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2.1 The Part You Can See

Visible light is only a portion of the electromagnetic radiation spectrum and is a rather complex subject. Despite this complexity, there are some fundamentals that you should know, and in this lesson you will learn some of the basics of light.

2.1 The Part You Can See

Visible light is only a portion of the electromagnetic radiation spectrum, and is a rather complex subject. Despite this complexity, there are some fundamentals that you should know. And in this lesson, you will learn some of the basics of light. When we use the word Light, most of the time we are referring to Visible Light, that is, light that is visible to the human eye. The kind of light that makes our sense of vision possible. Visible light is only a small part of a larger spectrum of electromagnetic radiation, which includes radio, Microwave, Infrared, the visible region that we perceive as light, ultraviolet, X-Rays and Gamma Rays. Electromagnetic radiation, including the visible spectrum, behaves like a wave and like a particle. Light ways that encounter an obstacle can interfere with each other which is called refraction. In photography you may be familiar with this concept. If you close down your aperture too far the light starts to create and interference pattern on your image sensor and your image will start to look softer. This is diffraction at work. Light also acts as a particle. In the late 19th and early 20th century, experiments with light and metals showed that light could cause electrons to be emitted, or ejected from metals. This mean that light acted like a particle, and these particles were called photons. Now this idea of light behaving like a wave and light behaving like a particle is all very, very complex stuff. How complex? Well, the particle nature of light is first explained by Albert Einstein, the greatest scientific mind in history. However, I think it's important to understand, at least at a very simple level, how light acts as a particle and a wave, because it will help explain how light interacts with matter. Sometimes it's easier to think about light interacting with matter as a particle and sometimes it's easier to think about light interacting with matter as a wave. This isn't some kind of scientific shortcut. It's more about using the explanation that makes the most sense to those of us who haven't spent our lives studying physics. When a wave passes through an opening or a slit, the wave radiates outward with a spherical pattern. This apparent change in direction is called defraction. If you do this with light, it creates a band of light on the wall behind the opening that's larger than the slit. With two slits, you would expect two bands of light on the other side. What happens is something much different. The light coming through the slit appears to interfere with itself, creating an interference pattern on the wall. This is called the double slit experiment, and in the early 1800s, this experiment helped to solidify the acceptance of the wave theory of light. In my experiment, I'm going to shine a green laser at two very narrow slits created by these three pieces of graphite taped so close together, they're almost touching. As you can see, when I push the graphite In front of the laser, that same interference pattern is created. This is not some kind of trick, and to show you that, I'm going to add some smoke to the equation so that you can see that the bands of light are coming from the graphite and the single laser beam. In this experiment, I'm going to show you the photoelectric effect. I have an aluminum can connected to a a steel wire that's connected to strips of aluminum foil. In order to show you how this works. I need to charge up the metal with electrons. To do this, I'm going to rub a plastic bag on a piece of PVC pipe. This will cause the pipe to steal some electrons from the bag. Then if I touch the pipe to the aluminum strips, the electrons will be transferred to the aluminum and they will repel each other. They do this because my cam and strip apparatus now has a net negative charge and like charges repel each other. Next I'm going to shine a UV light on the can. The light is now off, and you can see that holding it close to the can doesn't change the charge on the metal. Once I turn the light on and bring it close to the can, watch the foil strips. They start to relax. The UV light is causing the electrons to be ejected from the can. This neutralizes the charges, and the aluminum strips start to relax. Now, what would happen is I tried this with a very bright green laser? Nothing, because green light doesn't have the same energy as UV light no matter how bright it is. What about a super bright spot light? That's not going to work either because the amount of energy in light is related to it's frequency, not the quantity. Because UV light has a higher energy, compared to visible light, it will cause the electrons to be ejected from the can in this experiment. But lower energy, and thus lower frequency light, won't have any effect at all. So as Einstein explained, electromagnetic radiation is made up of photons. Photons are tiny bits of energy that travel through a vacuum at the speed of light. Photons have no mass and carry a certain quantifiable level of energy. Which is also related to the frequency of the electromagnetic wave. The electromagnetic field around the photon fluctuates from positive to negative and back to positive as the photons move through space. According to the wiki, this can be thought of as a self-propagating transverse oscillating wave. If you didn't study physics, which I didn't, this is probably a bit confusing. Let me try and unpack that for you. Self propagating refers to the fact that a changing electric field generates a changing magnetic field. And the changing magnetic field generates a changing electric field. The result is that it creates an infinite look that self-propagates through space. A transverse wave, is a moving wave that consists of oscillations occurring perpendicular to the direction of the wave. So if a wave is moving along the X-axis, oscillations have to be in the Y-axis or the Z-axis. As you may have guessed electromagnetic waves have two components. There is an electric field and a magnetic field that are oriented 90 degrees from each other and they are always in phase. In this example, the electric field is in the Y-axis and the magnetic field is in the Z-axis. The wave is traveling in the X-axis. As we look at more examples of how these waves work, it will get really confusing to look at both fields at the same time, and it's often good enough to just talk about the direction and orientation of the electric field. This wave has a lot of similarities to other waves that you might be familiar with, like water waves. The wave has crests and troughs. And the distance between the crests is called the wavelength. Frequency, is the number of complete wave cycles or wavelengths that pass a point in space In one second. Like many things in science this measurement has a name called Hertz. This was named after Heinrich Rudolf Hertz a German physicist who first conclusively proved the existence of electromagnetic waves. These waves were theorized by James Clerk Maxwell's electromagnetic theory of light. Unlike water waves, the faster and electromagnetic wave oscillates, or the higher the frequency, the more energy the wave has. In the visible spectrum, we see these differences in frequency as color. Red, has the longest wavelength and therefore, the lowest energy of visible light. Violet, is the highest frequency, and has the highest energy of visible light. Check out what is just above violet. Ultraviolet light. Ultraviolet light, has even more energy than visible light, and this is where electromagnetic waves start to become harmful to humans. As you move up the scale, you get into X-Rays and gamma rays, which, as you probably know, are even more harmful. As you move down the scale, just past visible light, you get into infrared, then microwaves, and finally long radio waves. Infrared, is often associated with heat. This is because infrared energy causes molecules to vibrate faster, and we feel this as heat. In fact, everything in the universe is emitting infrared light. You are emitting infrared energy right now. But you can't see it because, infrared, is outside the visible spectrum of light. Unlike other forms of energy, electromagnetic radiation does not need a medium to travel through. Electromagnetic radiation can travel through a vacuum. In photography, we are primarily concerned with the visible part of the spectrum, which, from this point forward, I am going to just refer to as, light. In the next few lessons, you'll learn about the essential property of light that are of most concerned to photographers in the start with brightness.

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