What are the characteristics of light?

Question

RE: https://www.shutterbug.com/content/characteristics-light-quantity-quality-color-direction

This article lists 4 characteristics of light:

1. Quantity
2. Quality
3. Color
4. Direction

Aren't size and distance also characteristics of light? Or do they somehow fall under the list above?

To add to the confusion, https://www.sekonic.com/united-kingdom/whatisyourspecialty/photographer/articles/the-characteristics-of-light.aspx doesn't even include "direction" as a characteristic of light.

This reference http://www.peachpit.com/articles/article.aspx?p=1963995 lists the following as the characteristics of light:

1. Direction
2. Intensity
3. Color
4. Contrast
5. Hardness

So, now I'm wondering what exactly are the characteristics of light.

Answer 1

What are the characteristics of light?

The list in the Peachpit article is the same one I came up with when I developed a syllabus for a beginner's photography course on my own. I later changed hardness to diffusion, then demoted contrast, explained below.

With regard to light, these are the characteristics I evaluate when making decisions about exposure and composition.

1. Direction - position of the source(s); where all of the light in the scene (incident and reflected) is coming from; the direction, definition, and brightness of shadows

2. Intensity - output level at the source, brightness of illuminated surfaces, relative brightness

3. Color - hue and color temperature, luminance

4. Diffusion - variance of angles of incidence

I don't consider hardness (the rate of transition from light to dark) or contrast (the difference between the darkest and lightest points) to be characteristics of light. They are the perceived effects of light that varies in direction and intensity, so I consider them functions of those characteristics.

Aren't size and distance also characteristics of light?

At first glance these are characteristics of light sources, not light itself.

Size and even "relative size" used to describe how large a source is are useless by themselves. It's more useful to think of the maximum angle at which light arrives at the subject. The greater the angle, the "larger" the source.

But distance is interesting. It's kind of a pseudocharacteristic of light, isn't it? As in "the distance from the source traveled by the light striking the subject", because that segment of the light beam has a unique property: the rate of falloff, defined by the inverse square law.

Answer 2

The physical properties of light are intensity (or quantity), direction, wavelength (or colour), polarity and coherence.

Intensity is in simple terms the brightness of light, at least if you ignore that the human eye is more sensitive to some colours than others. The human eye is most sensitive to green light, then red and least sensitive to blue light. Green light with the same intensity as blue light is perceived brighter.

Light is also 'travelling' in a specific direction as a ray. It can be reflected when bouncing of surfaces and can be refracted at the boundary between two transparent materials (e.g. air and glass) and change direction. Most light sources emit light in all directions at the same time, e.g. the sun, fire or light bulbs (ignoring the shadow cast by the socket). Using reflectors and lenses, many artificial sources of light are built to emit light in a more or less specific direction as a beam of light, e.g. flash lights or car head lamps.

The wavelength of the light is what we perceive as colour. Longer wavelengths are perceived as colours on the red side of the rainbow and with shorter wavelengths, we go through the colours orange, yellow, green, and blue until we reach the shortest wavelengths we are able to see.

Polarity and coherence are properties of light, to which the human eye is not directly perceptible, but they play a role in many physics applications. Controlling the polarity of light is e.g. important in LCD displays and coherence is relevant for lasers and holograms.

I am not sure what 'quality' in your first list is supposed to mean. Contrast and hardness are not direct properties of light, but more related to how we perceive the interaction between different sources of light.

Contrast is seen if two adjacent objects are reflecting light with different intensities, as with one dark and one bright object. Some times the term contrast is also used for different wavelengths (colour contrast), e.g. the edge between a red and a green object may be perceived as 'contrasty' even if both objects have the same brightness.

Light seems hard and will throw distinct shadows if an object is only lit with light coming from one direction, e.g. outside on a bright sunny day, where the sun is the only significant source of light. Light coming from many directions is considered soft or diffuse, as opposite to hard. Outside on a cloudy day, the light from the sun is scattered by the clouds and will come from many directions, not only the single point of the sun, and you will see that objects are casting no, or only very weak and soft shadows.