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Light is pretty mysterious: sometimes it acts just like a wave, interfering with itself and undergoing Doppler shifts and so on, and sometimes it acts like a particle, such as when it interacts with an electron. In this chapter we will explore the wavelike properties of light and leave the particle-like behavior to Chapter 16.

So light is a wave, like sound and water ripples. But sound involves the motion of air, and water ripples involve the motion of water, but what motion happens in a light wave? Well, it turns out to be a hard question. The answer is not "this molecule" or "that substance" but rather a combination of electric and magnetic fields. When all the material is removed from a piece of space, we call that piece of space a vacuum, but electric and magnetic fields are still there, and light disturbs these fields. That is why light can travel in a vacuum.

Here is another strange property of light, or perhaps it is a property of the space and time in which light travels. Its speed is the same c = 3.00 x 108 m/s to every observer. Think of how strange this is. If you are driving 60 mph down the freeway, a car may pass you going 75 mph, and a car on the other side of the yellow stripe may be going 60 mph in the other direction. From the point of view of your car, the first car is going 15 mph and the second is going –120 mph. This makes sense.

In the super fast space freeway, your spacecraft may be going 1.00 x 108 m/s and another spacecraft passes you going in the same direction going 2.00 x 108 m/s. A light beam passes both of you in the same direction going 3.00 x 108 m/s. From the point of view of your spacecraft, it is going (not 2.00 x 108 m/s but) 3.00 x 108 m/s, and from the point of view of the other spacecraft it is going 3.00 x 108 m/s. Go figure.

The speed of light is this mysterious c = 3.00 x 108 m/s only in a vacuum. In other materials the speed is a little slower.

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