Lab 3: Reflectance Spectroscopy
- Cool Cosmos. A page that focuses on the world of the infrared. Has lots of cool pictures taken with an infrared camera: people, animals, and more.
- USGS Digital Spectra Library. Reflectance spectra of many many minerals, and some other things, too.
- Blackbody Applet. A Java applet to demonstrate blackbody radiation (emitted light) at various temperatures. You can enter the temperature (in Kelvin) you want in the text box. To convert between Kelvin and Fahrenheit...
(temp. in deg.F) = (temp. in K) x 9/5 - 460
(temp. in K) = (temp. in deg.F) x 5/9 + 255
Emitted Light: When an object is hot enough to glow.
- All objects emit light, but only objects that are really hot emit light that is visible to our eyes. Objects at room temperature emit mainly in the infrared.
- As an object gets hotter, the wavelength at which it emits the most light becomes shorter. This is why something that is yellow-hot is hotter than something that is red-hot.
- As an object gets hotter, it also emits more light at all wavelengths. So when the heating coil on an electric stove is glowing bright orange, it is emitting more orange light, yellow light, blue light, red light, and even infrared light than it would when it is only glowing red.
- When we see an object emitting light, the color tells us how hot the object is.
Reflected Light: How we see objects not hot enough to glow.
- When we see an object due to reflected light, we see light from some other source that hits the object and is reflected off of its surface. If we turn off the lights, then we won't be able to see this object anymore.
- The color of objects seen in reflected light is not due to their temperatures, but rather the chemical properties of the material they're made of.
- A reflected-light object that is a bright color (like red, yellow, blue, or green) is made of material that reflects that color more than any other color.
- Objects that are lighter in color generally reflect more of each visible color than objects that are darker.
- Objects that reflect a lot of infrared or ultraviolet do not necessarily appear "black." Our eyes cannot see these colors, so they do not affect the color we see at all. If we have two objects with identical reflectance spectra, except for the fact that one reflects a lot of IR and the other reflects almost none, our eyes will not see any difference. Only how reflective the object is at visible wavelengths will tell us what color we see.
Reflectance Spectroscopy, or precisely what color is that object?
- Each material has its own unique reflectance spectrum. A red crayon and red ketchup are both similar colors, but if we look at how bright they appear at many different wavelengths, we will see differences.
- This technique, known as reflectance spectroscopy, is useful for looking for specific minerals on a planet's surface, or just seeing what a planet's surface is made of. Many space probes have and use reflectance spectrometers... it's much cheaper than landing with a portable chemistry set!
- One of the drawbacks is we can only use reflectance spectroscopy to see what is on the surface, because light will not travel down to layers that are covered up. So if I take the reflectance spectra of a Hershey bar and a Hershey's with almonds, in both cases I will just get the spectrum of chocolate.
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