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Have you ever wondered how a camera
works? Or why you need a flashlight to see in the dark?
Or how a magnifying glass can make things look bigger?
Or why a rainbow looks so colorful?
There are many devices that use the
visible electromagnetic waves we call light. Some of
these devices create light, some detect light, and
others manipulate the beams of light for some use.
Light passes through some materials
Light easily passes through air and
glass. Such materials are called transparent.
There are some materials that only allow
some of the light to pass through them. If only a
percentage of the light can pass through, the material
is called translucent.
Some materials only allow selected
wavelengths to pass through. Colored glass may block
some colors, while allowing only one or two specific
wavelengths to pass through it.
It is obvious that it doesn't pass
through materials like steel or wood.
The ray model of light
Light normally travels in a straight
line. Some useful properties of light can be studied by
considering light as a single ray or straight line:
reflection, refraction and dispersion
When incoming light reflects off an
object, the incident ray has the same angle as the
reflected ray, when measured from a perpendicular from
the point of reflection. The two angles are equal, and
this is called the Law of Reflection.
For example, when light hits a flat
mirror, the angle of reflection is the same as the
incoming angle.
Light rays are bent, or refracted, at
the boundary when passing from one transparent media to
another. For example, you have seen how light is bent
when it passes from water to air.
The amount of refraction depends on the
incident angle and the index of refraction of the
material. The index of refraction is defined as the
ratio of the speed of light in a vacuum to the speed of
light in the media.
In certain situations, when the
refracted angle is 90 degrees to the incident angle
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.
When visible light passes through a
glass prism, the light is broken into its colors. This
occurs because the index of refraction is different for
each color, with short wavelengths refracted more than
larger ones. Thus the different colors are bent at
gradually increasing angles, and the colors are spread
apart.
This property of separating a beam of
white light into a spectrum is called dispersion.
Light passing through a glass wedge or
prism will not only be bent by refraction, but will also
be dispersed into its spectrum of colors. In studying
light, Isaac Newton used a prism to discover that white
light was composed of the spectrum of colors.
The spectroscope is a device that is
used to determine the elements present in a luminous
object or gas. By examining the spectrum produced when
the emitted light is passed through a prism you can
determine the elements creating the light. Each element
has its own characteristic location and color in the
spectrum. With the spectroscope, light coming from our
sun and other stars has been investigated to determine
the elements present.
Lenses
The ray model of light is very useful in
explaining lenses.
A lens is a piece of glass or any other
transparent material with two curved surfaces, or with
one curved and one flat surface. A convex lens is a lens
that is thicker in the middle than at its edges.
Refraction through such a lens causes parallel light
rays to converge (meet) at a point called the principal
focus. The lens of your eye is a double convex lens that
focuses an image on the retina of the eye.
If a person is farsighted, the eyeball
is too short from front to back, and the point of focus
will fall behind the retina. Such people can see only
distant objects clearly. Glasses with convex lenses are
used to correct farsightedness.
A magnifying glass is a double convex
lens that behaves like two prisms connected together at
their bases. It can concentrate light rays from the sun
to such a degree that they can bum a hole in paper or
cause a fire.
A convex lens has a focal point. Light
coming from that point is refracted by the lens to
become parallel or vice versa. An object that is beyond
the focal point of a lens will be focused upside-down on
the other side of the lens.
Both a camera and the eye have a convex
lens which focuses the image upside-down on its
detector. The camera uses film to detect the image,
while the eye has its retina to detect the image.
A concave lens is thinner in the middle
than at its edges and causes light rays to diverge
(spread apart). If a person's eyeball is too long from
front to back, the point of focus will be in front of
the retina and the person will be nearsighted. Such
people can see only close objects clearly. Concave
lenses are used to correct near-sightedness.
The ordinary light microscope is used to
investigate tiny structures such as cells and bacteria
that would be otherwise invisible to the unaided eye.
The telescope also makes use of lenses to bring distant
objects into view.
Special characteristics of light
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.
Diffraction is the bending of light
around the edge of an object or spreading of light in an
arc after passing through a tiny opening.
Did you ever see waves hit a breakwater
and then sort of go around the obstacle at the edges?
The same thing happens with light at edges.
By looking at the tiny grooves in a
phonograph record, you can see another example of
diffraction. Look at a sharp angle, and you should see
how light is spread into various colors by the
diffraction.
Since light is a wave motion, it is
possible that if the waves are out of phase with each
other, that they could cancel out each other. Another
example is that is waves are of slightly different
wavelength, they could cause "beat frequencies." This is
similar to hold two tuning forks that are slightly
different near each other. They produce a throbbing
sound or slow beats.
Interference occurs when light passes
through two small slits or holes and produces an
interference pattern of bright lines and dark zones.
Oil slicks on a wet pavement are caused
by the interference of the light passing through and
reflecting off the ultra-thin oil film.
Polarized light is light that is allowed
to vibrate in only one direction. This is similar to
trying to vibrate a rope that passes through a picket
fence: the rope can only vibrate is a direction parallel
to the pickets.
Polarized light vibrates in one
direction only, in a plane. Certain materials can
polarize light, by means of reflection, or by
scattering. Only the transverse wave model can explain
polarization.
Polaroid sunglasses are used to cut down
glare. They filter out light that is reflected off
surfaces, but allow the other to pass through.
You have seen the interesting
characteristics of light and now understand the
fascinating things light can do |
