Seeing Invisible Light

Earlier this week it was in the news that the Herschel Space Telescope was shut down.  Herschel was being used to observe the sky in infrared light, which is light you can’t see that’s redder than red.

It wasn’t until William Herschel in 1800 (namesake of the space telescope) that infrared light was discovered.  Nowadays, it’s pretty easy to see.
Step 1: Get a TV or DVD player remote
Step 2: Activate your cell phone camera
Step 3: Point the business end of the remote at your cell phone and push the button.  You should see a purple* light.

What’s going on?  Well, your eyes are not sensitive to infrared light, but detectors like the one in your cell phone CAN see it.

There are actually quite a number of things we can’t see in visible light.
That’s why astronomers like using different techniques and instruments like the Herschel Space Telescope to study space. Different things about the universe show up in different colors.  While hot things (like the Sun) glow in visible light, cold things glow in infrared light – dim stars, planets, asteroid belts surrounding stars, and, if we go to lower-energy “thermal” infrared, you.  (Yes really).  As we go to lower and lower energies, we get terahertz waves, microwaves, and radio waves (including AM and FM).

On the other side, Ultraviolet is more violet than violet; with still more energetic light you’ll get X-rays, and finally gamma rays – here you would see the very hottest stars, pulsar pulses, superheated gas threaded throughout the galaxy, the final screams of matter entering a black hole.  THOSE highly energetic forms of light can give you cancer (microwaves cannot give you cancer).

As it turns out, your eye actually could be sensitive to some ultraviolet light.  The impressionist artist Monet was going blind from cataracts and had the lens of his eye removed… afterwards, he started painting everything with strange blue tints.
He also couldn’t focus his eye afterwards, so it wasn’t a clean win.  Otherwise, I suspect several of my astronomer friends would have already had their lenses removed…

* Why is it purple?  The colored filters in your camera that pick up red and blue light happen to also be sensitive to that particular frequency of infrared light given off by the remote; the camera says “ok, so what’s very red and very blue at the same time? Purple!” and presents you with a purple dot, even though the light is definitely not purple.


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