1 cm so fringe width W should be of order 0.1 cm.
$w=\frac{\lambda D}{d}orD=\frac{wd}{\lambda }=\frac{0.1\times {10}^{-2}\times 0.25\times {10}^{-3}}{700\times {10}^{-9}}$
D = 0.357 m