There’s no question that light plays “The” main role in photography. Without light there won’t be photography at all.
As photographers we all intuitively know, or learn, how to see and use light. We discover the property of different materials and how they affect light. We use those materials to modify existing light. We reflect, shape, tint, diffuse and mix light. In a way we are “Wizards of Light”.
In this article I’ll try to explain in a general way the physics of light, how they determine the way we see things and how we can use light and material properties in our advantage.
It is important to mention that Optics, the science behind our beloved lenses, is the study of light and the interaction with objects.
Measurement of light, in terms of perceived brightness to the human eye, is defined by the science of Photometry. I will not go into this topic at this time, but if you would like to read more about it you can start here.
What is visible light made of?
“Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum” (Wikipedia)
Electromagnetic radiation is a radiant energy formed by electromagnetic waves. Radiant means it’s transmitted to all directions. The portion of electromagnetic spectrum we will be talking is referred to as “visible light”. This is the range of waves, or frequencies, between infrared and ultraviolet.
Summarizing, visible light is made up of wavelengths, and each wavelength is a particular color. These wavelengths range from 700 nm at the red end of the spectrum to 400 nm at the violet end.
Properties of light
Primary properties of visible light are intensity, propagation direction, frequency or wavelength spectrum, and polarization, while its speed in a vacuum, 299,792,458 meters per second, is one of the fundamental constants of nature. (Wikipedia)
The definition above shows us that light can have different intensity (power), color (frequency), and direction. This last property is what allows us to modify direction by reflecting light on objects (mirror, diffuser, etc.).
Let’s see each one of these properties in more detail.
In other word, power or also called amount of light. As photographers we use this property to illuminate subjects to our desire. We use distance to control power, moving away to decrease and closer to increase amount of light. Or electronic circuitry to reduce the amount of power in our electrical sources (flashes, strobes, etc.).
This property defines the color of the light. We mostly work with white light, which is light that contains all wavelengths. But when using colored gels we are effectively modifying this property by allowing certain frequencies to be transmitted and filtering the rest. A red gel will filter all frequencies above the red part of the spectrum.
Light has a physical property, it has direction and it can bounce from objects. Photographers use this property for example when pointing the flash to the ceiling or walls to bounce the light. In this example we make light travel in a different direction than our subject is, make it bounce in the wall and return back in a diffuse mode.
And diffuse is the key word here. We’ll talk more about this property later in the article.
Light oscillates in more than one plane. To better explain this we can look at how we normally represent a light waveform:
Polarization is the property of waves when they oscillate with more than one orientation (Wikipedia)
Polarizing light is the action of partially filtering it to allow waves coming in one angle. To polarize light we use the polarization angle (see Brewster’s angle) to allow a certain portion of light to pass through while reflecting the rest.
This effect is specially prominent to photographers when it comes to shooting glass or other reflective materials. When light reflects from a glass it contains waves in multiple directions as shown in 2nd chart above. By polarizing this reflected light using a filter we are removing most of those waves and allowing only waves at a certain angle to pass through the filter. We achieve this by rotating the filter, thus changing Brewster’s angle.
The effect this has on subjects is the well known reduction of reflections. For example this allows us to shoot water and remove the reflection of the sky above.
Reflection is the change in direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originated. (Wikipedia)
There are two types of reflection, diffuse and specular. Diffuse reflection is the reflection of light from a surface such that an incident ray is reflected scattered at many angles. This is the type of reflection most material create and one photographers use a lot. Think about all those light modifiers we use to make light “soft”.
The other type of reflection is called Specular. This reflection is when all the light is reflected away in one single angle. Virtually all materials can give specular reflection, provided that their surface can be polished to eliminate irregularities. However, only metals can reflect light specularly with efficiency. All other common materials, even when perfectly polished, usually give not more than a few percent specular reflection. Photographers use this type of reflection to bounce already diffused light into the subject, think about those silver foldable reflectors.
Most real objects have some mixture of diffuse and specular reflective properties.
Reflection & color
In the real world almost all objects will absorb portion of the light and reflect the rest. This reflection of light depends of source wavelength and surface physical structure. When light hits an object it will absorb part of the light and reflect the rest. The physical structure (the space between its molecules) of the material from which the object is made of will determine which part of the light spectrum will be absorbed and which reflected. Because light is composed by a sum of different wavelengths the result effect is that we will see the reflected portion and identify it with the object’s color.
For example, white objects appear white because they reflect all wavelengths. Black objects appear black because they absorb all wavelengths of light.
When white light shines on a red object, all of the colors that form the white light are absorbed except red, which is reflected. This is why the object appears red!
Light Reflectance Value (LRV) is the total quantity of visible and usable light reflected by a surface in all directions and at all wavelengths when illuminated by a light source. (ref. British Standard BS 8300:2001/A1:2005)
LRV is a measurement that tells you how much light a color reflects, and conversely how much it absorbs. LRV runs on a scale from 0% to 100%. Zero assumed to be an absolute black and 100% being an assumed perfectly reflective white. Approximately speaking, the average blackest black has a LRV of 5% and the whitest white 85%. This is why we can still see black objects, humans can perceive down to that 5% of reflection and interpret it as deep black.
I hope this article gave you a glimpse of what light is, how it behaves and how we photographers can use it. I wrote it based on reading many articles, some directly referenced and other used as a source of knowledge. Below there’s a list of material I used during my research.
 http://en.wikipedia.org/wiki/Reflectivity [/alert]