Go outside on a bright and sunny day. Take a look at a flower or a tree. You can see that flower because light from the sun travels all the way to the flower. When the light reflects off the flower it then travels to your eye and you can see the flower. Remove the light from the sun and you just see blackness. Even at night humans can see things---but there has to be some type of light reflecting off objects to see. Sunlight reflected off the surface of the moon provides a surprising amount of light for most outdoor activities at night.
If you go inside a building at night, you might not be able to use the moonlight. In that case you need some artificial light source to see. Since this is the International Year of Light, let me go over the four common methods for creating artificial light along with the basic physics that makes them work.
You’ve used LED lights for quite some time. They are in your infrared (IR) remote control for your TV. They are the light source for the flash on your smartphone camera. There’s even a good chance that LED lights are used to make your computer screen visible. The LED started seeing real uses in the 1960s and today they are everywhere.
If the LED is a relatively new method for creating artificial light, why am I starting with this one first? In terms of physics, I think the LED might be the easiest to explain. Now wait, don’t get me wrong. The LED is still complicated---but it might still be the easiest device to explain.
There is one thing in common to all light production methods. They all deal with electrons changing energy levels. When we think of energy for macroscopic objects, we imagine that they could have any particular energy level. I can throw a tennis ball so that it has 10 Joules of kinetic energy or 10.1 Joules or any value in between. This isn’t exactly true and as we look at smaller and smaller things, it’s obviously not true. An electron in some type of system can only have certain energy levels.
Let’s look at the simplest case---the hydrogen atom which consists of just a proton and an electron. At its lowest energy level, the electron is at an energy level of -13.6 eV (electron volts is a unit of energy). If the electron moves to the next higher energy level, it would be at -3.4 eV (which is indeed higher than -13.6 eV. The electron in hydrogen can NOT be at an energy level between -13.6 and -3.4 eV. That’s just the way it is.
But what does this have to do with light? It turns out that when an electron makes the transition from a higher energy level to a lower energy level, it produces light. Also, the frequency of this light is proportional to the energy change. Humans perceive different frequencies of light (in the narrow spectrum of all electromagnetic waves) as different colors of light.
What does this have to do with the LED? The LED is a solid state device. This means that the process is not governed by a typical chemical reaction or mechanical method. The solid state device is a combination of two different semiconductor materials in which electrons can move about at different energy levels due to the periodic nature of the material. This produces an energy gap for electrons in the system. Yes, when electrons transition across this energy gap, they produce light, light of a particular color.
Early LED lights produced only infrared light (light with frequencies that humans can’t see). After that, we started creating red and then green LEDs. Finally, a blue LED was created (but carefully combining different semiconductors). With the blue LED you get two things. First, you can use red, green, and blue (RGB) lights to make video displays. Second, using blue LEDs and some other tricks you can make a white looking LED that can be used for lights.
What makes the LED light so great? First, they can be very small and robust. If you don’t run too much current through them, they last a very long time and they don’t break just by shaking them. Second, the LED light doesn’t get very hot when it is on. The less energy that goes into heating the device means that more energy goes to light. LED lights are much more energy efficient that other devices.
Is there a downside? Right now, the only downside is that they are a bit more expensive for larger applications. The price for these devices seems to be dropping fast though. Soon we might be using LED lights much more than we did in the past.
Fluorescent lights have been around for quite some time. They began to be popular for office and industrial settings in the 1950s, but now they are in most homes. Now we also have the compact fluorescent. As you can guess, this is just a fluorescent bulb that is small enough to fit in the sockets of traditional incandescent lights. But how do they work?
Let’s look at a similar light first---the neon lamp. Start with a glass tube that is filled with neon gas. Now apply a large voltage across the ends of the tube. The electric potential difference inside the tube will cause free electrons to accelerate and collide with the neon atoms. On collision, these electrons can excite electrons in the neon to higher energy levels. When the excited electrons in the neon atoms come back down to lower energy levels, they produce light.
Every atom has its own unique energy levels. This means that different gasses would produce different colors corresponding to the different energy levels. Neon has that classic red-orange. If you are excite a gas of mercury vapor, you get a different color (from different energy levels). These gases don’t just create one color of light, instead they make many different colors that correspond to different energy level transitions. You can see the individual colors by looking through a diffraction grating (a slide with many tiny lines on it). This is what that would look like for both the neon and mercury vapor.
Clearly we don’t want to use neon lamps for normal lighting. It’s just not the right color. Mercury vapor seems closer, but not quite right. Here is the trick for fluorescent lamps---fluorescence. Fluorescence is the process through which a material absorbs a particular wavelength (color) of light and re-emits a color with a longer wavelength. In the case of a fluorescent bulb, there is a coating on the inside of the glass that absorbs ultraviolet light (which you can’t normally see) and re-emits it as visible light. Yes, it’s a complicated process, but that’s how it works.
But why aren’t these fluorescent lamps (and compact fluorescents) as good as an LED light? There are a couple of disadvantages. First, in order to excite the gas you need a high voltage applied to the tube. To get this high voltage, a fluorescent lamp uses an electromagnetic ballast that take the normal household voltage and ramp it up to a higher level. This ramp up process isn’t perfect and produces heat in the process which means the lamp isn’t as energy efficient as the LED.
Another problem with the fluorescent lamp is the lifespan. If you crack the glass tube, the gas will escape and the light won’t work. The ballast can also fail and the elements inside the bulb eventually wear out. They don’t last forever.
Modern fluorescent bulbs produce appropriate colors and don’t flicker as much as older bulbs. This makes them an excellent replacement for the older incandescent bulbs.
The incandescent light might seem like the simplest light to explain. If you examine one carefully, you can see that there is not much to look it. Basically, it is just a wire inside a glass container. If you want to get a little more complicated, inside the glass bulb there are two wires that support a much tinier wire in between them---the tiny wire is called the filament.
Here is the basic operating principle. When you run electric current through a wire, it gets hot. The filament is just a wire that gets so hot that it glows. It’s that simple. But then, why the glass? The glass bulb serves one primary function---keep the air out. When a hot filament comes in contact with air, it will burn and melt. With a melted filament, you no longer have a working bulb.
That seems like a complete explanation, but why do hot things produce light? It turns out that all solids produce light. Yes, it’s true. Your pencil produces light. The apple on the countertop produces light. These everyday things produce light, but they produce light that you can’t see---light with wavelengths longer than the wavelength of red light. We call this light infrared. If you take an object and slowly increase its temperature, it will produce different wavelength of light. When it gets hot enough, the light will be in the visible range.
Here is a simple experiment. Turn on the stove in your kitchen, but don’t put a pot on it. Very soon, the eye of the stove will become hot (don’t touch it). As the temperature continues to increase, you will eventually see the eye glowing red. This is exactly what happens with the filament in the bulb. It is so hot that it doesn’t glow red, but yellow-white.
But what about energy levels? In the previous two sources, light was produced when an electron changed energy levels. Is that true in this case? Yes, the light you see from a filament is also produced by electrons transitioning between energy levels. The difference between a filament and a fluorescent bulb is that the filament is a solid. In solid materials, atoms interact with other atoms to slightly change nearby atom’s energy levels. The result is that you have many different atoms with many different energy levels. There is such a variety of energy levels that you get all possible transitions and all possible colors of light (once you have enough energy). When you combine all the possible colors of light, the human eye perceives this as white light.
Although the incandescent lightbulb is simple to make, it’s not the best device for light. The problem is that the bulb makes light by getting very hot. Very hot means very bad and wasted energy. Most of the energy you get from an incandescent bulb goes straight into thermal energy that you don’t want (unless you are using the lightbulb to heat things up).
You might think that fire is the simplest of all lighting sources. Yes, it is simple to create and simple to control. However, it is not so simple to explain. Much of the organic matter we see (like wood and coal and oil) contain carbon that is bound to other molecules. It turns out that this carbon can also make very strong chemical bonds with oxygen to form carbon dioxide. Although it takes some energy to pull a carbon away from its other bonds, the formation of carbon dioxide also produces extra energy. And this is the basic idea behind fire. With a little bit of starting energy, you can turn organic carbon and oxygen into carbon dioxide.
Where does the light come from? In the burning process, there is something other than carbon dioxide produced. It is generally called soot---but it is basically unburned pieces of material. This extra material gets caught in the along with hot air and rises above the combustion area. Since the soot is hot, it produces light in the visible spectrum just like a lightbulb filament or a hot stove eye.
Is fire more efficient than an incandescent lightbulb? Well, it is difficult to compare the two lighting methods. One runs on electricity and the other runs on carbon-based material (with no electricity). Of course the fire still produces lots of heat which may or may not be a good thing (depending on what you are using it for). The other issue with fire is that it produces carbon dioxide which isn’t really a good thing to have too much of. Oh, sometimes fire gets other things so hot that they also start interacting with the oxygen. Sometimes these other burning things are important things like your house. So overall, fire is nice but we can do better.
Let’s look at the key concepts:
- Electron energy levels are quantized. This means the electron can only have certain values of energy.
- When an electron moves from a higher to lower energy level light is produced.
- The frequency (and thus wavelength and color) of this light depends on the change in energy level.
- For LED, this electron energy transition happens because of an energy gap in a semiconductor material.
- In a fluorescent bulb, electrons transition energy levels in a gas. Also, some of the light produced is in the ultraviolet range. This light is converted to visible light through fluorescene.
- An incandescent bulb creates light by making a hot filament. In a solid, there are many different energy levels in order to produce all possible colors of light.
- Fire is just like a filament except that it’s not a filament. Instead, the light mostly comes from hot pieces of material that did not get burned.
That’s it. The four ways humans make light. Yes, these explanations are not complete. In order to really understand light, you would probably need to take several undergraduate courses in physics. Well, that would at least get you started.
