Light as a Particle

A radiometer in action. The radiometer is the bulb on the right. Note that the bulb is sealed: nothing material, including air, can get into the bulb. That means that the only thing that can get into the bulb and cause the fins to spin is light.

You may be aware that light behaves both as a particle and as a wave. This strange, dual nature of light is just the best way for us to understand it and explain its behavior. Most of the time, when light passes through or around things, it behaves like a wave. When light strikes a mirror, it reflects off of it, just like waves of water reflect when they strike a hard surface like a wall. When light passes through a lens, it refracts, just like when waves of water refract when they move from deep water to shallow water. When light reflects off of oil and creates a rainbow colored sheen, the light is interfering and summing with itself, in much the same way that two waves of water can overlap and interfere and sum with each other. The point is, most of the effects of light that we are familiar with stem from its wave-like properties, because we are observing how light moves through space and around objects.

The particle properties of light primarily affects how it interacts with matter. Now, most of the interaction of light with matter only has a noticeable effect on small particles. For example, electrons are small enough that light can impact them like a particle and “bounce” off of them. Think of pool balls striking each other: one ball is moving (the light particle, which is called a photon) and it strikes another ball (the electron), causing it to move.

Now, since light only really has an effect on very small particles, its property as a particle may be disguised: we do not see the effect because it happens at a very small scale. There are, however, some ways we can see or use the particle property of light. One of these is something that allows us to take digital photographs: the photoelectric effect.

When light is shined on a piece of metal, the light strikes the metal. Now metal, as opposed to other types of materials, has lots of “loose” electrons. It does not have extra electrons, but many of its electrons are loose within its structure. At the subatomic level, when the light strikes these loose electrons, it can strike them like pool balls, knocking some of the electrons off of the metal. This is called the photoelectric effect.

In a digital camera, the photoelectric effect is used by the camera’s sensors. Light strikes the sensors and knocks off some electrons. Those electrons set up a small current which is detected by the camera. The camera is thus sensing when light strikes the sensor, and ultimately interprets this as color in the image. Of course, things are much more complex than I have described it here, but basically, that is what is going on.

While digital cameras are a type of technology that rely on the particle nature of light to function, a neat demonstration of the particle nature of light can be seen in a radiometer. A radiometer can be seen in the video at the beginning of this post. Notice that the sails of the radiometer spin when the light is turned on. Why does this happen? First, consider what happens when light strikes a white surface. Most of the light will be reflected. That is why the surface appears white in the first place: it reflects all colors of light and our eyes interpret all colors of light as white. In other words, the light is reflecting (it is behaving like a wave) off of the white side of the sails. Now, what happens when the light strikes the black side of the sails? Very little light is reflected. That is why it appears black to us: most of the light is absorbed and very little light is reflected back. Our eyes interpret a lack of light as black. What happens when the black side of the sails absorbs the light? Well, the light, behaving as a particle, has momentum. When those light particles, those photons, are absorbed by the black color, they transfer their momentum to the black side of the sails. The result is that the sails get “pushed” by the light. The net result is that the sails begin to rotate, with the light “pushing” on the black side and the light reflecting off of the white side.

Now, if the light is “pushing” on the black side, why do you not feel “pushed” to the ground by sunlight when you step outside wearing dark clothes on a bright, sunny day? The reason is because the momentum of light is so small, it can only “push” a tiny bit. It is so small, in fact, that you would never notice it. Part of the reason it is enough to push the sails of the radiometer is because the sails are suspended on the tip of a needle. If you look carefully at the video, you may notice that the four sails are attached to a small, glass cap, and that cap is sitting on the tip of a needle. This provides minimal resistance, which allows the tiny “push” of the light to actually be able to move the sails.

While most effects of light that we can see and use reflect its wave-like nature, there are some technology and devices that can demonstrate its particle nature. The photoelectric effect, which is utilized by digital cameras, is one such piece of technology. The radiometer is a good demonstration piece that shows the effect of light particles on light and dark surfaces. While it may seem strange to us, the wave-particle duality of light is simply the way it works, and we can use both properties to our advantage in our technology.

Thoughts from Steven