A listener asks: Can spiders get caught in the webs of other spiders?
We’ve always been warned not to look at the sun, for fear of burning out the cells in our retinas. It turns out that even photodetectors—devices specially designed to measure light—can’t look at the sun either, but for a different reason: they’re rendered useless when flooded with the sun’s intense rays. In this Science Update, you’ll hear about a new photodetector that’s not afraid to stare the sun straight in the eye.
Turning a blind eye to the sun. I'm Bob Hirshon and this is Science Update.
An important part of our national defense involves scanning the sky for missiles and other airborne threats. But while the bright flare of an incoming rocket might be visible at night, it can easily be overwhelmed by the sun during the day.
That’s why Joe Campbell and his colleagues at the University of Texas at Austin are making a detector that’s blind to the sun. The group is part of The National Science Foundation’s Center for Synthesis, Growth, and Analysis of Electronic Materials.
And so what we’ve been doing is looking at it with new materials called aluminum gallium nitride. Now, this material system is just now being developed. It has been used to make blue light-emitting diodes and blue lasers.
Detectors made out of these materials would be sensitive only to ultraviolet light that doesn’t penetrate the earth’s atmosphere.
So if I want to track for a rocket, then looking in that wavelength range is very good because that shows the emission from the rocket, but it doesn’t detect the sun’s radiation.
The detector would essentially see fire from a rocket’s tail against a blacked-out sun—helping the military keep a more effective eye on the sky. For The American Association for the Advancement of Science, I'm Bob Hirshon.
Making Sense of the Research
It’s a hard concept to grasp, but the light that we can see represents just one part of the complete spectrum of light that exists. If our eyes were designed differently, we might have been able to see forms of light that we can’t see now, including infrared and ultraviolet light. And it’s just as possible that we wouldn’t be able to see the light from the sun.
That’s exactly how the aluminum gallium nitride detectors are meant to work. They’re looking for specific patterns of ultraviolet light that are associated with fire from rocket engines. If they can detect these ultraviolet light patterns, they might be able to spot incoming rockets and missiles when they’re high in the atmosphere.
But most ultraviolet light detectors can’t do this during the daytime. That’s because all the light from the sun drowns out the ultraviolet light from the rocket plumes. It’s like trying to hear a pin drop in a crowded train station: there’s too much interference. Because the aluminum gallium nitride detectors can’t see the light from the sun that we can see, they can focus entirely on the search for ultraviolet light.
Campbell says the technology is being developed exclusively for the military, in order to protect tanks and military transports from surprise attacks, and to guard against enemy missile strikes. But he says some researchers have talked about civilian applications for materials like these. One idea is to make a monitor for sunbathers. If you brought one with you to the beach, it could measure the amount of harmful ultraviolet light you’ve been exposed to, and let you know when you’ve had enough.
Now try and answer these questions:
- Why can we see the sun’s light and not ultraviolet light?
- Why is the sun such a problem for ultraviolet light detectors?
- How do the aluminum gallium nitride detectors get around this problem?
- Can you think of other situations in which one kind of noise, smell, taste, texture, or image drowns out the one you’re actually looking for? Are there any ways around that problem?
- Infrared light is another invisible kind of light radiation, which we detect as heat. Can you think of any reasons why people would want to “see” infrared light? (Think about what kinds of things give off heat.)
Learn about light at The Universe of Light, on the University of California at Berkeley’s Center for Science Education website.
For information on the entire electromagnetic spectrum, see this page by Lawrence Berkeley National Laboratory.
The Sun - A Multimedia Tour is a good introduction to the sun and its composition.