Electromagnetic radiation is emitted from all matter with a temperature
above absolute zero, and as the temperature increases, more radiation
and shorter wavelengths are emitted. Visible light is emitted from matter
hotter than about 700°C, and this matter is said to be incandescent. The
Sun, a fire, and the ordinary lightbulb are incandescent sources of light.
The behavior of light is shown by a light ray model that uses straight
lines to show the straight-line path of light. Light that interacts with matter
is reflected with parallel rays,moves in random directions by diffuse reflection
from points, or is absorbed, resulting in a temperature increase.
Matter is opaque, reflecting light, or transparent, transmitting light.
In reflection, the incoming light, or incident ray, has the same angle
as the reflected ray when measured from a perpendicular from the point
of reflection, called the normal. That the two angles are equal is called the
law of reflection. The law of reflection explains how a flat mirror forms a
virtual image, one from which light rays do not originate. Light rays do
originate from the other kind of image, a real image.
Light rays are bent, or refracted, at the boundary when passing from
one transparent media to another. The amount of refraction depends on
the incident angle and the index of refraction, a ratio of the speed of light
in a vacuum to the speed of light in the media. When the refracted angle
is 90°, total internal reflection takes place. This limit to the angle of incidence
is called the critical angle, and all light rays with an incident angle
at or beyond this angle are reflected internally.
Each color of light has a range of wavelengths that forms the spectrum from
red to violet. A glass prism has the property of dispersion, separating
a beam of white light into a spectrum. Dispersion occurs because
the index of refraction is different for each range of colors, with short
wavelengths refracted more than larger ones.
A wave model of light can be used to explain diffraction, interference,
and polarization, all of which provide strong evidence for the wavelike
nature of light. Interference occurs when light passes through two
small slits or holes and produces an interference pattern of bright lines
and dark zones. Polarized light vibrates in one direction only, in a plane.
Light can be polarized by certain materials, by reflection, or by scattering.
Polarization can only be explained by a transverse wave model.
A wave model fails to explain observations of light behaviors in the
photoelectric effect and blackbody radiation. Max Planck found that he
could modify the wave theory to explain blackbody radiation by assuming
that vibrating molecules could only have discrete amounts, or quanta,
of energy and found that the quantized energy is related to the frequency
and a constant known today as Planck's constant. Albert Einstein applied
Planck's quantum concept to the photoelectric effect and described a
light wave in terms of quanta of energy called photons. Each photon has
an energy that is related to the frequency and Planck's constant.
Today, the properties of light are explained by a model that incorporates
both the wave and the particle nature of light. Light is considered
to have both wave and particle properties and is not describable in terms
of anything known in the everyday-sized world.
