Introduction

Use an overhead projector to project an image of a slit on a screen. A diffraction grating will spread the white light passing through the slit into a spectrum from red through green to blue. The beautiful glowing colored spectrum will often bring an appreciative Aah! from a class. Place a colored filter on top of the slit and the colors of light that get through the filter will appear in their place in the spectrum.

Material

An overhead projector and white screen,
A holographic diffraction grating large enough to cover the entire exit hole of the projector. (Available from several places such as Learning Technologies Inc....)
A piece of cardboard larger than the diffraction grating in which to mount the diffraction grating, plus some sticky tape.
Thin opaque cardboard, large enough to cover the overhead projector. I use manilla folders.
Transparent, colored pieces of plastic.

Assembly

Cut a hole in the cardboard slightly smaller than the diffraction grating.
Tape the diffraction grating over the hole.

In the other piece of cardboard, cut a slit 4 to 6 mm (1/8 to 1/4 inch) wide and 10 cm long. When projected the slit should make a vertical line of light on the wall. Focus the image of the slit.

Place the cardboard with the slit in it on the overhead projector so that the slit is projected as a vertical line on the screen. Cover the entire overhead projector with cardboard so that no light gets out except the light that comes through the slit.

Hold the cardboard-mounted diffraction grating over the exiting light from the overhead projector, rotate it until two spectra of light appear to the right and left of the slit of light. Tape it in place. In this orientation the lines on the diffraction grating will be vertical. Rotate the overhead projector, or move the screen until the spectrum appears on the screen and the white image of the slit too.

Wider slits will produce a brighter spectrum, narrower slits will produce a sharper yet dimmer spectrum. I suggest you start with slits about 6 mm (1/4 inch) wide.

To Do and Notice

1. Notice the slit still appears white even after the diffraction grating is in-place, this is called the zero'th order diffraction.
Spectra of light appear on both sides of the slit, these are the first-order spectra. A second more widely spread spectrum may appear beyond the first, this is the second order spectrum. Ignore the second order for this activity.
In both first order spectra, blue light is near the white slit and red is farthest away.
Notice the colors that appear between blue and red. (see the Etc. for Newton's naming of these colors.) Children will often have heard the word ROYGBIV as a way to remember the colors in order. Red Orange Yellow Green Blue Indigo and Violet.

2. Cover the top half of the slit with a transparent yet colored piece of plastic, e.g. yellow. The full spectrum of the white light from the bottom of the slit appears to the side. Only the light colors which get through the filter appear to each side of the colored part of the slit. A yellow plastic filter will usually transmit red and green while blocking the blue. A magenta filter will pass both red and blue while blocking green, and a cyan filter will pass through green and blue while blocking red.

What’s Going On?

1. When light passes through a diffraction grating some is bent to the side. Each color of light is bent through a different angle: red light is bent through the largest angle while blue light is bent through the smallest. ( see the Two-slit Interference snack for more details on how light is bent by interference.) Each position in the spectrum corresponds to a wavelength of light. So each color in the spectrum is a human perception of a single wavelength of light.

2. Transparent plastic filters absorb a wide range of frequencies of light and allow other frequencies to pass through. Yellow filters absorb blue and allow green and red to pass through. Human eyes perceive the resulting sum of green and red as the color yellow. The energy of the light absorbed by the filter, warms the filter.

So What?

Diffraction gratings can be used to explore the light emitted by different sources such as the sun, distant stars, "neon" tubes, and fluorescent tubes. The spectrum of light tells us what elements the light sources are made of and also what absorbing elements there are between the light source and us. Dark lines appear in the spectrum of sunlight due to light absorb by atoms in the outer atmosphere of the sun.

Going Further

Replace the slit with a strip of manila folder, an anti slit, observe the resulting colors.
Visit the Anti-slit activity.

Etc.

1. The naming of the colors

Isaac Newton named the colors of the spectrum we use today. He made 7 names of the colors of light to correspond to the seven named musical notes in an octave. Instead of do-re-me-fa-so-la-te he named them ROYGBIV, red, orange, yellow, green, blue, indigo, violet.

Going Further

Project a spectrum and ask your students how many colors they see. Most, like Newton will pick a small number of colors, like 5 or 7.

2. Non-spectral colors

Notice that yellow and cyan appear in the spectrum of the slit however, magenta is not a color of the spectrum. This is because humans perceive yellow as the color produced by a single wavelength of light in the spectrum, they also perceive yellow as the mixture of two different single wavelengths, red and green. The same is true for cyan. No single wavelength of light produces the perception of magenta, red plus blue wavelengths must be combined. Therefore magenta is not a color of the spectrum. This means that white is not a color of the spectrum since at least three different wavelengths must be mixed to produce a perception of white.

The Project STAR spectrometer is available from Learning Technologies Inc., 40 Cameron Ave., Cambridge, MA 02140 phone 617-628-1459

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Scientific Explorations with Paul Doherty

© 1999

22 May 2000