Using a spectrometer

The project Star spectrometer can be used to look at the spectra of many different sources. It is available from Learning Technologies, for under $20.

You can also build your own spectroscope. www.exploratorium.edu/

Incandescent light

An incandescent light has a continuous spectrum with all visible colors present. There are no bright lines and no dark lines in the spectrum. This is one of the most important spectra, a blackbody spectrum emitted by a hot object. The blackbody spectrum is a function of temperature, cooler objects emit redder light, hotter objects white or even bluish light.

Fluorescent light

The spectrum of a fluorescent light has bright lines and a continuous spectrum. The bright lines come from mercury gas inside the tube while the continuous spectrum comes from the phosphor coating lining the interior of the tube.

Neon light

The simplest source of a neon light is a night light which says 1/4 watt on the package. these night lights have neon lights inside them. You can also find neon lights in the windows of businesses.
Warning: even though they are called neon lights the lights do not necessarily contain neon gas, some contain argon or other gasses to produce different colors. The red ones contain neon.
The spectrum of the neon light has several bright lines. The red lines are bright.
The line used by helium neon lasers, 632.8 nm wavelength, does not appear in the spectrum of a neon tube. It is too dim relative to the other lines.
The lines of light are produced when electrons in an excited state decays into a lower energy state. The change in energy of the electron between these two states is precise and results in the emission of light with a narrow range of energies, a spectral line.

DO NOT LOOK AT THE SUN! even with a spectrometer.

Sunlight

Look at sunlight by looking at a white surface in the sun. White paper works well.
The solar spectrum is a continuous spectrum of an incandescent gas.
Look closely and you will see fine dark lines crossing the solar spectrum.
These fine lines are fraunhofer lines. The dark lines are produced by gas above the surface of the sun which absorbs some of the incandescent light from the sun below. Each of these lines is produced by one atom or ion. However several lines may be produced by one atom. Two lines close together in the yellow are a famous pair of sodium lines.

Light emitting diodes, LEDs

These come in many colors from red, orange, yellow and green to blue.
In Light emitting diodes electrons in a higher energy conduction band drop into holes in a lower energy band. The energy lost by the electrons is emitted as light. Thus there is usually one brightest color of light that appears as a line in the spectrum of the LED. In addition to the bright line there is usually also a dimmer, continuous emission of lower energy light. This lower energy light is produced when electrons decay to or from impurity states between the main energy bands.
In a solid the well defined energy states of electrons that would appear in atoms of a gas are spread into energy bands.

Street lights

Mercury Vapor

Looks bluish.
Has many bright lines of mercury. Like those that appear in fluorescents.

Low Pressure sodium

Looks orange. Has yellow lines of sodium vapor.

High pressure sodium

Looks yellow. Has broad bands of light.

Computer Screen

Look at a white screen on a computer. Nootice the bright spectral emiission bands.

Compare the spectral bands on a liquid crystal display screen to those on a cathode ray tube display.

You can also look at :
Candles
Aurora

You can also look at lights through a diffraction grating without using a spectrometer.
Just hold the grating in front of your eyes and look through it at a light.
This only works for lights which appear to be small points of light or narrow lines of light that line up with the lines in the diffraction grating.
The diffraction grating spreads the light right and left when its lines are vertical. So look at a vertical line of light with the diffraction grating lines also vertical, i.e. the spectrum to the right and left. Look at horizontal lines with the diffraction grating horizontal, i.e. with the spectra above and below the light.

I usually place the diffraction grating in a plastic page protector to protect it from scratches and fingerprints.

Candle
A candle across the room works well. You will see the continuous spectrum of the incandescent carbon particles in the flame.

A linear filament incandescent lightbulb
Or a distant lightbulb

The continuous incandescent blackbody spectrum will appear.

Fluorescent light

Bare fluorescent bulb at a distance, not an incandescent fixture!
The source of light needs to be narrow to produce a good spectrum.
Spectral lines from mercury and a continuous spectrum from the phosphor.

Stars

Few stars are bright enough to trigger the color sensitive cones of your eyes. However those that are such as Sirius in the winter and Vega in the summer will have a continuous incandescent spectrum. If you look at stars through a telescope you will gather more light and be able to see their colors better. Hold the diffraction grating in front of a small telescope or behind the eyepiece of a large one.

Lightning

Lightning usually makes bright vertical lines. So hold the lines of the diffraction grating vertical to spread the spectra to the sides. Look at lightning and you will see the continuous spectrum from hot incandescent gas plus spectral lines from excited atmospheric gasses.

Light emitting diodes, LEDs

These come in many colors from red, orange, yellow and green to blue.
These can be viewed at a large enough distance that they are small. You will see a bright narrow band of light plus a broader dimmer band.

Sunlight

To see the solar spectrum never look at the sun.
Make a large black region using black paper or cloth.
Put a bright white line down this blackness. Look at the line through the diffraction grating. You will see a continuous spectrum. It is difficult to see the fraunhofer lines.

Lasers

Never shine a laser beam into your eye!
However, you can project a laser dot on a wall and look at the dot through a diffraction grating. You will see just one dot of light spread to either side of the original dot representing the single color of light produced by the laser.
You can also shine the laser through the diffraction grating at a distant white screen or wall. Once again a single dot of light will be diffracted to each side. Each single dot represents the single color produced by the laser.

Other

Some sources that will not work well with a diffraction grating.
aurora are too broad and diffuse to produce a good spectrum through a diffraction grating.
Fluorescents in fixtures are too wide also.

Learning Technologies, Inc., 59 Walden St., Cambridge, MA 02140

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Scientific Explorations with Paul Doherty

© 1999

24 May 2000