The word photography means painting, drawing or writing with light. It is impossible to make a photograph without light and the direction, intensity and quality of the light falling on and then reflected by our subject is what creates our images.
The direction from which light falls on out subject creates shadows, enhances or hides textures and details and creates reflections. Everything our eyes see and our camera records is reflected light unless we are looking directly at an energy source. A completely black object is black because it does not reflect any light. An object that does not have any light falling on it will also appear black; more on that later. This applies to both daylight and artificial light. We can change the direction light falls on our subject by either moving the subject or light source. You can experiment with the effects of lighting direction by using one model and one light.
Where neither moving the subject or the light source is possible, for example when photographing a building in sunlight, we need to wait for the Earth to move. Simply visiting locations when you know the sun will be in the right place can transform your photography.
Diffused light from a source is always less intense than light coming directly from the source. When clouds pass in front of the sun the light intensity decreases, place a diffuser in front of a bare light bulb and the light is dimmer. The intensity of light is important to us as photographers as it affects the technical aspects of image capture. In bright light we can use higher shutter speeds to freeze movement and smaller apertures for greater depth of field without sacrificing quality. In dim light we may struggle with long shutter speeds causing blur and camera shake and limited depth of field due to large apertures. We can boost the sensitivity of our sensors by increasing the ISO but this often reduces the technical quality of the image. To find out more download my free Beginners Guides to Exposure Settings
Probably the most difficult aspect to describe as it depends not only on the light source but also on anything between the source and the subject. In the latter part of the 19th century many artists worked in the south of France because of the quality of the light. In parts of the Mediterranean bright sunlight passes through relatively little atmospheric pollution and bounces off the sea, sandy beaches and a light coloured landscape creating the effect of clarity. Colours appear more intense and shapes are outlined by shadows tat are filled with reflected light.
Two terms you will often hear used to describe light are hard and soft. These refer to the type of shadows created. An un-diffused light source creates hard edged shadows (in the black and white photo above) whilst a diffused source creates soft edged shadows.
The reflective qualities of your subject also effect the light quality. A glossy surface will reflect a lot of light and appear shinny with bright highlight reflections of the light source known as specular highlights. This is why every creative nude photographers tool kit should include a bottle of baby oil. A matt surface will produce almost no specular highlights. These highlights give the viewer of a two dimensional photograph important visual clues as to the three dimensional nature of the subject. Compare a photograph where there are highlights in the subjects eyes to one where they are missing.