# Electromagnetic Waves

## Introduces waves consisting of vibrating electrical and magnetic fields and some of their properties.

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Electromagnetic Waves

Did you ever wonder how a microwave works? It directs invisible waves of radiation toward the food placed inside of it. The radiation transfers energy to the food, causing it to get warmer. The radiation is in the form of microwaves, which are a type of electromagnetic waves.

### What Are Electromagnetic Waves?

Electromagnetic waves are waves that consist of vibrating electric and magnetic fields. Like other waves, electromagnetic waves transfer energy from one place to another. The transfer of energy by electromagnetic waves is called electromagnetic radiation. Electromagnetic waves can transfer energy through matter or across empty space.

Q: How do microwaves transfer energy inside a microwave oven?

A: They transfer energy through the air inside the oven to the food.

### How an Electromagnetic Wave Begins

An electromagnetic wave begins when an electrically charged particle vibrates. The Figure below shows how this happens. A vibrating charged particle causes the electric field surrounding it to vibrate as well. A vibrating electric field, in turn, creates a vibrating magnetic field. The two types of vibrating fields combine to create an electromagnetic wave. You can see animations of electromagnetic waves at these URLs:

The electric and magnetic fields that make up an electromagnetic wave are perpendicular to each other.

### How an Electromagnetic Wave Travels

As you can see in the Figure above, the electric and magnetic fields that make up an electromagnetic wave are perpendicular (at right angles) to each other. Both fields are also perpendicular to the direction that the wave travels.   Therefore, an electromagnetic wave is a transverse wave. However, unlike a mechanical transverse wave, which can only travel through matter, an electromagnetic transverse wave can travel through empty space. When waves travel through matter, they lose some energy to the matter as they pass through it. But when waves travel through space, no energy is lost. Therefore, electromagnetic waves don’t get weaker as they travel. However, the energy is “diluted” as it travels farther from its source because it spreads out over an ever-larger area.

### Electromagnetic Wave Interactions

When electromagnetic waves strike matter, they may interact with it in the same ways that mechanical waves interact with matter. Electromagnetic waves may:

• reflect, or bounce back from a surface;
• refract, or bend when entering a new medium;
• diffract, or spread out around obstacles.

Electromagnetic waves may also be absorbed by matter and converted to other forms of energy. Microwaves are a familiar example. When microwaves strike food in a microwave oven, they are absorbed and converted to thermal energy, which heats the food.

### Speed of Electromagnetic Waves

All electromagnetic waves travel at the same speed through empty space. That speed, called the speed of light, is about 300 million meters per second (3.0 x 10 m/s). Nothing else in the universe is known to travel this fast. The sun is about 150 million kilometers (93 million miles) from Earth, but it takes electromagnetic radiation only 8 minutes to reach Earth from the sun. If you could move that fast, you would be able to travel around Earth 7.5 times in just 1 second!

### Wavelength and Frequency of Electromagnetic Waves

Although all electromagnetic waves travel at the same speed across space, they may differ in their wavelengths, frequencies, and energy levels.

• Wavelength is the distance between corresponding points of adjacent waves (see the Figure below). Wavelengths of electromagnetic waves range from longer than a soccer field to shorter than the diameter of an atom.
• Wave frequency is the number of waves that pass a fixed point in a given amount of time. Frequencies of electromagnetic waves range from thousands of waves per second to trillions of waves per second.
• The energy of electromagnetic waves depends on their frequency. Low-frequency waves have little energy and are normally harmless. High-frequency waves have a lot of energy and are potentially very harmful.

Which electromagnetic waves do you think have higher frequencies visible light or x-rays? Answer: X-rays are harmful but visible light is harmless, so you can infer that x-rays have higher frequencies than visible light.

Which electromagnetic waves do you think have higher frequencies: visible light or x-rays?

### Summary

• Electromagnetic waves are waves that consist of vibrating electric and magnetic fields. They transfer energy through matter or across space. The transfer of energy by electromagnetic waves is called electromagnetic radiation.
• An electromagnetic wave begins when an electrically charged particle vibrates. This causes a vibrating electric field, which in turn creates a vibrating magnetic field. The two vibrating fields together form an electromagnetic wave.
• An electromagnetic wave is a transverse wave that can travel across space as well as through matter. When it travels through space, it doesn’t lose energy to a medium as a mechanical wave does.
• When electromagnetic waves strike matter, they may be reflected, refracted, or diffracted. Or they may be absorbed by matter and converted to other forms of energy.
• All electromagnetic waves travel across space at the speed of light, which is about 300 million meters per second (3.0 x 10 m/s).
• Electromagnetic waves vary in wavelength and frequency. Longer wavelength electromagnetic waves have lower frequencies, and shorter wavelength waves have higher frequencies. Higher frequency waves have more energy.

### Vocabulary

• electromagnetic radiation: Transfer of energy by electromagnetic waves across space or through matter.
• electromagnetic wave: Transverse wave consisting of vibrating electric and magnetic fields that can travel across space.
• speed of light: Speed at which all electromagnetic waves travel through space, which is 3.0 × 10 m/s.

### Practice

For an excellent video introduction to electromagnetic waves, go to this URL: http://www.youtube.com/watch?v=cfXzwh3KadE

1. What surrounds you everyday and everywhere?
2. EM waves are like ocean waves in that they both transmit __________.
3. The number of crests that pass a given point in one second is the _________.
4. What is the difference between gamma rays and radio waves?
5. What part of the EM spectrum can we see?
6. EM waves interact with the molecules in objects so some wavelengths are _______ while others are absorbed.
7. Everything around us emits, reflects, and absorbs EM radiation differently based on its __________.

### Review

1. What is an electromagnetic wave?
2. How does an electromagnetic wave begin?
3. What can electromagnetic waves do that mechanical transverse waves cannot?
4. What is the speed of light across space?
5. How is wavelength measured?
6. How is the energy of an electromagnetic wave related to its frequency?

• 1.2.C.a

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