Light & Waves Atmospheric EffectsSpectra Home PageSpectral RegionsPROPERTIES OF LIGHT
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Q1. Describe the measurable properties of a light wave? Which of these properties can be perceived by humans? How is each of those properties perceived? Answer
Q2. Give a simple example that explains what light is. Answer
Q3. Define the four observable properties of a wave. Which two are most important for our observation of light? Answer
Q4. Using a simple model of light, describe what light is. Answer
Q5. Describe what happens when a light wave is emitted and received. What is a light wave? When we see light of different colors, what is different about the waves that allows us to distinguish the colors? Answer
Q6. When you see a light wave with your eye, what are the two properties of the wave you experience directly? Explain how they affect what you see. Answer
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Q1. How are different colors of light distinguished from each other (that is, what is different about them)? What is a spectrum? Answer
Q2. How is color defined? What is a spectrum? Which parts of the spectrum can astronomers observe from the ground? Answer
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Q1. In what ways are visible light and x-rays similar and different? Why can only one be observed from astronomical objects with ground-based telescopes? Answer
Q2. For any two regions of the light spectrum (other than the visual): name the region, compare the wavelength of the light to visual light, describe a common application or experience with the light, and describe the effect Earth’s atmosphere. Answer
Q3. For three regions of the light spectrum, name the region, describe in general terms how its wavelengths compare to the visual range of wavelengths, and describe how humans experience or utilize these waves. Answer
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Q1. Why is it important to be able to observe astronomical objects from above the atmosphere? Give two reasons. Answer
Q2. Which wavelengths of light can penetrate completely through our atmosphere? How can astronomers observe the other regions of the spectrum? Answer
Light & Waves Answers
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A1. A light wave can be described by its wavelength (distance from one crest to another), its amplitude (the height or strength of the wave), its speed (how fast it moves from one place to another), and its frequency (the rate at which it oscillates up and down). Humans see wavelength as color and amplitude as brightness.
A2. Imagine two electrons in an empty region of space. If one of the electrons moves up and down, the other electron will feel a changing force, pushing it down and up, as the first one changes its position. This changing force is light. The second electron is receiving the light emitted by the first one.
A3. The four observable properties of a wave are speed (how fast it moves from one place to another), wavelength (the distance from the crest of one wave to the crest of the next wave), amplitude (the height of the wave), and frequency (the number of times the wave oscillates up and down each second). When we see light, we sense wavelength as color and amplitude as brightness.
A4. If two objects with the same electric charge are brought close to each other, they will experience a repulsive electric force pushing them away from each other. If one of those charges is moved, the other can sense that motion from the change in the force it feels. If the first charge oscillates up and down, the second charge feels an oscillating electric (and magnetic) force. This sensing of the motion of the first charge by the second is called "detecting" light. Light is merely the changing electric force felt by the second charge.
A5. A light wave is generated when an electrically charged object oscillates, sending out a changing or rippling electric force. The light wave is received when these changing forces cause another electric charge to begin to oscillate. The changing electric forces is the light wave. We see different colors when we detect light of different wavelengths. Wavelength is the distance from the crest of one wave to the crest of the next.
A6. When you look at something, the color you see is a measure of the wavelength of the light and the brightness of the light you see is a measure of the amplitude of the light waves.
Spectra Answers
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A1. Different colors of light are distinguished by different wavelengths – the distance from the crest of one wave to the crest of the next. A spectrum spreads out the wavelengths of light from a light source so they can be observed separately. A rainbow visible in the sky after a rainstorm is an example of a spectrum.
A2. Color is defined as the wavelength of the light we observe. A spectrum displays the individual colors of light from a source in wavelength order. Only visible light and radio waves penetrate through Earth’s atmosphere to be observed on the ground. Some portions of the infrared also penetrate, but all other parts of the spectrum are absorbed by the atmosphere before they reach the ground.
Spectral Regions Answers
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A1. Both visible light and x-rays are forms of electromagnetic light – they are the same thing. They differ only in their wavelengths – x-rays are much shorted than visible light. X-rays cannot be observed from the surface of Earth because our atmosphere absorbs all x-rays before they reach the surface.
A2. Gamma rays are shorter than visible light and are completely absorbed by the atmosphere. They are used in radiation therapy to kill cancerous cells in tumors. X-rays are shorter than visible light and are completely absorbed by the atmosphere. Since they easily penetrate through soft body tissue, they are used to photograph the interior of the body to diagnose broken bones and other ailments. Ultraviolet light is just shorter than visual light. All but the wavelengths right next to the visual are absorbed by the atmosphere. The little bit of ultraviolet light that penetrates the atmosphere causes suntan and sun burn.
Infrared light is just longer than visual light. Most is absorbed in the atmosphere, although some wavelengths penetrate to high mountain tops. We feel infrared light emitted by hot objects as the heat of the object. Microwaves and radio waves are longer than visible light. A broad portion of both regions penetrates to the surface of the Earth. Both are now used for communication purposes. Microwaves are also used to cook food.
A3. Gamma rays are the shortest waves of light, about a million times shorter than visible light. They are used for medical treatment of cancer tumors in some forms of radiation therapy. X-rays are also quite a bit shorter than visible light, and are used for medical diagnostic purposes to "photograph" objects or organs inside the body. Ultraviolet light is just a bit shorter than visible light. Exposure of human skin to UV light causes a sunburn and sometimes causes genetic mutations. Infrared light is just a bit longer than visible light and is felt by us a warmth, for example from a hot object. Microwaves are still longer and are used both for communication and for cooking food in a microwave oven. Radio waves are the longest light waves, much longer than visible light. They are used for many kinds of long distance communication, for example, radio stations, television, and cellular telephones.
Atmospheric Effects Answers
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A1. Images viewed from the ground are always fuzzy due to turbulence in the atmosphere. Some wavelengths of light, such as x-rays, are completely absorbed in the atmosphere and can never be observed from the ground.
A2. Only visible light and radio waves penetrate completely through the atmosphere, although a tiny portion of the ultraviolet portion of the spectrum and some infrared light also can penetrate to the ground. Other wavelengths of light must be observed from above the atmosphere, using telescopes mounted in satellites orbiting Earth.