reflection

You see yourself in a mirror, catch the glare off a lake, and notice a faint ghost of yourself in a window at night. All of these are reflection: some of the light striking a surface bounces back instead of passing through. Every boundary between two different materials reflects at least a little light, and how much it reflects, and how neatly, tells you a great deal about the surface.

The fraction of light reflected at a surface is set by the mismatch in refractive index between the two materials. For light hitting straight on, the reflectance R is roughly ((n2 minus n1) divided by (n2 plus n1)) squared. For air (n about 1) meeting glass (n about 1.5) that gives about 0.04, so a clean glass surface reflects about 4 percent of the light and transmits the rest; this small loss at every surface is why camera lenses get anti-reflection coatings. Reflection also comes in two flavors: specular reflection off a smooth surface sends parallel rays back as a clear image (a mirror), while diffuse reflection off a rough surface scatters rays every which way (a sheet of paper), which is why matte things show no reflection of your face.

Metals are the great reflectors, bouncing back 90 to 99 percent of visible light, and this is no accident: their sea of free electrons responds instantly to light's electric field and re-radiates it right back out, so the light barely penetrates. That single fact explains why metals are shiny and opaque at once, and why we silver a mirror rather than glassing it. The color of the reflection can differ too, gold and copper reflect red and yellow more strongly than blue, which is why they look warm rather than silvery white. A practical note: reflection is not lost light in the way absorption is; it is redirected, and controlling it, boosting it for mirrors and solar concentrators, suppressing it for lenses, displays, and solar cells, is a whole branch of optical engineering.