Diffraction Grating - Wavelength of Light

PHY 201 & 231

Students separate light sources into component wavelengths using diffraction gratings. A series of measurements are made using a mercury light source and given wavelength values to determine the grating spacing. This value will then be used to determine the unknown constituent wavelengths of a helium source.

Equipment

Notes

Install Hg spectral discharge tube in high voltage fixture and then arrange for smallest possible distance between the tube and aperture slit. Place the slit mount at 0cm or 100cm, whichever is closest to the wall. The rail support leg on the slit/wall end should be placed under the scale to the inside of the slit mount so as not to interfere with getting the light fixture as close to the slit as possible. The holder for the diffraction grating should be 100 cm from the slit. Separate the boxes that support the light fixture enough for the end of the rail to slide between them. This allows the light to get as close as possible to the slit. Make sure the slit is high enough to be centered on the brightest skinny part of the bulb. Leave He spectral tube on lab bench in safe and secure location. Only the single slit apertures are used in this lab. Mount high-efficiency (750 lines/mm) diffraction gratings in clip rod taking care to prevent clip from contacting window. Rest the spectral tube fixtures on empty hooked mass holders (found on shelf X36) or on other stable supports flanking optics bench to allow bulb to be very close to slit. For example, the lens holder blocks on S135 and the small blocks on S20 together can support the lights too. Once the meter stick is held by a clip rod and leveled by the lab jack, tape down the end of the meter stick to the lab jack.