# the electromagnetic spectrum

## what is the electromagnetic spectrum

Electromagnetic radiation occurs in waves of different wavelengths and frequencies. Infrared light and visible light make up just a small part of the full range of electromagnetic radiation, which is called the electromagnetic spectrum. The electromagnetic spectrum is summarized in the diagram in Figure 21.7. On the far left of the diagram are radio waves, which include microwaves. They have the longest wavelengths and lowest frequencies of all electromagnetic waves. They also have the least amount of energy. On the far right are X rays and gamma rays. The have the shortest wavelengths and highest frequencies of all electromagnetic waves. They also have the greatest amount of energy. Between these two extremes, wavelength, frequency, and energy change continuously from one side of the spectrum to the other. Waves in this middle section of the electromagnetic spectrum are commonly called light. As you will read below, the properties of electromagnetic waves influence how the different waves behave and how they can be used.

## television

Television broadcasts also use radio waves. Sounds are encoded with frequency modulation, and pictures are encoded with amplitude modulation. The encoded radio waves are broadcast from a TV tower like the one in Figure 21.9. When the waves are received by television sets, they are decoded and changed back to sounds and pictures.

## microwaves

The shortest wavelength, highest frequency radio waves are called microwaves (see Figure 21.7). Microwaves have more energy than other radio waves. Thats why they are useful for heating food in microwave ovens. Microwaves have other important uses as well, including cell phone transmissions and radar, which is a device for determining the presence and location of an object by measuring the time for the echo of a radio wave to return from it and the direction from which it returns. These uses are described in Figure 21.10. You can learn more about microwaves and their uses in the video at this URL: (3:23). MEDIA Click image to the left or use the URL below. URL:

## light

Mid-wavelength electromagnetic waves are commonly called light. This range of electromagnetic waves has shorter wavelengths and higher frequencies than radio waves, but not as short and high as X rays and gamma rays. Light includes visible light, infrared light, and ultraviolet light. If you look back at Figure 21.7, you can see where these different types of light waves fall in the electromagnetic spectrum.

## gamma rays

Gamma rays are the most energetic of all electromagnetic waves. They can pass through most materials, including bones and teeth. Nonetheless, even these waves are useful. For example, they can be used to treat cancer. A medical device sends gamma rays the site of the cancer, and the rays destroy the cancerous cells. If you want to learn more about gamma rays, watch the video at the URL below. MEDIA Click image to the left or use the URL below. URL:

## x rays

X rays are high-energy electromagnetic waves. They have enough energy to pass through soft tissues such as skin but not enough to pass through bones and teeth, which are very dense. The bright areas on the X ray film in Figure also to screen luggage at airports (see Figure 21.14). Too much X ray exposure may cause cancer. If youve had dental X rays, you may have noticed that a heavy apron was placed over your body to protect it from stray X rays. The apron is made of lead, which X rays cannot pass through. You can learn about the discovery of X rays as well as other uses of X rays at this URL:

## x rays and gamma rays

The shortest-wavelength, highest-frequency electromagnetic waves are X rays and gamma rays. These rays have so much energy that they can pass through many materials. This makes them potentially very harmful, but it also makes them useful for certain purposes.

## ultraviolet light

Light with wavelengths shorter than visible light is called ultraviolet light. The term ultraviolet means "above violet." Ultraviolet light is the range of light waves that have shorter wavelengths than violet light in the visible spectrum. Humans cant see ultraviolet light, but it is very useful nonetheless. It has higher-frequency waves than visible light, so it has more energy. It can be used to kill bacteria in food and to sterilize laboratory equipment (see Figure 21.12). The human skin also makes vitamin D when it is exposed to ultraviolet light. Vitamin D is needed for strong bones and teeth. You can learn more about ultraviolet light and its discovery at this URL: MEDIA Click image to the left or use the URL below. URL: Too much exposure to ultraviolet light can cause sunburn and skin cancer. You can protect your skin from ultraviolet light by wearing clothing that covers your skin and by applying sunscreen to any exposed areas. The SPF, or sun- protection factor, of sunscreen gives a rough idea of how long it protects the skin from sunburn (see Figure 21.13). A sunscreen with a higher SPF protects the skin longer. You should use sunscreen with an SPF of at least 15 even on cloudy days, because ultraviolet light can travel through clouds. Sunscreen should be applied liberally and often. You can learn more about the effects of ultraviolet light on the skin at this URL: MEDIA Click image to the left or use the URL below. URL:

## infrared light

Light with the longest wavelengths is called infrared light. The term infrared means "below red." Infrared light is the range of light waves that have longer wavelengths than red light in the visible spectrum. You cant see infrared light waves, but you can feel them as heat on your skin. The sun gives off infrared light as do fires and living things. The picture of a cat that opened this chapter was made with a camera that detects infrared light waves and changes their energy to colored light in the visible range. Night vision goggles, which are used by law enforcement and the military, also detect infrared light waves. The goggles convert the invisible waves to visible images. For a deeper understanding of infrared light, watch the video at this URL: MEDIA Click image to the left or use the URL below. URL:

## visible light

The only light that people can see is called visible light. It refers to a very narrow range of wavelengths in the electromagnetic spectrum that falls between infrared light and ultraviolet light. Within the visible range, we see light of different wavelengths as different colors of light, from red light, which has the longest wavelength, to violet light, which has the shortest wavelength. You can see the spectrum of colors of visible light in Figure 21.11. When all of the wavelengths are combined, as they are in sunlight, visible light appears white. You can learn more about visible light in the chapter "Visible Light" and at the URL below.

## kqed seeing cells in 3d

Scientists in Berkeley have developed a powerful new microscope which uses X rays to scan a whole cell and in a manner of minutes, generate a 3D view of the cell and its genetic material. This groundbreaking tool is helping to advance research into the development of biofuels, the treatment of malaria and it may even help to more rapidly diagnose cancer. For more information on X ray microscopes, see http://science.kqed.org/quest/video/x-ray-micros MEDIA Click image to the left or use the URL below. URL:

## instructional diagrams

This diagram shows light waves of varying lengths, and some of their characteristics. The red line illustrates the wavelengths. Above that is a bar showing which light waves penetrate the Earth's atmosphere. Below the red line are the names of the different types of light, with their wavelength measured in (m). The illustrations of physical objects are to show scale. Below that is a diagram of the different light frequencies, measured in Hertz. Below that is a measure of the temperatures at which these light waves are most commonly emitted.

The diagram shows different kinds of waves. Visible light is the part of the electromagnetic spectrum that humans can see. Visible light includes all the colors of the rainbow. Each color is determined by its wavelength. Visible light ranges from violet wavelengths of 400 nanometers (nm) through red at 700 nm. There are parts of the electromagnetic spectrum that humans cannot see. This radiation exists all around you. You just can't see it! Every star, including our Sun, emits radiation of many wavelengths. Astronomers can learn a lot from studying the details of the spectrum of radiation from a star. Many extremely interesting objects can't be seen with the unaided eye.

## questions

Radio waves have the least amount of energy of all electromagnetic waves.

``````-->  a. true

b. false
``````

Which electromagnetic waves are used for cell phone signals?

``````a. X rays

-->  b. microwaves

c. gamma rays

d. none of the above
``````

Visible light has higher-frequency waves than ultraviolet light.

``````a. true

-->  b. false
``````

Which choice lists electromagnetic waves in the correct sequence from higher to lower frequencies?

``````a. microwaves, infrared light, visible light

b. ultraviolet light, X rays, gamma rays

-->  c. X rays, ultraviolet light, visible light

d. radio waves, microwaves, infrared light
``````

Cell phone transmissions are carried by microwaves.

``````-->  a. true

b. false
``````

Electromagnetic waves that have the least amount of energy are

``````-->  a. radio waves.

b. infrared light.

c. visible light.

d. ultraviolet light.
``````

What color does the shortest wavelength of visible light appear to the human eye?

``````a. red

b. yellow

c. orange

-->  d. violet
``````

Radar stands for radio detection and recovery.

``````a. true

-->  b. false
``````

Visible light consists of a very wide range of wavelengths.

``````a. true

-->  b. false
``````

X rays are used for

``````a. tracking storms.

b. killing bacteria.

-->  c. screening luggage at airports.

d. two of the above
``````

You should protect your skin from ultraviolet light even on cloudy days.

``````-->  a. true

b. false
``````

The only use of X rays is to make images of bones and teeth inside the body.

``````a. true

-->  b. false
``````

Gamma rays cannot pass through bones and teeth.

``````a. true

-->  b. false
``````

Gamma rays can be used to destroy cancer cells.

``````-->  a. true

b. false
``````

Radar is used for tracking storms.

``````-->  a. true

b. false
``````

Ultraviolet light has shorter wavelengths than visible light.

``````-->  a. true

b. false
``````

Which of the following choices lists electromagnetic waves from lower to higher frequencies?

``````a. radio waves, infrared light, microwaves

b. ultraviolet light, infrared light, X rays

-->  c. infrared light, ultraviolet light, gamma rays

d. visible light, microwaves, ultraviolet light
``````

Television broadcasts cannot pass through the ionosphere.

``````a. true

-->  b. false
``````

Which electromagnetic waves have a wavelength about as wide as the nucleus of an atom?

``````a. radio waves

b. infrared light

c. ultraviolet light

-->  d. gamma rays
``````

``````a. carry more information.

b. be heard more clearly.

c. pass through the ionosphere.

-->  d. travel to more distant receivers.
``````

Television broadcasts encode pictures with amplitude modulation.

``````-->  a. true

b. false
``````

Television broadcasts encode pictures by changing the

``````a. frequency of radio waves.

-->  b. amplitude of radio waves.

c. wavelength of radio waves.

d. speed of radio waves.
``````

Infrared light is used to sterilize surgical instruments.

``````a. true

-->  b. false
``````

X rays cannot pass through lead.

``````-->  a. true

b. false
``````

Which type of electromagnetic waves are used for radar?

``````a. ultraviolet waves

-->  c. microwaves

d. X rays
``````

Visible light with the longest wavelength appears to be

``````a. violet.

b. green.

c. blue.

-->  d. re
``````

Uses of ultraviolet light include

``````a. killing bacteri

b. sterilizing surgical instruments.

c. making vitamin D by the skin.

-->  d. all of the above
``````

electromagnetic waves with the greatest energy

``````a. radio waves

b. infrared light

c. ultraviolet light

d. microwaves

e. electromagnetic spectrum

f. visible light

-->  g. gamma rays
``````

full range of wavelengths of electromagnetic radiation

``````a. radio waves

b. infrared light

c. ultraviolet light

d. microwaves

-->  e. electromagnetic spectrum

f. visible light

g. gamma rays
``````

electromagnetic waves with wavelengths between infrared and ultraviolet light

``````a. radio waves

b. infrared light

c. ultraviolet light

d. microwaves

e. electromagnetic spectrum

-->  f. visible light

g. gamma rays
``````

light with the shortest wavelengths

``````a. radio waves

b. infrared light

-->  c. ultraviolet light

d. microwaves

e. electromagnetic spectrum

f. visible light

g. gamma rays
``````

electromagnetic waves with the longest wavelengths

``````-->  a. radio waves

b. infrared light

c. ultraviolet light

d. microwaves

e. electromagnetic spectrum

f. visible light

g. gamma rays
``````

light with the longest wavelengths

``````a. radio waves

-->  b. infrared light

c. ultraviolet light

d. microwaves

e. electromagnetic spectrum

f. visible light

g. gamma rays
``````

radio waves with the highest frequencies

``````a. radio waves

b. infrared light

c. ultraviolet light

-->  d. microwaves

e. electromagnetic spectrum

f. visible light

g. gamma rays
``````

## diagram questions

What nm are gamma rays located based on the diagram?

``````-->  a. 0.0001nm

b. 1000 nm

c. 1 cm

d. 0.01 nm
``````

Which part of the spectrum are humans capable of seeing?

``````a. gamma rays

-->  b. Visible light

c. xrays

d. infrared
``````

Which form of electromagnetic radiation has the shortest wavelength but the most energy?

``````a. Ultraviolet

c. Visible light

-->  d. Gamma rays
``````

How many kind of rays are described in the diagram?

``````-->  a. 5

b. 3

c. 2

d. 4
``````

If something were visible at 210 nm, what light would that be?

``````-->  a. ultra-violet

b. xray

c. gamma ray

d. infrared
``````

What kind of rays come from the lowest frequency wavelength?

``````a. Gamma

b. Infrared

c. Microwave

``````

What is between Infrared and X-ray?

``````-->  a. Ultraviolet

c. Microwave

d. Gamma-Ray
``````

Which is the largest type of wavelength?

``````a. Microwave

b. Gamma-ray

c. Infrared

``````

According to the diagram, how many light(s) appear between 5000K to 300,000,000K degrees?

``````-->  a. 3

b. 4

c. 1

d. 5
``````

How long are radio wavelengths?

``````a. 800nm or a single cell

b. 1mm or a pin head

-->  c. 10cm or a hand width

d. 0.1mm or a human hair
``````

How many types of rays are there?

``````-->  a. 8

b. 6

c. 7

d. 5
``````

What is the wavelength of radio waves?

``````a. 10^-2 meters

b. 10^-8 meters

c. 10^-5 meters

-->  d. 10^3 meters
``````

How many centimeters is the smallest radio wave?

``````-->  a. 100

b. 1000

c. 0.1

d. 10
``````

What has the lowest wavelength on the chart?

``````a. Microwave

c. Visible

d. X-Ray
``````

Which of these can be seen by the human eye?

``````a. Gamma rays

-->  b. Visible light

c. Infrared

``````

What is the wavelength in centimeters for microwaves?

``````-->  a. 1

b. 10^2

c. 10^-2

d. 10^4
``````

Which of the following is the shortest wavelength?

``````-->  a. Gamma ray

b. Infrared

c. X-ray

``````

What happens to the energy as the wavelength increases?

``````a. It increases

-->  b. It decreases

c. It stays the same

d. It increases rapidly
``````

What electromagnetic wave lies between UV and gamma rays?

``````a. Visible light

b. Infrared

-->  d. X-Rays
``````

What happens to wavelength as energy increases?

``````a. Wavelength stays the same

-->  b. Wavelength decreases

c. Wavelength increases

d. Wavelength varies
``````

How many are rays in this diagram?

``````-->  a. 2

b. 5

c. 1

d. 3
``````

What is the shortest wavelength for x-rays?

``````-->  a. 0.01 nm

b. 1 cm

c. 10 nm

d. 1000 nm
``````

Which has the longest wavelength?

``````a. Ultraviolet

b. Gamma Ray

-->  c. Microwave

d. Infrared
``````

How many types of rays form an electromagnetic spectrum?

``````a. 3

-->  b. 7

c. 6

d. 5
``````

Which of these is true the shorter the wavelength?

``````a. Lower energy

-->  b. Higher energy

c. Higher amplitude

d. Lower frequency
``````

Which wave has the highest energy?

``````a. X-ray

-->  b. gamma ray

d. Ultraviolet
``````

What do you call waves that have a frequency of 10^15 Hz?

``````a. Gamma ray

b. X ray

-->  c. Visible Light

``````

That is the name of the wave that is in-between Gamma ray and Ultraviolet light?

``````a. Radio wave

b. Infrared

c. Visible light

-->  d. X-ray
``````

How many Types of wavelengths exist in the electromagnetic spectrum?

``````a. 3

b. 6

-->  c. 7

d. 1
``````

What comes after microwave?

``````a. x-ray

c. gamma ray

d. visible
``````

What range covers x-rays?

``````a. 1cm-1m

-->  b. 0.01nm - 10nm

c. 400nm-600nm

d. 0.01nm-100nm
``````

What electromagnetic wave type lies between visible light and radio waves?

``````-->  a. Infrared

b. X-Rays

c. Gamma

d. UV
``````

What ray is between the x-ray and infrared?

``````-->  a. UV ray

b. none

d. gamma ray
``````

What is the shortest wavelength for x-rays?

``````a. 10 nm

-->  b. 0.01 nm

c. 1 cm

d. 0.01 cm
``````

Which rays are sound rays?

``````a. Gamma