Rainbow Science Project
Make a Spectroscope
A spectroscope is a device that separates light into its individual colors in a rainbow-like band called a spectrum. The spectrum of each chemical element looks slightly different, so scientists use spectroscopes to see what elements make up the sun and stars. Try making your own spectroscope and turn any light in your house into a rainbow!
What You Need:
- Cardboard tube from a toilet-paper roll
- Black construction paper
- Diffraction grating
What You Do:
- Line the inside of the cardboard tube with black construction paper.
- Cut two circles out of the construction paper. Make them slightly bigger than the cardboard tube. In the middle of one of the circles, cut a square with sides about 1.5cm long. Tape the diffraction grating over the square hole, and then tape the circle over one end of the cardboard tube with the diffraction grating on the inside.
- The center of the other circle needs to have a very narrow slit for light to enter through. Since cutting this can be difficult, just make a rectangular hole about 2.5cm long. Then tape two rectangles of black paper over the hole, leaving a narrow slit between them. It’s a good idea to cut your rectangles from the edge of the paper so you are sure to have straight edges to use for the sides of the slit. (The right-hand photo shows this step using a white circle so you can easily see how to line up the rectangles.)
- Hold the slotted circle over the other end of the cardboard tube and look through the grating end at a light source. You should see a color spectrum on the inner side of the tube. (There might be one on either side of the slit.) It may be very narrow; turn the slotted circle until the spectrum widens out, and then tape the circle in place.
You now have a working spectroscope! Use it to look at several different types of light: a normal incandescent light bulb, fluorescent light, LED light, a glow stick, even sunlight. (But be very careful – do NOT look directly at the sun through your spectroscope!) You can also look at the flame of a match or candle, if you have someone else hold it for you.
Do you see a difference in each light’s spectra? Incandescent light bulbs and sunlight will produce a continuous spectrum, where all the colors merge smoothly into each other. (Stars actually emit a dark-line spectrum, which has the colors broken up by dark lines. Only very precise spectroscopes can see the dark lines, however, so the sun looks like a continuous spectrum.) A fluorescent light will produce a bright-line spectrum, which has bright lines separated by dark spaces. Try drawing each spectrum with colored pencils and comparing them. You can also try varying the width of the slit – does that change the appearance of the spectrum?
Each of the colors that light is made of has its own wavelength, which reflects and refracts at its own angle, different from all the other colors. When light hits the diffraction grating, it is reflected back onto the wall of the spectroscope. All the little grooves on the grating separate the colors so they reflect at their different angles. The beam of light hits the diffraction grating at one angle, but since each color bends back at a different angle, they are spread out along the spectroscope wall, allowing you to see them. Keep reading to find out more about the visible spectrum of light, and get answers to some classic science questions!
Visible Light Science Lesson
Why is the sky blue?
One of the most beautiful things about our world is a blue sky on a clear, sunny day. If you’ve seen pictures of the Apollo astronauts on the moon, you might have noticed that the sky above them was black as night even in bright sunshine. What makes the difference? Why is Earth’s sky blue?
Unlike the moon, the earth is surrounded by an atmosphere. The atmosphere is a mixture of gasses, mostly nitrogen and oxygen. The way the sun’s light travels through the atmosphere makes the sky colored.
Why blue? It doesn’t look like it, but light is made up of several different colors, like you see in a rainbow. Each of these colors travels in a wave, but the wavelength (distance between the tops of each wave) varies. Red light has a long wavelength, while blue light has a much shorter wavelength. When light from the sun enters our atmosphere, the waves collide with gas molecules. The longer wavelengths, like red and yellow, pass straight through and appear to us as “regular” sunlight. Shorter wavelengths, like blue, bump into the gas molecules and scatter in different directions. Some of it still makes it through directly, but the rest is reflected back to our eyes from all directions, so the whole sky looks blue.
You can see similar light scattering by mixing half a teaspoon of milk with a large (quart-size) jar of water. In a darkened room, shine a flashlight through the jar and look at the water. It should have a bluish tint, because the milk particles are scattering the blue light from the flashlight just like the gas molecules in our atmosphere do.
Maybe you’re asking a follow-up question: why are sunsets pink and orange? When the sun is low in the sky, near the horizon, its light has to travel through a lot more atmosphere to reach us. The blue light is scattered so much in the extra atmosphere that none of it reaches our eyes from that direction, leaving us to see the beautiful reds and oranges there instead. Sometimes clouds or air pollution can make a sunset even more red because the particles in the cloud help scatter away the shorter wavelengths.
Add a little more milk to your jar – do the extra milk particles allow you to see an orange tint? Try looking in the side of the jar directly opposite where the flashlight is. This is like looking at the sun on the horizon.
Why do stars twinkle?
If we lived on the moon, we probably wouldn’t know the song, “Twinkle, Twinkle, Little Star.” Just like blue sky, a star’s twinkle is a result of Earth’s atmosphere. Stars don’t twinkle in space! As the light from a star enters the atmosphere, it hits gas molecules and scatters. Since the star is so far away, we only see a tiny beam of light from it. This beam gets scattered away from our eyes and then back into them almost like it is blinking on and off. It happens so fast that it just looks like it is twinkling. (Planets are closer to us and send more light; if some of the light beams are scattered away, others still get through to us, so planets don’t usually twinkle.)
Stars twinkle more when they are close to the horizon, because the light has to travel through more atmosphere before it reaches our eyes. Weather can affect how much stars twinkle, too. Cold air scatters more light than warm air, because molecules are closer together in cold air, making it harder for light to pass through without interference. (Think about how it’s harder to walk in a straight line through a crowd of people than through an empty room!)
The temperature variation in the atmosphere affects what astronomers call “seeing.” If there is a lot of variation, even a perfectly clear night will be bad for stargazing with a telescope. You might be able to see a twinkling star clearly with your naked eye, but an astronomer will say it’s “bad seeing” because it’s hard to study a star that just won’t stop flickering! Next time you’re outside on a clear winter night, look for twinkling stars near the horizon. If you have a telescope, you may even see the stars changing colors because their wavelengths are scattering in different directions.