Reflection and transparency

€‹Taking a photograph is an act of catching rays of light, exactly as they are at a certain moment. Here, the light comes from the sun and goes straight through the window. The window is see-through, or transparent. It lets most of the visible light through, and you can see right through. Glass and water are transparent.

Some kinds of plastics are too. Even minerals can be transparent, like crystal - a see-through piece of rock. But most materials are not transparent. Wood, concrete, stone, and steel are all completely non-transparent, or opaque materials. Then there are materials that are partially transparent, like frosted glass and a few plastics.

The rays of light pass through them, but not straight through. Such materials are sometimes called translucent. They let light through, but you can't see through them. When light hits a non-transparent, opaque material, some of the light is absorbed and some is reflected. In fact, everything you see, except when you look right at a light source, is reflected light.

Most surfaces are a tiny bit uneven. Even if they feel smooth when you touch them, they are actually rough enough for the light to bounce off them in every possible direction. This is called diffuse reflection. An entirely smooth surface on the other hand - a calm lake in the morning, a piece of polished metal, or a mirror - reflects light regularly. This is called a specular reflection.

Look at a specular reflection and you'll see an image of the light that hit the smooth surface - a mirror image. Your image in a mirror appears to be as far into the mirror as you are standing away from it. When a beam of light hits a mirror at an angle, it bounces off it at the same angle. Draw a line perpendicular to the mirror. We call this line, the normal.

Now, measure the angle between the incoming ray of light - the angle of incidence - and the angle between the outgoing ray of light and the normal - the angle of reflection - and you'll notice that it's the same angle. The angle of incidence is equal to the angle of reflection. €‹When you take a photograph, it usually looks best when you use a diffuse light source. This could be either reflected light or light that has passed through a translucent material. Let the sunlight reflect off a white wall and you'll get a nice picture without dark shadows. Or tape some translucent plastic film over the window to obtain cozy diffuse light and avoid curious glances.