f 720-nm and 620-nm light passes through two slits 0.68 mm apart, how far apart are the second-order fringes for these two wavelengths on a screen 1.0 m away

0.0003 m = 0.3 mm

Explanation:

For constructive interference in the Young's experiment.

The position of the mth fringe from the central fringe is given by

y = L (mλ/d)

λ = wavelength = 720 nm = 720 * 10⁻⁹ m

L = distance between slits and screen respectively = 1.0 m

d = separation of slits = 0.68 mm = 0.68 * 10⁻³ m

m = 2

y = 1 (2 * 720 * 10⁻⁹ / (0.68 * 10⁻³) = 0.00212 m = 2.12 mm

For the 620 nm light,

y = 1 (2 * 620 * 10⁻⁹ / (0.68 * 10⁻³) = 0.00182 m = 1.82 mm

Distance apart = 2.12 - 1.82 = 0.3 mm = 0.0003 m