White Light Dispersion on a Diffraction Grating

Demonstration of the dispersion of continuous spectrum light on a diffraction grating.

White light with a continuous spectrum produced by a thermal source passes through a slit and strikes a diffraction grating. The central (zero-order) maximum remains white, since all wavelengths constructively interfere in the forward direction. For higher-order maxima, the diffraction angle is directly proportional to wavelength.

This results in the separation of light into its constituent wavelengths, and we observe the spectrum on a screen. The shortest wavelengths (blue/violet) deviate least from the center; the longest wavelengths (red) deviate most.

Experimental setup. Halogen lamp with power supply, lens f = +100 mm with mounted slits, lens f = +50 mm, diffraction grating with 500 or 1000 lines per millimeter, screen.

Detail of screen with 500 lines/mm grating. The central maximum is white; the first-order side maximum shows the spectrum of the incoming light.

Detail of screen for 500 lines/mm grating and a narrower slit passing halogen light.

Diffraction grating with 1000 lines per millimeter. The spectrum in the first-order side maximum is now twice as wide.

Detail of the spectrum created by the 1000 lines/mm grating.