All right ladies and gentlemen, today we’re performing an experiment! More of a demonstration, really, but one that’s very easy and will impress your friends. You will need:
1) One remote control.
2) One camera phone.
The vast majority of remote controls operate via an infrared light-emitting diode situated at the front of the device. You press a button, and the diode lights up in a particular pattern of pulses corresponding to the button you pressed. A little sensor at the front of the TV detects these pulses and converts them into the electric signals that tell the TV to switch from Monday Night RAW to Mythbusters. (Or whatever your preference may be.) Your eyes can’t see this light, because your eyes contain cellular chemistry that only reacts with a particular slice of the electromagnetic spectrum. Infrared light happens to be too low in frequency to set off the chemicals in your eyes, and so it’s simply not seen.
The people who manufacture digital cameras don’t use those same chemicals, because what works in a biological environment is an engineering nightmare in an electronic device. Digital cameras use one of a few different technologies that generally speaking don’t involve chemical changes – diodes and photosensitive capacitors tend to be the major players. As such, they don’t generally do quite the same things as your eyes. One can approximate the other pretty well, but there’s some exceptions. The one we’re exploiting is this: the sensors in digital cameras are usually sensitive to infrared light. But the internal electronics in your camera have no way of knowing this, and so it displays the infrared light as the visible light it wrongly thinks it’s detecting.
So if I take a picture of the front of a remote control with no button pressed, I see something like this:
And if I take a picture with me holding down one of the buttons:
The heretofore invisible infrared light shows up as a blaze of visible light that may range from blue to pink depending on the specifics of your camera. Interestingly the prominence of this effect is usually inversely proportional to the quality of your camera. Higher-quality cameras are interested in the most faithful reproduction of the visible light scene and so they include special filters to remove the infrared light before it reaches the sensor. If you try this with a quality dedicated digital camera rather than one in a cell phone you may not see the effect at all. (Hence doing the experiment with a camera phone)
While there’s not a whole lot of other things that emit a lot of IR at this wavelength, there are a few. One of them is the thermal radiation of a very hot object. Here’s a camera phone picture of a stove just when a very dim red light starts to be visible with the naked eye:
You may be able to find other examples. Please feel free to share them, but be careful not to burn your house down!