


Optical Instruments
The reflection and refraction we’ve dealt with so far
have focused only on light interacting with flat surfaces. Lenses
and curved mirrors are optical instruments designed to focus light
in predictable ways. While light striking a curved surface is more
complicated than the flat surfaces we’ve looked at already, the
principle is the same. Any given light ray only strikes an infinitesimally
small portion of the lens or mirror, and this small portion taken by
itself is roughly flat. As a result, we can still think of the normal
as the line perpendicular to the tangent plane.
The four basic kinds of optical instruments—the only instruments
that will be tested on SAT II Physics—are concave mirrors, convex
mirrors, convex (or converging) lenses, and concave (or diverging)
lenses. If you have trouble remembering the difference between concave
and convex, remember that, like caves, concave mirrors and lenses
curve inward. Convex lenses and mirrors bulge outward.
General Features of Mirrors and Lenses
Much of the vocabulary we deal with is the same for all
four kinds of optical instruments. Before we look at the peculiarities
of each, let’s look at some of the features they all share in common.
The diagram above shows a “ray tracing” image of a concave
mirror, showing how a sample ray of light bounces off it. Though
we will take this image as an example, the same principles and vocabulary
apply to convex mirrors and to lenses as well.
The principal axis of a mirror or lens is
a normal that typically runs through the center of the mirror or
lens. The vertex, represented by V in
the diagram, is the point where the principal axis intersects the
mirror or lens.
The only kind of curved mirrors that appear on SAT II
Physics are spherical mirrors, meaning they look like someone sliced
off a piece of a sphere. Spherical mirrors have a center of
curvature, represented by C in
the diagram, which is the center of the sphere of which they are
a slice. The radius of that sphere is called the radius of
curvature, R.
All rays of light that run parallel to the principal axis
will be reflected—or refracted in the case of lenses—through the
same point, called the focal point, and denoted by F on
the diagram. Conversely, a ray of light that passes through the
focal point will be reflected parallel to the principal axis. The focal
length, f, is
defined as the distance between the vertex and the focal point.
For spherical mirrors, the focal length is half the radius of curvature, f
= R/2.
Concave Mirrors
Suppose a boy of height h stands
at a distance d in front of a concave
mirror. By tracing the light rays that come from the top of his
head, we can see that his reflection would be at a distance from the mirror and it would have a height . As anyone who has looked into a spoon
will have guessed, the image appears upside down.
The image at is a real image:
as we can see from the ray diagram, the image is formed by actual
rays of light. That means that, if you were to hold up a screen
at position , the image of the boy would be projected
onto it. You may have noticed the way that the concave side of a
spoon can cast light as you turn it at certain angles. That’s because
concave mirrors project real images.
You’ll notice, though, that we were able to create a real
image only by placing the boy behind the focal point of the mirror.
What happens if he stands in front of the focal point?
The lines of the ray diagram do not converge at any point
in front of the mirror, which means that no real image is formed:
a concave mirror can only project real images of objects that are
behind its focal point. However, we can trace the diverging lines
back behind the mirror to determine the position and size of a virtual
image. Like an ordinary flat mirror, the image appears to
be standing behind the mirror, but no light is focused on that point
behind the mirror. With mirrors generally, an image is real if it
is in front of the mirror and virtual if it is behind the mirror.
The virtual image is right side up, at a distance from the vertex, and stands at a height .
You can test all this yourself with the right kind of
spoon. As you hold it at a distance from your face, you see your
reflection upside down. As you slowly bring it closer, the upsidedown
reflection becomes blurred and a much larger reflection of yourself
emerges, this time right side up. The image changes from upside
down to right side up as your face crosses the spoon’s focal point.
Convex Mirrors
The focal point of a convex mirror is behind the mirror,
so light parallel to the principal axis is reflected away from the
focal point. Similarly, light moving toward the focal point is reflected
parallel to the principal axis. The result is a virtual, upright
image, between the mirror and the focal point.
You’ve experienced the virtual image projected
by a convex mirror if you’ve ever looked into a polished doorknob.
Put your face close to the knob and the image is grotesquely enlarged,
but as you draw your face away, the size of the image diminishes
rapidly.
The Two Equations for Mirrors and Lenses
So far we’ve talked about whether images are real or virtual,
upright or upside down. We’ve also talked about images in terms
of a focal length f, distances d and , and heights h and . There are two formulas that relate these
variables to one another, and that, when used properly, can tell
whether an image is real or virtual, upright or upside down, without our
having to draw any ray diagrams. These two formulas are all the
math you’ll need to know for problems dealing with mirrors and lenses.
First Equation: Focal Length
The first equation relates focal length, distance of an
object, and distance of an image:
Values of d, , and f are
positive if they are in front of the mirror and negative if they
are behind the mirror. An object can’t be reflected unless it’s
in front of a mirror, so d will always
be positive. However, as we’ve seen, f is
negative with convex mirrors, and is negative with convex mirrors and with
concave mirrors where the object is closer to the mirror than the
focal point. A negative value of signifies a virtual image, while a positive value
of signifies a real image.
Note that a normal, flat mirror is effectively a convex
mirror whose focal point is an infinite distance from the mirror,
since the light rays never converge. Setting 1/f = 0,
we get the expected result that the virtual image is the same distance
behind the mirror as the real image is in front.
Second Equation: Magnification
The second equation tells us about the magnification, m,
of an image:
Values of are positive
if the image is upright and negative if the image is upside down. The
value of m will always be positive because the
object itself is always upright.
The magnification tells us how large the image is with
respect to the object: if , then the image is larger;
if , the image is smaller; and if m = 1,
as is the case in an ordinary flat mirror, the image is the same
size as the object.
Because rays move in straight lines, the closer an image
is to the mirror, the larger that image will appear. Note that will have a positive value with virtual
images and a negative value with real images. Accordingly, the image
appears upright with virtual images where m is
positive, and the image appears upside down with real images where m is
negative.
Example

How far from the mirror should she set up a screen
in order for her image to be projected onto it?
The question tells us that d = 40 cm
and f = 30 cm. We can
simply plug these numbers into the first of the two equations and
solve for , the distance of the image from the mirror:
Because is a positive number,
we know that the image will be real. Of course, we could also have
inferred this from the fact that the woman sets up a screen onto
which to project the image.
How tall will her image be on the screen?
We know that d = 40 cm,
and we now know that = 120 cm,
so we can plug these two values into the magnification equation
and solve for m:
The image will be three times the height of the woman,
or m tall. Because the value of m is
negative, we know that the image will be real, and projected upside
down.
Convex Lenses
Lenses behave much like mirrors, except they use the principle
of refraction, not reflection, to manipulate light. You can still
apply the two equations above, but this difference between mirrors
and lenses means that the values of and f for
lenses are positive for distances behind the lens and negative for
distances in front of the lens. As you might expect, d is
still always positive.
Because lenses—both concave and convex—rely on refraction
to focus light, the principle of dispersion tells us that there
is a natural limit to how accurately the lens can focus light. For
example, if you design the curvature of a convex lens so that red
light is focused perfectly into the focal point, then violet light
won’t be as accurately focused, since it refracts differently.
A convex lens is typically made of transparent
material with a bulge in the center. Convex lenses are designed
to focus light into the focal point. Because they focus light into
a single point, they are sometimes called “converging” lenses. All
the terminology regarding lenses is the same as the terminology
we discussed with regard to mirrors—the lens has a vertex, a principal
axis, a focal point, and so on.
Convex lenses differ from concave mirrors in
that their focal point lies on the opposite side of the lens from
the object. However, for a lens, this means that f > 0, so
the two equations discussed earlier apply to both mirrors and lenses.
Note also that a ray of light that passes through the vertex of
a lens passes straight through without being refracted at an angle.
In this diagram, the boy is standing far enough from the
lens that d > f. As we can see, the image
is real and on the opposite side of the lens, meaning that is positive. Consequently, the image appears
upside down, so and m are
negative. If the boy were now to step forward so that d
< f, the image would change dramatically:
Now the image is virtual and behind the boy on the same
side of the lens, meaning that is negative. Consequently,
the image appears upright, so and m are
positive.
Concave Lenses
A concave lens is designed to divert light
away from the focal point, as in the diagram. For this reason, it
is often called a “diverging” lens. As with the convex lens, light
passing through the vertex does not bend. Note that since the focal
point F is on the same side of the
lens as the object, we say the focal length f is
negative.
As the diagram shows us, and as the two equations for
lenses and mirrors will confirm, the image is virtual, appears on
the same side of the lens as the boy does, and stands upright. This
means that is negative and
that and m are
positive. Note that h > , so m <
1.
Summary
There’s a lot of information to absorb about mirrors and
lenses, and remembering which rules apply to which kinds of mirrors
and lenses can be quite difficult. However, this information is
all very systematic, so once you grasp the big picture, it’s quite
easy to sort out the details. In summary, we’ll list three things
that may help you grasp the big picture:
 Learn to draw ray diagrams: Look over the diagrams of the four kinds of optical instruments and practice drawing them yourself. Remember that light refracts through lenses and reflects off mirrors. And remember that convex lenses and concave mirrors focus light to a point, while concave lenses and convex mirrors cause light to diverge away from a point.
 Memorize the two fundamental equations: You can walk into SAT II Physics knowing only the two equations for lenses and mirrors and still get a perfect score on the optical instruments questions, so long as you know how to apply these equations. Remember that f is positive for concave mirrors and convex lenses, and negative for convex mirrors and concave lenses.
 Memorize this table: Because we love you, we’ve put together a handy table that summarizes everything we’ve covered in this section of the text.
Optical Instrument  Value of d ´  Real or virtual?  Value of f  Upright or upside down?  

Mirrors (and f are positive in front of mirror)  Concave d > f  +  Real  +  Upside down 
Concave d < f  –  Virtual  +  Upright  
Convex  –  Virtual  –  Upright  
Lenses (and f are positive on far side of lens)  Convex d > f  +  Real  +  Upside down 
Convex d < f  –  Virtual  +  Upright  
Concave  –  Virtual  –  Upright 
Note that when is positive, the image
is always real and upside down, and when is negative, the image is always virtual
and upright.
SAT II Physics questions on optical instruments are generally
of two kinds. Either there will be a quantitative question that
will expect you to apply one of the two equations we’ve learned,
or there will be a qualitative question asking you to determine
where light gets focused, whether an image is real or virtual, upright
or upside down, etc.
