Waves Traveling the Universe

Anticipation guides are useful for activating students' prior knowledge about science concepts and focusing students' attention on key content in a text. They are one of the strategies from SERP's Reading to Learn in Science (RTLS), all of which can be helpful to science teachers when the lesson requires some student engagement with text.

Before going through the lesson, have students decide whether they think each statement is true or false. After today's lesson, you will have students reassess which statements in the anticipation guide are true and false, noting evidence from the lesson.

Anticipation Guide

What I think:

Humans can see all light. TRUE • FALSE

All radiation hurts humans. TRUE • FALSE

If something makes electromagnetic radiation, it glows. TRUE • FALSE

Waves transport energy, not matter. TRUE • FALSE

All waves need something, a medium, to travel through. TRUE • FALSE

Radio waves are a kind of light wave.  TRUE • FALSE

Radio waves move more slowly than the speed of sound. TRUE • FALSE

Look at the logarithmic nature of the electromagnetic spectrum by starting with the prefixes used for radio frequencies.

Ask:

Have you ever heard of a gigahertz?

A lot of wireless devices use frequencies around 2.4 gigahertz. Can you think of any of them?

Possible answers:

Some cordless phone, bluetooth headsets, baby monitors, car alarms, and even microwave ovens all use frequencies of about 2.4 GHz.

Paraphrase:

On the low-frequency end of the electromagnetic spectrum, radio frequencies are measured in hertz, kilohertz (kHz), megahertz (MHz), and gigahertz (GHz). One gigahertz is a wave that cycles one billion times more per second than a one hertz wave.

A 1-GHz electromagnetic wave is about 30 cm (or a foot) long. A 1 Hz wave is how far light travels in one second, which is about 300,000 km, or about three-quarters of the way to the Moon.

Ask:

Which end of the electromagnetic spectrum has waves that travel at faster speeds?

Intended answer:

This is a trick question. Assuming the EM waves are traveling through the same medium or through a vacuum, they are all traveling at the same speed, the speed of light, abbreviated as "c" by physicists, as in the famous equation E=mc2.

Ask:

Which end of the radio wave spectrum has the shortest wavelength: frequencies that are measured in hertz or gigahertz?

Intended answer:

The wavelengths get shorter as the frequency gets higher. So the gigahertz end of the radio wave spectrum has waves with a shorter wavelength.

Radio waves frequencies

 hertzkilohertzmegahertzgigahertz

1,000,000,000 Hz = 1,000,000 kHz = 1,000 MHz = 1 Ghz

 a 1 Hz wave is
300,000 km longa 1 kHz wave is 300km longa 1 MHz wave is 300m longa 1 GHz wave is 30cm long

The lower the frequency, the longer the wavelength. The shorter the frequency, the shorter the wavelength.

All electromagnetic waves travel at the speed of light.

Pass out the 3-meter lengths of string / twine / yarn / rope to the groups. Have them hang them up between two spots at the same height: like the backs of two chairs, the tops of two tables, along a white board using magnets or bulletin board using tacks, etc.

Pass out the gray scale cards. Students should cut them up and place them equidistant along the clothesline, about 20 cm (8 inches) apart, from least to greatest frequency.

Take a look at one card. Explain:

The Hz stands for "hertz" or cycles per second, the frequency of the waves. Explain that 10^6 means 1,000,000 hertz, so that in a radio wave of 1000 kHz, a million radio wave crests pass a given point in one second.

Walk from one end of the spectrum model to the other, from the radio waves at 10^6 to the gamma rays at 10^20. Explain:

The frequencies along this spectrum go up VERY fast. It's a logarithmic scale. This gamma ray end of the spectrum we're modeling has a frequency that is 100,000,000,000,000 or one hundred trillion times faster, with a wavelength that is a hundred-trillionth the length of a radio wave at the other end of the spectrum.

Ask:

Which end of your clothesline has the lowest frequency?

Intended answer:

The end with radio waves labeled 106 Hz or 1000 kHz.

Ask:

Which end of your clothesline has the highest frequency?

Intended answer:

The end with radio waves labeled 1020 Hz or 100 EHz.

Point to two adjacent cards on the clothesline (such as the cards marking 10^6 and 10^7). Ask:

How much faster are the frequencies at this point in the clothesline than at this point?

Intended answer:

The clothesline is logarithmic, so each frequency marker is 10 times larger (or 10x smaller depending on which direction you're moving in the spectrum.

Critique the model of the electromagnetic spectrum that is often shown in textbooks, with a curved line representing a decreasing wavelength. Paraphrase:

The frequencies in the electromagnetic spectrum vary widely from one end to the other. There would be no way to show in a drawing like this that the wavelength is 100,000,000,000,000 times longer on this radio wave end of the spectrum (left) than the gamma ray end (right).

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Decide whether you want the students to recreate the cards as sticky notes or to cut up and arrange the cards. (See Teacher Tips.) If students are copying the information onto sticky notes, this activity will probably take about twice as much time.

Paraphrase:

We use radio waves from 30 Hz to 300 GHz to communicate wirelessly. Most signals are below 5 GHz.

Radio waves are part of the electromagnetic spectrum. Even though we can't see them, they are a type of light. All EM waves are light waves.

Radio waves have the longest wavelengths of any electromagnetic wave, and they are especially useful for communication.

Like all electromagnetic waves, radio waves do not need a medium to transmit energy. We use them to communicate.

When radio waves overlap and are at very similar frequencies, "interference" happens. This occurrence sounds like noise or a jumble of one or more signals, or even a dropped signal. Interference happens when devices don't contain their signals well, a problem complicated by the fact that many gadgets are trying to use a small bit of spectrum.

Ask students to take the cards and put them in order by frequency along a long table or by attaching to the string or rope using clothespins / paper clips.

Look for crowded parts of the spectrum. Point out that the electromagnetic spectrum is continually re-allocated to new uses.

Television broadcast frequencies have been reassigned for use by satellite TV and a wide range of consumer electronics. Each device uses a very narrow sliver of the EM spectrum.

Note: New technologies may have emerged since this lesson plan was written, so you may choose to add other technologies and their frequencies, and remove obsolete technologies from the activity.

Fun fact: The frequency 2.4 GHz is set aside by the Federal Communications Commission as a narrow part of the spectrum that most consumer electronics occupy. Engineers call it "junk spectrum." It's the frequency that vibrates water molecules, which is why microwave ovens use the same frequency. In some homes and offices, a re-heated lunch may make a Skype call drop on a WiFi signal.

Fun fact: Public safety communication dispatcher radio frequencies (for police, fire, and emergencies) in the 30–50 MHz range penetrate everything. They are not stopped by rain or buildings or anything that often blocks other radio waves, so this range is a very precious part of the spectrum.

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If students are copying the information onto sticky notes, this activity will probably take about twice as much time.

Paraphrase:

Radio waves are just one part of the electromagnetic spectrum. What's on the rest of it?

Ask students to take the cards and put them in order by frequency along a long table or by attaching them to the string or rope using clothespins / paper clips.

While these representative emitters of different frequencies of waves won't overlap the way that the radio spectrum overlapped, students should get a sense of the growing danger with smaller radio waves.

Paraphrase:

Notice how waves on the electromagnetic spectrum with frequencies faster than visible light are dangerous in high doses: ultraviolet light, x-rays, and radioactivity.

Starting somewhere in the UV range, waves are called "ionizing." The more our cells are exposed to ionizing radiation, the more likely it is our cells will be damaged, possibly mutating into cancer. That's why we protect ourselves from dangerous wavelengths of ultraviolet light and all kinds of high-frequency radiation.

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In an optional diversion, you can explore how "Spaceship Earth" protects earthlings—both human and other animals—from many of the harmful waves that travel across the universe. This same protective quality of the ionosphere is something we exploit to send EM signals around the earth.

Show the slide that illustrates which waves traveling the universe can penetrate the atmosphere and ionosphere.

Paraphrase:

Electromagnetic waves criss-cross the universe. Many of these can easily damage our DNA and kill us. The Earth is surrounded by a protective envelope that keeps many of these harmful waves out of our livable zone. Earth's atmosphere stops most types of electromagnetic radiation from space from reaching Earth's surface. This illustration shows how far into the atmosphere different parts of the EM spectrum can go before being absorbed. Only portions of radio and visible light reach the surface.

Image Credit: STScI/JHU/NASA

Show the slide that illustrates how we use the atmosphere to send radio waves around the world by bouncing it off the ionosphere.

Paraphrase:

The protective shell that keeps some electromagnetic waves out also keeps them in. High frequency waves pass through the ionosphere and escape into space while the low frequency waves reflect off the ionosphere and essentially "skip" around the Earth to get from one place to another.

Image Credit: NASA's Radio JOVE, Solar and Planetary Radio Astronomy for Schools

Now that students have read the text, have them decide again whether they think each statement is true or false, and note evidence from the lesson that backs up their claims.

• Humans can see all light. FALSE (Only a thin sliver of the EM spectrum is visible to the human eye: visible light from red to violet, which combine to make white light.)
• All radiation hurts humans. FALSE (Not all radiation is harmful.)
• If something makes electromagnetic radiation, it glows. FALSE (All types of waves on the EM spectrum are considered EM radiation. While some EM waves are visible to the human eye, a very thin sliver of the EM spectrum "glows.")
• Waves transport energy, not matter. TRUE
• All waves need something, a medium, to travel through. FALSE (Only mechanical waves need a medium in order to propagate.)
• Radio waves are a kind of light wave. TRUE
• Radio waves move more slowly than the speed of sound. FALSE (All electromagnetic waves travel at the speed of light in a vacuum. A medium slows them down. But it would not slow down so much as to be slower than a mechanical wave like sound, whose speed changes depending on the properties of the medium it is traveling through.)

Want to learn more about anticipation guides? They are one of the strategies from SERP's Reading to Learn in Science (RTLS).