Light

Understanding its basic features and properties is invaluable to know when illuminating a subject or using lenses

Light is at the foundation of photography. Understanding the fundamental principals of photography and the equipment you use, will allow you to have broader and more flexible approach to achieving the picture you want in any lighting condition.

In this section we will also show you how and why surfaces and subjects look the way they do. More importantly we will demonstrate how it is bent with glass to create a usable image and how this is best achieved with a photographic lens.

But What Exactly is Light ?

Simply put, it is a stream of energy radiating from the sun or another radiant source.

It has four main characteristics: Its first attribute is that it travels on a straight line which can be seen clearly in the "beam" of sunlight in the below picture.

Its second attribute is that it travels in a wave like motion similarly to when a smooth water surface is disturbed. The different wavelengths give us the sensation of different colours.

It also moves at very fast speed, but one of its properties is that it moves less fast in air, and even slightly slower in denser substances such as water or glass.

And lastly, it consists of energy particles, also called "photons" and the more intense the light, the more photons it contains.

---

The light's wavelength spectrum is quite large but the human eye is only sensitive to a narrow part of it which is approximately between 400nm to 700 nm (nm stands for nanometre and is one millionth of a millimetre) as progressively violet, blue, green, yellow and red.m

When all colours are present together , the light appears as "white". However, if only some wavelengths are present, it appears as coloured i.e. between about 400nm to 450nm, it is seen as dark violet which changes to blue if wavelengths are changed to 450 - 500nm etc.

To learn more about the colour of light, often referred to as colour temperature and how it can be adjusted to get the image you want, review our White Balance section.

---

We have established that it travels from a radiant source in straight lines and in all directions. As a result of that if a subject is illuminated by a direct light from a relatively compact source such as the sun in a clear sky, a bulb or a small flash unit, then it will cast a harsh, sharply defined shadow. In practice, this is the type of shadow you will get at noon on a sunny day.

However, light from a larger area source which can be simple formed by inserting a large sheet of tracing paper, is causing it to scatter into new straight lines in all directions from every part of its large surface thus defusing the light. The same would happen with sunlight on an overcast day. This gives the subject a soft, graduated shadow.

---

When light reaches a surface - anything from a face, or a building, or a landscape - what happens next depends on the texture, tone and colour of the surface as well as the angle and colour content of the light itself.

There are three types of materials - opaque, transperant and translucent. If the light strikes an opaque material - metal or a brick for example - some of it is reflected and some is absorbed (turned into heat). The darker the material, the less light is reflected and the more is absorbed.

The colour of the material also has an impact as it will selectively reflect wavelengths of the colour it has and absorb most of the others present in the light. The appearance of the colour that is reflected of the material varies with the colour of the light source. For example, if a white light hits a opaque material that is blue in colour, then a blue colour will be reflected. However, if a red light hits the same opaque blue material, then the material will appear black. This is demonstrated in the graphic below.

The reason we need to know of such effects is to help us with the use of colour filters later on.

The surface of the material also has an impact on the way light is reflected. A mat surface such as paper or dry skin scatters the light evenly and the angle of the light does not make much difference to that.

However, if the surface is smooth and shiny, it almost acts like a mirror and reflects it back in one direction. If the light is angled though, it gets reflect off in the same angle from which it arrived. This is important to remember so that you position your camera viewpoint in such a way so that it bounces glare light away when photographing a highly reflective surfaces.

When the light hits a transparent material such a clear glass, plastic and water, it is transmitted directly, while translucent materials such as paper or clouds diffuse it. In both cases, if the material is coloured it will pass more light of these wavelengths than other kinds.

The last property, I would like to bring your attention to is what happens to light when it passes obliquely from one transparent material to another of different density - from air to water for example. It will slow when passing from air through a denser material such as water. If the light reaches the water obliquely, the wavelengths slow unevenly and bend (refract) slightly steeper into the water. You can see this at work in the picture below where the straight stick is put into clear water; it looks bent at the water surface.

This is quite an important property of light though which allows lenses to bend it and form images.

The intensity of the light that hits a subject is also a subject of the distance between the two. The closer is the subject to the source, the brighter it will be illuminated. Doubling the distance between the two will result in four times as much illumination.

All of the above described effects are the optical aspects of seeing that our brain interprets for us which allows us to identify properties of an object or a scene without us having to touch or feel them.

And photographing is all about that...

So How Can We Make Light Form an Image?

Let's say we place an object and illuminate it and just put a tracing paper towards it. The piece of paper will receive a jumble of uncontrolled light rays reflected from all part of the object which is not very useful.

One way to create some order is to restrict the light by adding an opaque screen with a pinhole between the object and the paper. Since light travels in straight lines, those rays from the top of the object will pass through the hole and can only reach the bottom part of the paper. And light from the lower parts of the object only reaches the top of the paper. This forms a crude upside-down image on the paper, as seen below.

The best way to improve the image that appears on the paper is to use a converging lens rather than a pinhole which can bend the beam of diverging light coming from the object so that it converges to a point of focus. The point of focus here will depend on the bending (refracting) power of the lens which is shown by its focal length as well as the distance between the lens and the object.

---

---