Materials that are not completely transparent either absorb light or reflect it. In
absorbing materials, such as dark colored cloth, the energy of the oscillating electrons
does not go back to the light. The energy instead goes toward increasing the motion of
the atoms, which causes the material to heat up. The atoms in reflective materials,
such as metals, re-radiate light that cancels out the original wave. Only the light
re-radiated back out of the material is observed. All materials exhibit some degree of
absorption, refraction, and reflection of light. The study of the behavior of light in
materials and how to use this behavior to control light is called optics.
Refraction is the bending of light when it passes from one kind of material into
another. Because light travels at a different speed in different materials, it must change
speeds at the boundary between two materials. If a beam of light hits this boundary at
an angle, then light on the side of the beam that hits first will be forced to slow down or
speed up before light on the other side hits the new material. This makes the beam
bend, or refract, at the boundary. Light bouncing off an object underwater, for instance,
travels first through the water and then through the air to reach an observer's eye. From
certain angles an object that is partially submerged appears bent where it enters the
water because light from the part underwater is being refracted.The refractive index of a
material is the ratio of the speed of light in empty space to the speed of light inside the
material. Because light of different frequencies travels at different speeds in a material,
the refractive index is different for different frequencies. This means that light of different
colors is bent by different angles as it passes from one material into another. This effect
produces the familiar colorful spectrum seen when sunlight passes through a glass
prism. The angle of bending at a boundary between two transparent materials is related
to the refractive indexes of the materials through Snell's Law, a mathematical formula
that is used to design lenses and other optical devices to control light.
Reflection also occurs when light hits the boundary between two materials. Some
of the light hitting the boundary will be reflected into the first material. If light strikes the
boundary at an angle, the light is reflected at the same angle, similar to the way balls
bounce when they hit the floor. Light that is reflected from a flat boundary, such as the
boundary between air and a smooth lake, will form a mirror image. Light reflected from a
curved surface may be focused into a point, a line, or onto an area, depending on the
curvature of the surface.
Scattering occurs when the atoms of a transparent material are not smoothly
distributed over distances greater than the length of a light wave, but are bunched up
into lumps of molecules or particles. The sky is bright because molecules and particles
in the air scatter sunlight. Light with higher frequencies and shorter wavelengths is
scattered more than light with lower frequencies and longer wavelengths. The
atmosphere scatters violet light the most, but human eyes do not see this color, or
frequency, well. The eye responds well to blue, though, which is the next most
scattered color. Sunsets look red because when the sun is at the horizon, sunlight has
to travel through a longer distance of atmosphere to reach the eye. The thick layer of
air, dust and haze scatters away much of the blue. The spectrum of light scattered
from small impurities within materials carries important information about the
impurities. Scientists measure light scattered by the atmospheres of other planets in
the solar system to learn about the chemical composition of the atmospheres.

The first successful theory of light wave motion in three dimensions was
proposed by the Dutch scientist Christiaan Huygens in 1678. Huygens suggested that
light wave peaks form surfaces like the layers of an onion. In a vacuum, or a uniform
material, the surfaces are spherical. These wave surfaces advance, or spread out,
through space at the speed of light. Huygens also suggested that each point on a
wave surface can act like a new source of smaller spherical waves, which may be
called wavelets, that are in step with the wave at that point. The envelope of all the
wavelets is a wave surface. An envelope is a curve or surface that touches a whole
family of other curves or surfaces like the wavelets. This construction explains