Physics Photo of the Week – April 16, 2004

Warren Wilson College

Hydrogen emission spectra and sun spectra

The top part of the photo is the spectrum of a glass tube filled with hydrogen that is excited with a high voltage. It resembles a neon sign, except the glass is straight and filled with hydrogen gas and not neon. The discharge tube is off to the right side of the photo. The camera is aimed at a dark wall and fitted with a diffraction grating so that the spectrum is made visible.

The main features to notice in the hydrogen spectrum are the bright emission lines. The red line at the left, the blue-green line in the right center, and the violet line is the second from the right. On the extreme right is an ultraviolet line that human eyes cannot see, but is barely detectable by a digital cameera. These visible lines in the hydrogen spectra are called the Balmer lines of hydrogen. The triumph of quantum mechanics of the 20th century was able to explain the physics of this emission.

The bottom part of the spectrum is that of the sun. It is obtained by photographing a slit in the window blind with a similar grating set-up. The sun was obtained simultaneously with the hydrogen spectrum in the physical science class on Tuesday, April 12, 2004 – a very cloudy day. Notice how the bottom spectrum is a continuous blend of all colors, but if you look closely, you can see some dark lines – especially coincident with the bright lines of the hydrogen. The dark lines are called absorption lines, they are characteristic of all stars, and are caused by absorption of light by the elements in the stars' atmospheres. Notice that there are several other dark lines for elements other than hydrogen.

Click here for the archived Physics Photo of the Week.

Donald F. Collins