Atmospheric Windows
Published on Jun 8, 2006 at 1:27 pm.
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Light is electromagnetic radiation. That means that light is composed of coupled oscillating electric and magnetic fields moving in wave fashion. Scottish physicist James Clerk Maxwell is generally given credit for figuring this out in the mid-Nineteenth Century. Maxwell took a set of equations derived from the work of Gauss, Faraday, and Ampere. He then added one small term to one equation — a term that just seemed that it had to be there simply from symmetry arguments so that said equation looked similar to another one. Then, using a bit of calculus, you can show that these equations can be combined to produce an equation that describes coupled electric and magnetic waves. Further, the equation shows that the speed of that wave is fixed by two constants: the permitivity and permeability of space. The reciprocal of the square root of the product of these constants turns out to be the speed of light. Clever chap, he was.
Anyway, all light is the same: electromagnetic radiation. The only difference is the frequency with which the waves oscillate. Since they all move at the same speed, the freqeuncy of oscillation is also related to the wavelenth: the higher the frequency, the shorter the wavelength, and vice versa. The higher the frequency, the more energy the light has, and so its interaction with matter is a bit different. For the visual range, red light has the longest wavelenths (lowest frequency and energy) and violet light has the shortest wavelengths (highest frequency and energy). For the average human, the longest wavelength light that you can see is about 700nm, and the shortest wavelength of light that you can see is about 400nm. For the average human, that is. That implies that non-average humans can see something different? Yes. When we do spectroscopy labs with my students, we find that when working at the extremes of the visual range, some students can see certain spectral lines and others can not. Now, it could be that the ones that claim that they can not see those lines are simply not looking closely enough, as they are extremely difficult to see. However, they might simply not have the right visual range.
William Herschel, a rather famous astronomer, is generally credited with the discovery that light extends beyond the visual range. He was working with prism “measuring the temperature of light.” Huh? Well, his idea was that everyone knows that if you sit out in the Sun, the sunlight warms you. Yet, a prism shows that the light is composed of many different colors of light. Isaac Newton showed that if you use a prism to break light into colors, and then use another prism on just one color of light, it does not break up any more. So, the colors are basic things. Herschel wanted to know if the warming effect of sunlight were due to certain colors in particular, a combination of them, or just what. So, he used prism to break sunlight into its constituent colors, and then stuck a thermometer into each color of light to measure its temperature. OK, so the idea of the temperature of light is a bit squirley, but the idea that different colors might react differently is fine. Well, when he was done, he set his thermometer down just past the red end of the spectrum. He started to write up his findings, and he chanced to look at his thermometer. It was getting warmer! Yet, as he saw, there was not light falling on it. He placed something between the prism and the thermometer, and the temperature fell again. Removing the obstruction, the temperature rose again. Herschel correctly deduced that there must be some form of invisible light beyond the red end of the spectrum. Since the red was the least bent light coming from the prism, the light beyond red was even less bent. So, we call this infrared light. Soon thereafter, German physicist Johann Ritter discovered light beyond the violet end of the spectrum. This light is bent more than violet, which is the most deflected visual light coming through the prism, so we call it ultraviolet light.
Now your eyes don’t see these forms of light, but some insects can. Wasps, bees, and similar insects use the orientation of the Sun to navigate. On cloudy days, the clouds block and scatter the infrared and visual light, so you look up and you can’t see where the Sun is. However, much of the ultraviolet will pass through water, so the clouds don’t affect it as much. This is why you can get sunburned on a cloudy day. This is why you can get sunburned even in a swimming pool, even if you are under water most of the time. It isn’t the heat that burns you, it is the UV. Since the clouds don’t block the UV from the Sun, the bees and wasps can still see where the Sun is in the sky, even if you can’t. This is also why flowers pollinated by insects are generally very reflective in the ultraviolet.
So, water in the atmosphere blocks and scatters infrared light. Not all ultraviolet makes it through the atmosphere, either. Most ultraviolet, in fact, is blocked by ozone in the Earth’s stratosphere. This is good, because without that ozone, the UV intensity on the ground would be lethal. The Sun is a very powerful UV source. However, lots of other things in the sky emit ultraviolet light. Many emit infrared light. Many things emit those forms of light and not visual light. So, in order for astronomers to see these objects, we need to have telescopes and instruments capable of detecting those forms of light. Well, most of the water vapor is fairly low in the atmosphere, so situating telescopes on top of very tall mountains gets better infrared reception. Telescopes in high flying aircraft are even better. But, we don’t have any mountains tall enough to get above the stratospheric ozone, and aircraft don’t fly that high either. Extreme altitude balloons help, and sounding rockets into Earth’s mesosphere are even better. But even better than than is putting an ultraviolet telescope into space.
Ultraviolet, infrared, and visual light are not the only forms of electromagnetic radiation. Even longer wavelength than infrared are radio waves. Some radio waves are absorbed or reflected by the upper atmosphere, but many make it through. So, that means that we now have two types of electromagnetic radiation that make it through the atmosphere pretty good: visual (and near visual) light, and some radio waves. We call the range of wavelengths that make it through the atmosphere “atmospheric windows,” since we can look through the atmosphere in these wavelenthgs to see the rest of the universe.
There are other forms of electromagnetic radiation. Shorter wavelength even than ultraviolet you get X-rays, and even shorter you get gamma rays. Neither pass very far through the atmosphere. You can measure their penetration in meters, or less. So, it would be hopeless to set up a backyard X-ray telescope. There is no choice but to put X-ray and gamma ray telescopes into orbit. In fact, even visual light is distorted by passing through the atmosphere, so it is helpful to have a visual telescope outside of Earth’s atmosphere. Such telescopes exist.
The Hubble Space Telescope is often called the Space Telescope. That is FAR from the case. At this time, there are a dozen or so space based telescopes. The Hubble is simply the biggest and most expensive, and it gets the most press attention. The others, though, are doing fantastic work. Astronomers have been working, though, on compensating for the effects of Earth’s atmosphere on the light passing through the atmospheric windows. With adaptive and active optics, we can now generate images from the ground about as good as we can get from Hubble, but only for visual light. Hubble is still the instrument of choice for near IR and near UV observations. And, for farther IR or UV, there are other space based telescopes, such as the Spitzer Telescope for infrared or the Galaxy Evolution Explorer (GALEX) for ultraviolet. In fact, there have been several dozen space based telescopes!Â
-Astroprof
