Visible and Invisible Light
Title: Visible and Invisible Light
Author: Andrew Berger
Basic idea: Not all light can be seen by our eyes, but other detectors can sense it and make use of it, and it's not so different from visible light.
Jargon:Infrared, ultraviolet (explained in text)
CCD (charge coupled device) detector array
Deeper explanation: The bulb in a TV remote control emits infrared light. We can use a
video camcorder to look at the bulb, because the detector in the camcorder responds to infrared light and show it on the screen.
Importance in science:
Nature: various animals can see outside of the visible range
Astronomy: detecting radiation from outer space and absorption of that
light by the atmosphere
Engineering: using invisible beams of light for communication, signaling,
Sensing
Everyday examples:
Remote controls for security ("tripping the beam"), garage door safety, elevator doors
Extended discussion and demonstration:
Background: The various light-sensing cells in our retinas have pigments that absorb light most efficiently over the range of about 400 to 700 nm. The chemical properties of these pigments thereby define this range of wavelengths as what we call visible light.
Outside of our human-centered perspective, there is nothing fundamentally unique about this range. It is not even true that no other wavelengths can be seen by the eye; very bright light with slightly shorter or longer wavelengths (e.g. 800 nm) can easily be perceived.
Light exists at both longer and shorter wavelengths than the visible. Shorter wavelengths are called ultraviolet ("above violet"); longer wavelengths are called infrared ("below red"). Our eyes do not readily perceive these wavelengths, either because our retinal pigment doesn't absorb them enough or because other portions of the eyeball absorb them before they reach the retina. Unsurprisingly, however, the ultraviolet and infrared regimes play important roles elsewhere in the animal and plant kingdoms, in astronomy, and in human science and engineering.
In this activity, we will explore a familiar device that uses infrared light, namely the infrared remote control. Because our biological eyes can't see this light, we will use a videocamera as an "artificial eye" that can focus and detect the infrared radiation, and a
TV monitor as an "artificial brain" that allows us to see images that include contributions from infrared light.
[Technical note: the camcorder uses a charge coupled device (CCD) array, which is most sensitive in the visible but does have decreasing responsivity in the near-infrared out to about 1100 nm. Usually such cameras have an IR-blocking filter to reduce sensitivity to light you wouldn't normally see. The "night vision" setting of the typical videocamera does one or more things: (a) shines an IR source of light out from the camera, (b) physically removes the filter (if it exists) from in front of the CCD, (c) processes the pixel information in such a way to be more biased towards IR sensitivity (it is still plenty sensitive to visible light).]
Equipment:
Big stuff:One or two video camcorders (preferably with "night vision" option), TV or other monitor for display, connection cable for camcorder to output to TV. Alternatively, a webcam and computer will also work, though “night vision” won’t be available.
Small specialized stuff: IR remote control [either for the TV or (preferably) something
else], Rainbow Peephole or other diffraction grating, small thermal and/or
LED-based light sources (e.g. cell phone).
Small around-the-house stuff: paper, tape, ruler, pencil, water, two clear mugs or cups
Seeing IR sources on the TV.
1. Aim IR remote at students, show that they can't see the light directly. Show visible light sources for contrast. If students each have diffraction gratings, look at the light sources with the gratings.
2. Connect one videocamera to TV set, set to “night vision” mode. Darken room (not crucial). Aim remote at camcorder, show IR clicker (from TV) lighting up brightly when shined directly at camera. [If you only have the TV clicker, push some setting that does nothing (e.g. controls a cable box) or doesn't affect the screen output (e.g. "mute").
3. Shine clicker on people (now it helps to have the room darkened). If you hold the remote close enough to a person, the flicker off the person should be bright enough to see on the TV. Students can look back and forth between the illuminated student and the TV, emphasizing that they can't see the light with their biological eyes but the artificial eye of the camcorder can see it.
4. If there's a second videocamera available, turn it on and run it in IR mode, with the IR source on. Use second camera as an IR flashlight, shining on various people in the room. This effect is more dramatic than the relatively weak clicker. Switching from IR to daytime mode turns the IR source on and off, which is also a neat effect.
5. Look at thermal or LED light (e.g. cellphone) in the same way. They also will show up on the TV.
What's the wavelength of the clicker? (It's about 1000 nm.)
1. Turn the TV into a vis-NIR spectrometer! Tape the peephole (or other grating) over the lens of the camcorder.
2. Tape a piece of tracing paper or overhead transparency over the screen (the less scattering the better). Draw a center dot and some lines radiating outward with hashmarks.
3. Stand back and aim a thermal (broadband) light source at the camcorder so that its TV image hits the center dot. Use daytime mode so the rainbow is more evident. If your only broadband light source is a large flashlight, cover most of the aperture so it becomes a smaller source. Have someone else stand at the TV screen and mark down the location of the edges of the diffracted visible spectrum. Mark these points as 400 and 700 nm. (If the light on the screen is too bright, dim the source brightness by placing a filter in front—probably a polarizer is available.)
4. Aim the IR remote at the screen and hold down a signaling button. Have somebody mark down location of the diffracted IR spot. Extrapolate linearly out from the 400 and 700 nm marks to figure out the wavelength of the remote. You get pretty close to 1000 nm, give or take 100.
Something transparent can be an IR absorber.
1. I used my VCR remote control for this so as not to change the TV picture, but using the mute button on the TV control is fine (just don't turn the TV off or change the channel). Back off some distance and/or place something over the clicker to make it weaker. For me this worked for my VCR remote at a distance of about 15 feet, with no other attenuation. Aim clicker at TV/VCR; you can see the flash on the TV (I keep the grating in place, but that's not necessary) and the VCR turns on/off. It helps if you can operate the clicker on a flat surface so you can leave it in place.
2. Get two glass mugs, one empty and one with water. Place empty mug in front of clicker (again the benefit of the flat surface). The remote should still work (this could take a little tinkering, depending upon the mug), and you can still see the flash on the TV screen. Now place the water mug in front, and now the signal can't be seen on screen and doesn't activate the VCR.
3. If desired, show absorption spectrum of water (data here downloaded from website data of Kou et al., plotted using Matlab), confirming significant absorption in this region. The units of absorptivity are log scale; 1 cm pathlength through water at the peak of this absorption leads to a decrease in light by a factor of exp(-absorptivity * 1 cm).
Safety warnings: don't stare for too long at the light sources.
Give away: Rainbow peepholes
References: ?? (plenty of stuff on internet)
E-day Logistics
Equipment for demonstrations: as listed above (need source of water ahead of time)
Electrical requirements: Webcam and laptop system demonstrated
Other contingencies: none
Location: dark
Demonstrators: 1