Appliance Science: The bright physics of light and color

Stop for a moment, and look around you. Look at the colors of the objects, the brown of wood and the green of leaves. What makes these colors? You probably learned at school about how colors are created by the different frequencies of light, and how these different frequencies are absorbed or reflected by an object to create the colors that you see. An apple is green because it reflects the green light, absorbing the other colors. That is true -- but it's only part of the story.

The other part of the story of the colors around you is the source of the light, and how different light sources produce light that changes the way you see colors. Let's take a look at the science of color, and how the type of light bulb you use changes the colors of your home.

The visible light that you see is composed of different frequencies of light, with that frequency determining the color that you see. Red light, for instance, has a wavelength of between 610 and 700 nanometers (nm) at one end of the visible spectrum. At the other end, violet light has a frequency of between 410 and 450nm, with the other colors lying in between.

I remember the sequence of colors from the schoolyard mnemonic of "Richard Of York Gave Battle In Vain," for red, orange, yellow, green, blue, indigo and violet. You might remember it from the US version using the name "ROY G. BIV." Outside of this range are colors that your eye can't see, but which some cameras can: infrared at one end and ultraviolet at the other. Far beyond these are the other types of radiation, such as gamma and x-rays beyond the ultraviolet and radio waves beyond the infrared.

The color of objects is defined by which frequencies of light they reflect, and which they absorb, but there's a wrinkle: Objects can't reflect a particular color of light if it isn't there. And, sure enough, the different sources of light that you see differ hugely in what colors of light they emit.

Take sunlight, for instance. On a sunny day, the light that reaches your eyes contains a broad range of colors all across the spectrum. Below, we have used a spectrometer to split this light into its component colors, and then measured how much of each there is. On the graph, the higher the line, the more of that particular color of light there is.

As you can see, this is pretty flat for most of the spectrum: the amount of red to yellow light is mostly the same, while the amount of blue, indigo and violet is less. That's because of a phenomenon called Rayleigh scattering, where blue light is bounced around more than other frequencies by water particles in the atmosphere (which, incidentally, is why the sky looks blue).

Now, let's look at the spectrum of a conventional incandescent light bulb, where the light is created by heating a filament with electricity. This looks slightly different, with more of the red and orange. This helps to explain the rather orangey look of photos taken indoors, although that gets rather complicated (as we'll see in the next column).

Here is the spectrum from a compact fluorescent (CFL) bulb, which is very different again. Look at the number of peaks and troughs in here: rather than producing a wide range of colors, this bulb produces lots of light in certain narrow bands of color. That is because of how it works: A fluorescent bulb uses high-voltage electricity to excite mercury vapor, which then gives off certain colors of light. This, in turn, excites phosphors on the inside of the bulb tube, which emit other colors. Although this produces a wide range of colors, there are big holes in the spectrum: some colors simply aren't there.

Next is the spectrum of an LED light bulb from Feit Electric that plugs into a standard light socket to replace an incandescent bulb. The LED has fewer peaks and troughs than the CFL, because it uses several phosphors that produce a wider spread of colors. But there are still things missing here: look at the big gap in the blue end of the spectrum, where there is very little light between about 450 and 475nm.

So, it's clear to see that each of the light sources here produces light with different characteristics, with a different mix of colors -- and with different colors missing. What does all of this mean? It means that the color of the light around you is more complex than you might think, which, in turn, has a profound impact on how you see the world around you, and how you perceive color.

Imagine, for instance, looking at an object that is deep blue in color, which has a wavelength of about 475nm. Sunlight contains light at this wavelength, but the LED and CCFL lights don't. And an object can't reflect light that isn't there. So what will happen? Will the object be invisible? Probably not. The combination of light source, reflected colors and how your eye perceives color means that the process is more complicated than it first appears. We'll discuss this more in my next column.

One additional thing to note here: We've found that the spectrum of light bulbs varies a lot, even those of the same type. One LED bulb may produce a quite different spectrum to another, even those from the same manufacturer. So, don't be surprised if your spectra look somewhat different than the ones above.

Incidentally, all of the spectra you see in this article were captured using a spectrometer that costs just $45 (around £30 or AU$65) from the Public Lab project. I would strongly recommend getting one of these to anyone who is interested in color and light: it's a great way to capture and analyze light for the home scientist. And the Public Lab project is a great non-profit that is looking to democratize science by helping people measure the world around them, by doing things like look for oil contamination in water and creating their own maps.