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When white light passes through a glass prism, blue light is refracted more strongly than red light, resulting in dispersion; glass has a higher index of refraction for blue light than for red light. While all colors of light travel at the same speed in a vacuum, light of different colors travels at different speeds through a medium such as glass. The refractive index of a material represents the ratio of the speed of light in a vacuum to the speed of light in the material. Electromagnetic waves or light do not only come in the visible colors that you can see in a rainbow or in light dispersed by a prism. Electromagnetic radiation encompass a broad spectrum of waves, from low-frequency radio waves and microwaves to high-frequency x-rays and gamma rays. The electromagnetic spectrum is used in many different modern technologies. 
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dispersion, index of refraction, electromagnetic spectrum, radiation, spectrum, gamma rays, x-rays, ultraviolet light, infrared radiation, microwaves, radio waves
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Review problems and questions |
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- Describe one piece of technology that is based on each of the following wavelength regimes of the electromagnetic spectrum: x-ray, visible, infrared, microwave, and radio.
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Examples will vary. Here are some possible answers:
X-ray: x-ray machine in hospitals for diagnosing bone fractures.
Visible: light bulb.
Infrared: thermal imaging camera for detecting heat leaks in a house.
Microwave: cellphone broadcast.
Radio: radar for detecting airborne planes. 
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- Compare photons detected by a CCD digital camera and a thermal imaging camera. Which photons have a longer wavelength? Which have a higher frequency?
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Visible photons detected by a CCD digital camera have higher frequency and shorter wavelengths than the infrared photons detected by the thermal imaging camera.
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- Astronomical observations at 1 THz can only be made at high elevations where there is little water vapor. What wavelength is this light? In what part of the electromagnetic spectrum does it fall?
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Answer: 3×10−4 m, which falls in the microwave region of the electromagnetic spectrum.
Solution: One terahertz is 1012 Hz. Find the wavelength for this light: This wavelength is in the microwave part of the electromagnetic spectrum.
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- How might a doctor use visible radiation to diagnose a patient?
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A doctor can use visible radiation to examine the outside of a person to identify injuries and symptoms of diseases. A doctor shines visible light into your throat when you say “ah.”
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- Dog ears can hear frequencies of sound that are higher than a human can hear. If an animal could see frequencies of light higher than humans can, what spectrum would they be able to see?
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Ultraviolet rays have a higher frequency than visible light. Bees can see into part of the ultraviolet spectrum, and thereby they can see a higher frequency of light than can humans.
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- Many people have radios in their car. When they listen to them, are they listening to radio waves? Explain.
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No, human ears cannot hear electromagnetic waves. Radios detect radio waves coming from radio towers and then use them to create sound waves that car passengers can hear.
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- Calculate the speed of light in crown glass for the following:
- violet light (n = 1.532)
- red light (n = 1.513)
Which travels faster through crown glass, violet light or red light?
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Answer: - The speed is 1.958×108 m/s.
- The speed is 1.983×108 m/s.
Red light travels faster through crown glass.
Solution: The refractive index for a material is n = c/v, so the speed of light in the material is - For violet light:
- For red light:
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- During a talk show segment a medical guest recommended the use of “safe” UVB (ultraviolet B) tanning beds for getting vitamin D. Vitamin D is important for human health, yet many Americans are deficient in the vitamin. The World Health Organization recommends against using a tanning bed for cosmetic purposes and classifies the beds as “carcinogenic to humans.” Research this issue and evaluate the validity of this doctor’s claims that you should not “believe all the negative hype on tanning beds.” Cite specific evidence for your conclusion.
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Answers may vary. Tanning beds typically emit 97% UVA (320–290 nm) and 3% UVB (400–320 nm) radiation. UVA radiation causes skin cancer and photoaging, while UVB radiation is responsible for sunburns as well as vitamin D production. A few minutes of sun exposure daily at midday provides sufficient UVB for vitamin D synthesis. The tanning bed therefore adds substantial risks from the UVA exposure while only providing minimal UVB exposure needed for producing vitamin D. The doctor’s claims appear misguided.
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