# Physics of Colors – Tinted Simplicity of Nature

Mastering colors in photography is an art only proficient photographers can boast. From a physics point of view, however, color is well known since the beginning of the 20th century. Starring in this brief physical explanation are: electromagnetic radiation, photons, wavelength and energy.

When a charged particle is at rest relative to an inertial observer, the observer measures an electric field. But when the same charged particle is in motion relative to the observer, in addition to the electric field another field is observed, which is named magnetic field. These two fields together are called the electromagnetic field. Energy is required to set up an electromagnetic field. This energy remains constant for a static electromagnetic field (i.e. a field that does not change with time). When the field is time dependent, the electromagnetic energy changes with time. These time variations give rise to an electromagnetic wave, propagating at the speed of light. Such a wave carries the energy of the electromagnetic field, and this energy is referred to as electromagnetic radiation.

A photon is the quantum of electromagnetic energy and it is entirely determined by the frequency of the radiation. The mathematical defining relation of the photon, relating it to the frequency and to the wavelength of the electromagnetic relation is: E = hf = hc/l where “E” is the photon’s energy, “h” the Planck’s constant, “f” the frequency, “c” the velocity of light and “l” the wavelength. There is a one-to-one correspondence between energy, frequency and wavelength.

Electromagnetic radiation at different wavelength (or energy, or frequency) takes on different names. For instance, if the wavelength is greater than 0.1 meter the radiation is called “radiofrequency”; if it is between 1 millimeter and 1 micron it is called “infrared”; between 0.1 micron and 1 nanometer it is “ultraviolet”; between 1 nanometer and 0.1 angstrom “X-rays” and, finally, if greater than 0.1 angstrom “gamma rays”.

As we all know, photography deals with visible electromagnetic radiation (light). This is a very narrow region of the electromagnetic spectrum, namely between 4000 and 8000 angstrom. Our eye is able to reveal electromagnetic radiation only in this region, and it perceives different wavelengths as different colors. For instance we perceive electromagnetic radiation at 8000 angstrom as red, at 5600 as yellow, at 5000 as green, at 4500 as blue and at 4000 as violet. Black and white are not colors: black is the absence of light while white is the superposition of all the colors together.

All the things in our world rarely emit or reflect light at just one wavelength. They usually do that over a range of frequencies. The curve of the energy distribution, that is the energy of light emitted or reflected versus the wavelength, may have any arbitrary shape. One well-known curve of this type is the blackbody radiation’s. If the object we are looking at has an energy distribution curve with a pronounced maximum at a certain wavelength, we’ll probably see that object with the color corresponding to that wavelength. For instance, our Sun emits over the entire visible spectrum, but it has a maximum corresponding to the yellow wavelength, and that’s why we see it yellow. This depends on how our eyes react to light.

Here is yet another example. An object can have two maxima in its energy distribution. This happens, for instance, if we draw a splash of color on white paper with blue and then with yellow. We will see it as green. It is not surprise. Indeed, look at the wavelengths aforementioned: green lies midway between yellow and blue. But, again, from a physical point of view a real green with its unique wavelength has nothing to do with something with two maxima centered on yellow and blue. This is only our vision system’s interpretation.

So, the colors of nature and the struggle we make for capturing them boils down to how our eyes see electromagnetic radiation at different wavelengths.

Andrea Ghilardelli runs an online photo retouching service. To get your pictures beautifully retouched and for articles about photography, please visit his site: www.ilghila.com.