If you'd like to know more about the essential role that light plays in photography, then you’ll love our free course, A Photographer's Guide to Light. In this lesson, you’ll learn about light absorption and reflection.
Noticing Absorption and Reflection
A white object appears white because it reflects almost all of the light that strikes it, and a black object appears black because it absorbs almost all of the light.
But what actually happens to the light energy? There are several explanations, most talking about what happens when a photon hits a single atom or molecule, but it’s not really relevant to photography because you won’t be photographing the interaction between one photon and an atom.
Another thing to consider is that materials made out of the same atoms can look vastly different. Take graphite and diamonds, both of which are made up of nothing but carbon. Graphite is somewhat reflective and not transparent, and diamond is reflective and highly transparent.
It makes more sense to talk about material on a larger scale. When light interacts with material, it's interacting with the collective structure of that material, so depending on the structure, you can have different interactions with light.
Non-transparent objects will reflect and absorb the visible light that hits them. Even when it comes to the most reflective objects, some of the light will be absorbed and with objects that have a high absorption, some of the light reflects. A black object looks black because it's reflecting very little of the light that hits it: that light energy is transformed into heat.
Consider for a moment that all matter is vibrating at the molecular level. This is why everything has a temperature; even very cold things are vibrating, just more slowly. When light hits matter, some of that energy that is not reflected is at a particular frequency that causes the molecules of an object to vibrate.
These molecules interact with neighbouring molecules and the result is thermal energy. All light at that particular frequency is gone and it's transformed into heat.
Something different happens with reflection. Some of the light that strikes an object will be at just the right frequency to cause the electrons on the object to move to a higher energy level. Once that energy from the light is absorbed by exciting those electrons to that higher energy level, the light energy is gone. The energy of the photon comes in and is momentarily absorbed and then re-emitted back out of the molecule.
Absorption and reflection are how you see colour. White light isn't really white, white light is photons of all colours traveling together — our eyes perceive this as white light. If white light falls onto an object that's orange, we’ll see orange light being reflected back to our eyes.
This means that nearly all of the other colour frequencies are being absorbed and transformed into thermal energy. ‘Nearly,’ because it's likely that some of the other frequencies besides orange are probably being re-emitted, but at a much lower level; so low that they have very little or no effect on the colour that we see.
Another possibility with absorption and reflection is for the light energy to cause the molecules to vibrate and move an electron into a higher energy level. As some of that light energy was transformed into vibrational motion and lost, the electron may emit a photon at a lower energy state than what originally came in. This is roughly how fluorescence works.
In fluorescence, a higher energy — also known as a higher frequency light — hits an object, but the object doesn't re-emit that light at the same frequency. Instead, it re-emits a photon of a lesser energy in the visible spectrum.
Most of the time you’ve seen this with ultraviolet light. This is because ultraviolet light is higher energy than all the other frequencies in the visible spectrum, and it's invisible to us so the effect is much more dramatic. It essentially makes things look like they're glowing in the dark.
However, this can also happen with just about any light frequency higher than red. As fluorescence is the emission of a lower-energy photon it makes sense that you can't get anything to fluoresce with red light because there isn't anything lower than red in the visible spectrum.
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About the Authors
David Bode created the video course that includes this lesson. Dave is an expert on video and audio production, and he lives in the upstate NY area. He works as a camera operator, editor, inventor, motion graphics designer, recording engineer, and studio musician.
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