31.2 - Light rays
Light ray: A straight line used to represent the
path of light.
Light rays are used to analyze devices such as mirrors and lenses. Light emanating
from a point in a particular direction is treated as moving in a straight line. You can think
of this as modeling light with a collection of laser beams, each ray moving in a straight
and narrow line until it intersects a surface.
You can see an example of light rays depicted on the right, where a penguin views
himself in a mirror. Several rays are shown emanating from a point on the penguin’s
foot. We show the full path of a single ray to illustrate how the penguin can see his foot
reflected in the mirror. The light ray starts at the foot, reflects at the mirror and then
reaches his eye. The arrowheads indicate the direction of the light’s travel.
The penguin’s feet are not the source of the light. That might be the Sun or an electric
lamp. But for the purposes of analyzing the mirror, we will proceed as though the light
rays originate from the object whose image we intend to analyze.
Light is a complex phenomenon. Treating it as propagating in a straight line does not explain why a beam from a flashlight spreads out as it
travels; other concepts are needed there. But to understand mirrors and lenses, light rays prove very useful.
Light rays
Represent light with straight lines
Used to analyze mirrors, lenses31.3 - Mirror basics
At the right, you see a penguin looking at himself in a planar mirror. A planar mirror is a
flat, smooth surface that reflects light. A typical planar mirror is manufactured by coating
one side of a sheet of glass with a metallic film. It is designed to reflect as much light as
possible and to create distinct images. Either by looking at yourself in a planar mirror or
referring to the illustration in Concept 1, you can observe four essential properties of an
image produced by this kind of mirror:
- Your image is the same size as you are. The variable h is used to represent its
height. - Your image appears as far behind the mirror as you are in front of it. Your
distance is called the object distance, and how far the image is behind the mirror
is called the image distance. The object distance, represented by do, is positive
by convention. When an image is on the opposite side of a mirror from the object,
its distance di from the mirror is negative by convention. - The image has front-back reversal. If you are facing north, then your mirror image
is facing south. If this reversal did not occur, you would see your back. - The image is not reversed either right-left or up-down. If you look in a mirror and
raise your left hand, its reflection will rise up on your left. Your head also still
appears on the top of the image. The image is not inverted.
Mirrors do create the illusion of left-right reversal. For instance, if you raise your left hand, it will appear on your left but on the image’s “right,”
because your image is facing the opposite direction than you are. This is akin to “stage left” or “stage right” directions for actors, which are
based on the perception of what is left or right for the audience, not for the actors themselves as they face the audience. Their “left” and “right”
are reversed because they are facing in the opposite direction than the audience, just as your mirror image is.
You cannot easily read normal writing in a mirror (unless you are as gifted as Leonardo da Vinci, who wrote in “mirror writing” so that others
could not easily decipher his work). Mirror writing does have its uses: An emergency vehicle may have “ ” written on its front so that
the text can be readily read when seen in the rearview mirror of a car.
Planar mirror object and image
Same height
Equally far from mirror
Image is right side up