One of my teachers said rather simplistically: "We need light to see". That could not be more true. Not only do we need it to see, it affects what we see and how we see it. Look at the example photo found here and then mouse over it to see it photographed at a different time of day.

One one hand, here you see a building photographed in the early morning. You can see the quality of the light from the sun is strong, casting dark, directional shadows that are sharp with a clear outline.

The night time image has a different source of light than the sun. It has a different color and cast than the day time picture. Its shadows are also different due to the different light sources.

Light also affects different surfaces in distinct ways, like the way the water in this image reflects like a mirror, the grass and foliage absorbs the light and reflects back the color green and is softer than the hard surfaces of the building, etc.

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