A higher frequency—and thus a shorter wavelength—corresponds to a wave with more
energy. Though all waves travel at the same speed, those with a higher frequency oscillate
faster, and a wave’s oscillations are associated with its energy.
Visible light is the part of the electromagnetic spectrum between roughly 400 and 700
nanometers ( 1 nm = m). When EM waves with these wavelengths—emitted by the
sun, light bulbs, and television screens, among other things—strike the retina at the back
of our eye, the retina sends an electrical signal to our brain that we perceive as color.
Classical Optics
“Classical” optics refers to those facts about optics that were known before the adoption
of the wave model of light in the nineteenth century. In Newton’s time, light was studied
as if it had only particle properties—it moves in a straight line, rebounds off objects it
bumps into, and passes through objects that offer minimal resistance. While this
approximation of light as a particle can’t explain some of the phenomena we will look at
later in this chapter, it’s perfectly adequate for dealing with most commonplace
phenomena, and will serve as the basis for our examination of mirrors and lenses.
Reflection
When people think reflection, they generally think of mirrors. However, everything that
we see is capable of reflecting light: if an object couldn’t reflect light, we wouldn’t be able
to see it. Mirrors do present a special case, however. Most objects absorb some light,
reflecting back only certain frequencies, which explains why certain objects are of certain
colors. Further, most objects have a rough surface—even paper is very rough on a
molecular level—and so the light reflected off them deflects in all different directions.
Mirrors are so smooth that they reflect all the light that strikes them in a very predictable
and convenient way.
We call the ray of light that strikes a reflective surface an incident ray, and the ray that
bounces back a reflected ray. The angle of incidence, , is the angle between the
normal—the line perpendicular to the reflective surface—and the incident ray. Similarly,
the angle of reflection, , is the angle between the normal and the reflected ray.