A webquest on Electromagnetic Radiation and where you find it

Answers to the What do you Know Section

Website questions at http://amazing-space.stsci.edu/resources/explorations/light

• Everything, including you, radiates light. • Light travels in waves, as does sound and ripples on a pond. • Wavelengths of light can be as small as the size of an atom or as

big as the height of a mountain. • Our eyes are only sensitive to the wavelengths of visible light. • Each color of visible light that we see has a different wavelength.

Our eyes respond differently to each wavelength. That is how we perceive color.

• Starlight is a mixture of many different colors because it is made up of many different wavelengths.

• The amount of light emitted at each wavelength depends on the temperature of the object.

• Wavelength and energy are related. The shorter wavelengths carry more energy than do longer wavelengths.

1. Bees have eyes that see ultraviolet radiation as well as visible light. Moths, on the other hand, only see visible light, like us. If you were a flower who wanted only bees and not moths to visit you, what could you do to make sure bees (and not moths) get the message?

2. Snakes' eyes are sensitive to infrared radiation. This makes them good hunters at night in the dark. Why?

3. The Sun puts out most of its light at visible wavelengths. As creatures on earth evolved, they developed eyes that respond to the short wavelengths of visible light. Imagine that you are a being on a planet whose Sun puts out most of its light at radio wavelengths, and you don't see the wavelengths of light visible to us on Earth. Do you think you would see the "colors" of radio light?

4. Our atmosphere is turbulent--wind and heat from the ground move the air around. This causes the light from stars to twinkle when we look at them from the ground. Do planets twinkle?

5. Humans are nearly blind to most wavelengths of light. If you lost your ability to see visible light, but could choose another wavelength region of the electromagnetic spectrum, what would you choose? Describe how you would see the world around you.

6. How would you prepare yourself to view the light from a solar eclipse as compared to the light coming from a lunar eclipse?

7. The water molecules in our atmosphere stop infrared rays that come from the Sun and other stars. If you built a telescope on the ground to study infrared light coming from distant stars, where would you locate it and why?

• Wavelength, frequency, and energy are related.

• The greater the energy, the shorter the wavelength and the higher the frequency.

• Shorter wavelengths carry more energy than do longer wavelengths.

1. Ultraviolet light penetrates our skin and gets stopped (absorbed) by the tissues just beneath it. X-rays penetrate all the way through skin and muscle and are finally stopped by bones and denser objects. Why does X-ray light get farther into our bodies than ultraviolet?

2. When you tune your radio (it receives radio light!) to a particular station, the dial shows the frequency at which the station broadcasts. For example, to listen to WROC you turn the dial to 101 Megahertz (101 million cycles per second). Instead of displaying the frequency, could the dial on the radio just as well show wavelength? Why?

3. What if we lived on Venus instead of Earth? The dense clouds forever blanketing that planet would block all starlight. Would we ever know that there are such things as stars?

4. Why is the order of colors in a rainbow always the same? For example, why is red never right next to blue??

•Temperature and color are related. •The hotter the object, the bluer it appears.

•Hot stars look blue because they emit more blue wavelengths than wavelengths of other colors.

1. At a temperature of 37 C (98.6 F), your body radiates mostly at infrared wavelengths. Imagine that you visit a strange planet where the creatures' body temperature is so cold that they radiate mostly at radio wavelengths. Their sun, however, is just like ours. Would you be able to see them? Why? hint: You radiate at infrared wavelengths that our eyes are not sensitive to, yet you can see your friends. Why?

2. The clothes you are wearing today probably have lots of colors: red, green, blue. Does that mean that that they have different temperatures? For example, if you're wearing a red sweater does that mean it has a different temperature than your green socks? Does your red sweater put out red light like a red star does?

3. Most people would say that stars are very hot. However all stars do not have the same temperature. Some stars are much hotter than others. Scientists use color as one way to tell the temperature of a star. Why is it that our Sun sometimes appears to be different colors (yellow, red, and orange)? Does this mean that it changes its temperature?

1. Answer: Since flowers and bees evolved together, they each depend on being able to find each other. Therefore, they advertise using patterns on their petals that only bees can see.

2. Answer: Because the body temperature of most animals that snakes hunt is in the range of 37 degrees C, the peak wavelength of light that they give out is in the infrared. At night, without the benefit of daylight shining on an animal, the snake can see its prey by the infrared light it gives off. Rattlesnakes have two sets of eyes. One set that we identify as regular eyeballs is used for detecting things in visual light. Another pair located below its regular eyes is sensitive to infrared light.

3. Answer: Light in the visible spectrum is broken into different wavelengths that we see as color. If we lived on a radio light world and had the ability to see radio waves, we would probably describe them as colors. Radio light just like visual light is broken into different wavelengths. It ranges from a few centimeters to six football fields in length. We would need to have a very different set of eyes to detect such waves in any detail.

4. Answer: Have you ever seen a pencil sticking out of a glass of water? The part in the water shifts position from the part in the air because of refraction. If you stir the water up, then you'll see the image of the pencil through the water move around. This is like what happens to starlight when it passes through our atmosphere if the air is moving around a lot. It causes the star's light to move around just a little. This is twinkling. We see the planets in the night sky being brighter than the stars. And also, the planets are so much closer to us than stars, so the planets are really little disks from our view whereas the stars are points of light. The larger the disk, the more the image resists twinkling. When the atmosphere is particularly turbulent, though, planets seem to twinkle, too--especially when low on the horizon where the light has to travel through more air to reach the ground.

5. Answer: ANSWERS WILL VARY. For example, a possible choice is infrared; in that case, the student would see most mammals and birds and hot rocks that soak up sunshine and then re-emit the light in the infrared.

6. Answer: A lunar eclipse can be viewed without any special precautions. The shadow that you see moving across the lunar surface is made by the Earth blocking the sunlight. Why do you see the moon at all? After all, the moon does not give off optical light since it is far too cold. You see the moon because Sunlight reflects off its surface. During a lunar eclipse the moon crosses the shadow that the Earth makes.

• A solar eclipse happens when the moon moves in front of the Sun, relative to us on Earth. Looking at a solar eclipse with your naked eyes is the same as looking at the Sun. Not all the Sunlight is blocked during a full eclipse, and if you are looking at it when the Sun first starts to reappear, you can burn the light-sensitive tissue in your eye, the retina. Damage caused to the retina by looking at the Sun is permanent.

7. Answer: The higher you go in the atmosphere, the less water there is. Astronomers reduce the amount of atmospheric water they have to look through by constructing infrared telescopes in high mountains, such as the Keck Observatory in Hawaii at 14,000 ft. Another solution is to put infrared telescopes in space, like IRAS in the 1980's and SIRTF to come soon.

1. Answer: The amount of energy carried by an electromagnetic wave depends on its frequency: the higher the frequency, the more energetic the wave. X-ray light has a higher frequency than ultraviolet light so it has more energy and can travel farther through denser material than ultraviolet light before being stopped.

2. Answer: The use of frequency units (cycles per second) to describe radio waves is just a matter of tradition. Wavelengths could be displayed on your radio dial and would work just as well. (As long as the radio stations identify their wavelength setting! Otherwise, you'd have to do the conversion yourself to know what wavelength to tune to!)

3. Answer: Even though we would not be able to see stars in the visible spectrum, it still could be possible to detect their presence by using another wavelength in of the electromagnetic spectrum. Since radio waves, for example, would penetrate the Venusian atmosphere without trouble, we could detect things in the sky that give off energy in radio wavelengths.

4. Answer: Colors in a rainbow will always appear in the same order because light waves arrange themselves by their wavelength and frequency. Since violet waves are just a little shorter than blue and blue waves are just a little shorter than green, blue will always be between violet and green. Each color’s wavelength is just a little shorter than the previous one so green is next to yellow, yellow is next to orange and orange is next to red.

1. Answer: Well, people here on Earth radiate mostly in the infrared but our eyes do not see infrared light, yet we see our friends with no trouble--thanks to the Sun. Sunlight reflects off things. That's how we see people, mountains and cars. On the strange planet, as long as your eyes were sensitive to visible light and the strange planet's Sun radiated visible light, you'd see the creatures, no problem.

2. Answer: There's a difference between reflected light (sunlight reflecting off your clothes) and light that a body radiates because of its temperature. You see a red sweater as red because the sweater reflects red wavelengths better than yellow, orange or blue wavelengths. But if the sweater is at body temperature (like when you wear it!) then it radiates its own light mostly in the infrared.

3. Answer: No, the Sun does not change its temperature from sunrise to sunset! The changing color of the Sun is due to a change in the thickness of the atmosphere that the sunlight travels through. At noon, the Sun is right overhead and the sunlight goes through the least amount of atmosphere to reach us on the ground. At sunset, the sunlight travels through the greatest amount of atmosphere. The more atmosphere the light travels through, the more dust particles there are. Dust particles absorb short wavelengths of light (the blue end of the spectrum) and scatter them out of our line of sight. The redder wavelengths pass through unhindered by the dust. Since blue wavelengths are being removed due to scattering, the Sun appears red to us!