READ: Wavelength and Frequency

1. Wavelength

Nobody really has such colorful eyes! The colors were added digitally after the photo was taken. They represent all the different colors of light. Light is a form of energy that travels in waves. Light of different colors has different wavelengths.

Defining Wavelength

Wavelength is one way of measuring the size of waves. It is the distance between two corresponding points on adjacent waves, and it is usually measured in meters. How it is measured is a little different for transverse and longitudinal waves.

• In a transverse wave, particles of the medium vibrate up and down at right angles to the direction that the wave travels. The wavelength of a transverse wave can be measured as the distance between two adjacent crests, or high points, as shown in the diagram below.

• In a longitudinal wave, particles of matter vibrate back and forth in the same direction that the wave travels. The wavelength of a longitudinal wave can be measured as the distance between two adjacent compressions, as shown in the diagram below. Compressions are the places where particles of the medium crowd close together as the energy of the wave passes through.

At the following URL, watch the animation to see examples of wavelength. Also, get a feel for wavelength by playing with the wave generator:

http://earthguide.ucsd.edu/wav/wavelength.html

Wavelength and Wave Energy

The wavelength of a wave is related to the wave’s energy. Short-wavelength waves have more energy than long-wavelength waves of the same amplitude. (Amplitude is a measure of how far particles of the medium move up and down or back and forth when a wave passes through them.) You can see examples of transverse waves with shorter and longer wavelengths in the Figure below.  

Q: Of all the colors of visible light, red light has the longest wavelength and violet light has the shortest wavelength. Which color of light has the greatest energy?

A: Violet light has the greatest energy because it has the shortest wavelength.

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2. Frequency

Imagine making transverse waves in a rope, like the person in the sketch above. You tie one end of the rope to a tree or other fixed point, and then you shake the other end of the rope up and down with your hand. You can move the rope up and down slowly or quickly. How quickly you move the rope determines the frequency of the waves.

What Is Wave Frequency?

The number of waves that pass a fixed point in a given amount of time is wave frequency. Wave frequency can be measured by counting the number of crests (high points) of waves that pass the fixed point in 1 second or some other time period. The higher the number is, the greater the frequency of the waves. The SI unit for wave frequency is the hertz (Hz), where 1 hertz equals 1 wave passing a fixed point in 1 second. The figure below shows high-frequency and low-frequency transverse waves. 

Q: The wavelength of a wave is the distance between corresponding points on adjacent waves. For example, it is the distance between two adjacent crests in the transverse waves in the diagram. Infer how wave frequency is related to wavelength.

A: Waves with a higher frequency have crests that are closer together, so higher frequency waves have shorter wavelengths.

Wave Frequency and Energy

The frequency of a wave is the same as the frequency of the vibrations that caused the wave. For example, to generate a higher-frequency wave in a rope, you must move the rope up and down more quickly. This takes more energy, so a higher-frequency wave has more energy than a lower-frequency wave with the same amplitude. You can see examples of different frequencies in the Figure below (Amplitude is the distance that particles of the medium move when the energy of a wave passes through them.)

3. Amplitude

Wave Amplitude

Wave amplitude is the maximum distance the particles of the medium move from their resting positions when a wave passes through. The resting position of a particle is where the particle would be if the wave had not traveled through the material. The crest is the highest point of a wave and the trough is the lowest point. Wavelength is the distance from one point on a wave to the same point on the next wave. In the picture it shows a wavelength from the trough to the trough of the next wave. A wavelength however could also be measured from the crest to crest.

Energy and Amplitude

The greater the amplitude the “stronger” the wave.  The opposite is also true. The exact meaning of "strength" depends on the type of wave. For example, a sound wave with a large amplitude is a loud sound.

Below are three different sketches of waves.What do you notice that is similar and different about each of the waves in the sketches? 

• Describe the similarities and differences between the waves using the words amplitude and energy.
• Which wave has more energy?
• Which wave has less?
• How do you know?

A student wanted to use a rope as a model to show that waves can have different amplitudes and wavelengths. The student tied a rope to a tree and then moved the rope up and down at two different speeds.

The faster you move the rope up and down, the more energy that is being transferred to the rope therefore amplitude is higher.

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4. Summary and Vocabulary

Summary

• Wavelength is one way of measuring the size of waves. It is the distance between two corresponding points on adjacent waves, usually measured in meters.
• The wavelength of a transverse wave can be measured as the distance between two adjacent crests. The wavelength of a longitudinal wave can be measured as the distance between two adjacent compressions.
• Short-wavelength waves have more energy than long-wavelength waves of the same amplitude.
• Wave frequency is the number of waves that pass a fixed point in a given amount of time.
• The SI unit for wave frequency is the hertz (Hz), where 1 hertz equals 1 wave passing a fixed point in 1 second.
• A higher-frequency wave has more energy than a lower-frequency wave with the same amplitude.

Vocabulary

• wavelength: Distance between two corresponding points of adjacent waves, such as the distance between two adjacent crests of a transverse wave.

• hertz (Hz): SI unit of wave frequency, where 1 hertz equals 1 wave passing a fixed point per second.
• wave frequency: Number of waves that pass a fixed point in a given amount of time.
• amplitude is the maximum distance the particles of the medium move from their resting positions when a wave passes through.
  

CK-12 Foundation, Physical Science. http://creativecommons.org/licenses/by-nc-sa/3.0/