Suggestions to Use the Night Spectra Quest Diffraction Grating

Diffraction gratings are neat scientific tools that can show you which individual colors of light that reach your eyes from various light sources, such as sunlight, fluorescent bulbs, street lamps, neon signs, incandescent bulbs, etc.

Light is an electromagnetic wave that travels at the very fast speed of approximately 3×108 m/s away from its light source. It may, for all practical purposes, be composed of a single wavelength, λ, in which case it appears to be of a single color. (This is the case, for example, with red and green laser pointers, with which you may be familiar). A pictorial representation of such a monochromatic light wave, illustrating how the electric and magnetic fields change along the direction of propagation of the wave, is shown below:

electomagnetic wave

Light from a particular source may instead be composed of a mixture of many different wavelengths, in which case its apparent color is that which is perceived by the eye when it encounters this mixture (for example, equal amounts of red light and green light appear to us to be yellow).

A diffraction grating allows us to separate light into these constituent colors, or wavelengths, and the resulting spectrum in turn provides useful information about the light source itself. For example, the range of wavelengths emitted by an incandescent (filament) light bulb is very different from that from an indoor fluorescent lamp, which in turn is very different from that of a street lamp, and the diffraction grating can make these differences very apparent.

The diffraction grating in your hand consists of clear plastic sheet on which thousands of thin parallel lines have been ruled. They are so fine you can barely see them with a microscope. When a beam of light passes through this diffraction grating, the straight-through beam is the same color (and therefore the same mixture) as the original source of light, but at other, specific, angles to the incident light beam the constituent colors of the light are each emitted, as shown below for the case of white light (all colors).


To optimally use your diffraction grating to examine a light source it is best to find light that comes from a narrow slit or a point (for example, a long fluorescent tube or a small lamp that is relatively far away. (Note that looking for too long at bright sources of light, such as intentionally looking at the sun, can cause eye damage, so take care!).

In the picture below, the observer is looking at a horizontally-oriented fluorescent tube and holding the grating close to his eye in the orientation shown (so the grating rulings are also horizontal). At various angles an array of colored lines (the spectrum) is then visible. Each of these colors of light is present in the original (what appears to be white) light source.

using the grating

On one side of the grating card are shown some examples of spectra from various light sources. Spectrum a) is that from an incandescent lamp such as a car headlight, with a spectrum consisting of a complete continuum of colors. The spectrum of the fluorescent lamp (spectrum b) contains a bright line spectrum on top of a continuous spectrum. The lines correspond to specific (or quantized) wavelengths of light emitted by mercury atoms excited by collisions with electrons within the tube, while the continuum is due to light emitted from a white phosphor coating on the inside walls of the tube (and explains why you can't see through a fluorescent tube, even when it is turned off). The fact that only specific, and characteristic, wavelengths are emitted from an atom such as mercury both confirms the validity of quantum theory and demonstrates how this technique can be used to unambiguously detect particular atoms, molecules, or other materials within a given sample. Spectra (c) - (i) are example spectra from discharge tubes with no or little phosphor coating in which particular atoms are excited by electronic collisions, and these each emit light with a characteristic "fingerprint".

To find out more about light, quantum theory, electric discharges or spectroscopy, contact us.

Buy the Night Spectra Quest Diffraction Grating:
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