An element emits light at two nearly equal wavelengths, 577 nm and 579 nm If the light is normally incident on a diffraction grating with 2000 lines/cm., what is the distance between the 3rd order fringes of the two wavelengths on a screen 1 m from the grating?

d = width of slit = 1 / 2000 cm = 5 x 10⁻⁶ m

Distance of screen D = 1 m.

wave length λ₁ and λ₂ are 577 x 10⁻⁹ and 579 x 10⁻⁹ m. respectively.

distance of third order bright fringe = 3.5 λ D/d

for 577 nm, this distance = 3.5 x 577 x 10⁻⁹ x 1 / 5 x 10⁻⁶

=.403 m = 40.3 cm

For 579 nm, this distance = 3.5 x 579 x 1 / 5 x 10⁻⁶

= 40.5 cm

Distance between these two = 0.2 cm.