Resonance: Definition & Transmission of Waves
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.
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.
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 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.
Resonance causes the amplitude of that frequency to increase so much that people hear a loud, single-frequency sound from your trumpet. That sound is only one of many frequencies that you’re producing. But, it’s the only one we hear because it’s the only one that is amplified by its resonance with the air column.
Have you ever tried making a wine glass sing? You can do this just by wetting your finger and sliding it around the rim of the glass. The motion causes small vibrations because your finger is actually sticking and slipping on the glass in an alternating pattern.
The slip-stick effect creates many frequencies of sound waves, one of which will resonate with the wine glass itself. A clear, ringing tone comes out of the glass, a sound which is the same as the glass’s resonant frequency. Some people can use these tones to create beautiful music!
Now, if you want to, you can try breaking the glass with your voice! Just find the resonant frequency, sing the note as loudly and clearly as you can, and wait for the wine glass to break apart. The high-amplitude vibrations you create with your voice will cause even higher vibrations in the glass. At some point, the glass will be vibrating so much it won’t be able to keep its shape. The vibrations will deform the glass to the point of breakage, and you can impress your friends with your talents! Just make sure you pick up the broken glass afterward.
The principle of resonance affects how we perceive sound and light waves. All objects possess a natural or resonant frequency at which they tend to vibrate. When vibrations from one object match the resonant frequency of another object, the two are said to resonate because the first object amplifies the vibrations of the second object.
Resonance in light waves results in absorption of the light frequency. When no resonance is present, then the light is transmitted through the object. For sound waves, resonance results in a loud sound that matches the resonant frequency of the instrument. Resonance in either case is always caused because one object vibrates at the resonant frequency of another.
Following this lesson, you’ll be able to:
- Define resonance and resonant frequency
- Explain why some objects appear black, while others are clear, due to resonance
- Describe how loud sounds are created using resonance