Waves and Light

Waves

In this chapter, we are going to explore waves—disturbances that transfer energy. We’ll start in this quiz by building an understanding of what waves are. With the mention of the word "wave," you may be inclined to think of the waves in Earth’s oceans. Out in the open ocean, surface waves travel through the water, transferring energy. But waves are much more common in our everyday experiences than you might anticipate. Ocean waves are familiar to us, but they’re not the only kind of waves!

Waves

Waves are traveling around you all of the time as light and sound! But you cannot see these waves travel. To understand waves, we’ll need a method of visualizing them as they move. Luckily, an easy way to do this is with a rope. We can move the rope and then observe how the (kinetic) energy moves across it.

Suppose you have a rope in your hand and you raise your hand up, and then back down to its original position. The energy from moving your hand travels across the rope as a single wave pulse, causing the rope to move as shown:

In this scenario, the rope is the medium or the substance (made of matter) that carries the wave. It’s important to understand that a wave is a disturbance that transfers energy as it travels through a medium. The medium moves because of the wave, but it is not the wave. The wave is the transfer of your hand’s kinetic energy across the rope! Waves like this one—which require a medium to carry them—are known as mechanical waves. On the other hand, electromagnetic waves (such as visible light) do not need a medium to travel and therefore can travel in empty space. It’s a good thing too, otherwise we wouldn’t be able to see light from stars (including our own)! Light is capable of traveling across billions of light-years of space. We’ll learn more about electromagnetic waves in the chapter on light. For now, we’ll start with mechanical waves.

Waves

We observe mechanical waves on a daily basis, including sound! Sound is simply the energy something releases when it vibrates (moves back and forth). This energy—which we interpret as sound—travels through the air and to our ears as waves!

Outer space is generally considered to be a vacuum—a space with no matter. If we consider space to be a complete vacuum, how does this impact the propagation of sound waves?

Remember: Sound waves are mechanical waves. That means that they can only travel through a medium.

                 

Waves

As it turns out, sound waves and most other waves we observe are quite different from our example wave pulse on the rope. Most waves are periodic in nature. But what does periodic mean?

When something is periodic, it repeats at specific time intervals or periods. For example, a period in school often lasts 45 minutes and repeats throughout the day. A year has a period of approximately 365 days, while a week has a period of 7 days. Any motion that repeats itself after a specific time period is considered to be periodic.

Which of the following are periodic motions?

                 

Select one or more

Waves

A wave pulse is a single disturbance, but a periodic wave will continually repeat at regular time intervals.

Which of the following waves are periodic in nature?

Hit the PLAY button!

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Select one or more

Waves

If you have studied trigonometry, wave D from the last example should look familiar. It’s what we know in mathematics as a sine curve.

But there is something else that is special about wave D—it oscillates. An oscillation is a back and forth pattern of motion between two states about a central position or equilibrium point. For example, a rocking chair is oscillating when it moves back and forth between two states about the same central position. Anything that moves back and forth between two states around a central position oscillates. Oscillations can repeat at fixed time intervals, but it is not necessary for them to be periodic. Which of the following are examples of oscillatory motion?

                 

Select one or more

Waves

It’s easy to confuse periodic motion and oscillatory motion. Let’s make sure to distinguish the two. When we say something is periodic, we are talking about time. The moon’s orbit is periodic because it takes the same amount of time to repeat itself. On the other hand, when we say something oscillates, we’re talking about how it moves in space. A pendulum or rocking chair both move back and forth between two states about a central position. Therefore, they are oscillating.

Motion can be periodic. Motion can be oscillatory. And, motion can be both. In fact, many waves you observe are periodic oscillations. This means that they repeat every period and move about a fixed location or axis. However, many signals you commonly use—such as radio, sound, and light pulses—aren’t periodic, but they can be built from periodic waves! In fact, this is the basis of modern communication. In the rest of this chapter, we’ll build our understanding of waves from these periodic building blocks.

Periodically oscillating waves

Periodically oscillating waves

Waves

There is another important quality of waves that we’re missing, however. Let’s see if we can understand it through observation. Watch these surface water waves. Carefully follow the movement of a wave as it travels across the screen. Then observe the movement of the duck. Consider how their relative motion compares. What does this tell us?

Surface water waves periodically oscillate

Surface water waves periodically oscillate

                 

Waves

In this quiz, we explored waves, which are disturbances that transfer energy but not matter. We learned that waves can either be electromagnetic or mechanical. Unlike electromagnetic waves such as light, mechanical waves need to be carried by matter (the wave’s medium). Finally, we learned about periodic and oscillatory motion—both of which can characterize the movement of waves.

Waves disturb their medium. But they ultimately transport energy, not matter.

Waves disturb their medium. But they ultimately transport energy, not matter.

In the next quiz, we’ll examine two main types of mechanical waves that behave periodically—transverse and longitudinal waves—and learn more about their structure.

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