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Resonance: Definition & Transmission of Waves

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  • 0:05 Introduction to Resonance
  • 0:44 Resonant Frequency
  • 1:56 Transmission and…
  • 3:48 Music and the…
  • 6:09 Lesson Summary
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Lesson Transcript
Instructor: April Koch

April teaches high school science and holds a master's degree in education.

This lesson describes how sound and light waves are affected by the principle of resonance. Learn how resonance occurs through the vibrations and resonant frequencies of objects. You'll investigate everyday examples of resonance and learn how to break a wine glass with your voice!

Introduction to Resonance

How do people make beautiful music with wine glasses? How do you break a wine glass by singing loudly in front of it? Sound waves allow us to do some pretty neat things when we know how to use them. Light waves, too, interact in special ways with the objects around them.

The behavior of sound and light waves explains why we hear sounds from musical instruments and why we see color and objects. A trumpet increases the amplitude of a sound wave. A colored object increases the amplitude of a light wave.

These changes in amplitude are caused by an important principle called resonance. In this lesson, we'll talk about resonance and how it affects the transmission of sound and light.

Resonant Frequency

We already know that waves originate from vibrations. Sound waves come from mechanical vibrations in solids, liquids, and gases. Light waves come from the vibration of charged particles.

Objects, charged particles, and mechanical systems usually have a certain frequency at which they tend to vibrate. This is called their resonant frequency, or their natural frequency.

Some objects have two or more resonant frequencies. You know when you drive on a bumpy road and your car begins to bounce up and down? Your car is oscillating at its resonant frequency; or really, the resonant frequency of the shock absorbers. You may notice that when you're riding in a bus, the bouncing frequency is a little bit slower. That's because the bus's shock absorbers have a lower resonant frequency.

When a sound or light wave strikes an object, it is already vibrating at some particular frequency. If that frequency happens to match the resonant frequency of the object it's hitting, then you'll get what's called resonance. Resonance occurs when the amplitude of an object's oscillations are increased by the matching vibrations of another object.

This relationship is difficult to imagine without an example. So, let's explore the subject of resonance further in the context of light waves.

Transmission and Resonance of Light Waves

Let's take a typical light wave. We'll say it's a stream of white light that comes from the sun. And, let's take a dark object, like a western rat snake slithering through your yard.

The molecules in the snake's skin have a set of resonant frequencies. That is, the electrons in the atoms tend to vibrate at certain frequencies.

The light coming down from the sun is white light. So, it has not just one but many wave frequencies. It has frequencies of red and green, blue and yellow, orange and violet. Each of these frequencies strikes the snake's skin.

And, each frequency makes a different electron vibrate. The yellow frequency resonates with the electrons whose resonant frequency is yellow. The blue frequency resonates with the electrons whose resonant frequency is blue. So, the snake's skin, as a whole, resonates with the sunlight.

The snake appears black because its skin absorbs all frequencies of sunlight.
Light Waves Snake Example

When light waves resonate with an object, they cause the electrons to vibrate with high amplitudes. The light energy is absorbed by the object, and we don't see that light coming back to us at all. The object appears black. Since a western rat snake absorbs all the frequencies of sunlight, then it appears as a black snake.

What if an object does not absorb any of the sunlight? What if none of its electrons resonate with the light frequencies? If resonance does not occur, then what you'll get is transmission, the passing of light waves through an object.

The glass appears clear because it does not absorb any sunlight.
Light Waves Glass Example

The light still causes vibrations of the electrons. But, because it doesn't match the electrons' resonant frequencies, the vibrations are very small and they pass from atom to atom all the way through the object. An object with no resonance would exhibit zero absorption and 100% transmission. So, the object in this case wouldn't be white; it would be clear, like glass or water.

We'll talk more about transmission and absorption in a later lesson. For now, let's switch gears and talk about how resonance works in sound waves.

Music and the Resonance of Sound Waves

Resonance for sound works the same as it does for light. When one object is vibrating at the resonant frequency of a second object, then the first object causes the second one to vibrate with a high amplitude.

Let's say you're going to play the trumpet. You press your lips to the trumpet's mouthpiece and get your fingers ready. When you play, your lips vibrate against the mouthpiece, creating a whole bunch of low-amplitude sound waves at many different frequencies. The sounds from your lips are very soft, so nobody really hears them. But, one of those frequencies you're producing will resonate with the air molecules inside the trumpet.

When you set your fingers to play one note, you created a column of air that measures a certain length and width. This air column has its own resonant frequency, and it matches one of the frequencies that are coming from your lips. The energy from your vibrations is absorbed by the air column. It is amplified by the air column, and it makes a loud sound. If you change your fingering, the column resets, and now you have a different resonant frequency to match.

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