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What Is Frequency? - Definition, Spectrum & Theory

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  • 0:00 Definition of Frequency
  • 0:47 Frequency in Light
  • 1:45 Frequency in Sound
  • 2:38 Energy and Frequency
  • 3:25 Lesson Summary
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

In this lesson, you will learn what frequency is, and how that is observed by our senses both in terms of light (the electromagnetic spectrum) and sound. A short quiz will follow.

Definition of Frequency

Have you ever wondered why the sky is blue? Or, more to the point, why anything has a color in the first place? Your father's voice might have been deep, and your mother's voice not so deep. Whether it's colors, or the notes on a piano, these things are all explained by frequency.

Frequency is a property of a wave. We are surrounded by waves every day. Light is an electromagnetic wave, and the sound of the fan in your computer is a sound wave. A wave is a vibration that carries energy with it. The frequency of a wave is the number of waves that pass by each second, and is measured in Hertz (Hz). For example, a sound wave might have a frequency of 450 Hz.

Frequency in Light

Light is an electromagnetic wave; it is a vibration in electric and magnetic fields. Because of this, it can travel through the vacuum of space, and thank goodness for that! If that wasn't the case, we would get no heat from the sun, and life on Earth would be impossible.

There are more wavelengths than we see with human eyes. A full continuous spectrum would include radio waves, microwaves, infrared, ultraviolet, x-rays, and gamma rays. The diagram on your screen shows the full electromagnetic spectrum and everything it contains.

The Electromagnetic Spectrum

Frequency tells you the color of light. High frequency light waves are at the blue end of the spectrum, and low frequency light waves are at the red end of the spectrum. Our eyes detect the frequency of the light, and our brains interpret it as color. Some philosophers have thought that perhaps color itself only exists in our brains; unfortunately, we have no way of knowing for sure.

Frequency in Sound

Sound waves are wave vibrations in the air. When you slap your hand on the desk, it causes the desk to vibrate, which causes the air to vibrate. The air particles hit each other until they reach your ear, making your ear drum vibrate. Your ear drum then sends a signal to your brain. It's your brain that interprets these vibrations.

Frequency tells you the pitch of sound. High frequency sound is high pitched, and low frequency sound is low pitched. Again, one might ask the question of whether pitch is just the way our brain interprets the waves, or if it's a real thing.

Every musical note you could play on the piano is a different frequency. In fact, if you've ever used a tuning fork, you might notice that the frequency of the note (in Hertz) is stamped on it for reference. Our ears can hear frequencies between 20 Hz and 20,000 Hz.

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