Absorption lines in the spectrum of sunlight

Demonstration

Using a spectrometer to see absorption lines.

Apparatus and Materials

• Spectrometer
• High-dispersion prism (flint or lead glass)
• Convex lens, +6D to +10D
• Plane mirror
• Lens holder

Health & Safety and Technical Notes

The spectrum should be projected onto a screen rather than viewed down the telescope part of the spectrometer, to minimize the risk of stray sunlight being directed into the observer's eye.

Read our standard health & safety guidance

If the dark absorption lines do not appear, the slit needs to be made narrower. See the manufacturer's guide book for details of setting up the spectrometer.

Procedure

1. Set up the prism spectrometer in the usual way to give a good spectrum of white light on a small screen. With the slit narrowed down very considerably, direct sunlight into the slit with a plane mirror. The lens is used to converge the light to a focus 20 to 30 mm in front of the slit.
2. Darken the room so that the spectrum does appear to be a bright one, and ask students to view the spectrum one at a time. Arrangements have to be made to ensure that the sunlight continues to fall on the slit.

Image courtesy of the Department of Physics and Astronomy at Dartmouth College, New Hampshire

Teaching Notes

• The Sun produces an absorption spectrum, with dark lines across its spectrum. Chemical elements in the Sun's corona absorb specific wavelengths of light so their electrons are excited to higher energy levels. Emission takes place equally in every direction, with the result that the intensity of light in the Earth's direction is much reduced.
• The dark lines are called Fraunhofer lines. These are very important in astrophysics as they reveal the composition of the outer layers of stars. Slight shifts in the positions of the lines indicate the speed of the stars' approach or recession.

This experiment was safety-tested in January 2007