Cracking the SAT Physics Subject Test

Be Positive

We will always treat the

object’s position as being

the positive side. For

mirrors, if the image is

produced on the same

side as the object, it is

considered a positive

distance. If an image is

produced on the opposite

side, it is considered a

negative distance.

Concave mirrors will

always have a positive

focal length (meaning the

focal point is on the same

side as the object).

Convex mirrors will always

have a negative focal

length (meaning the focal

point is on the opposite

side of the object).

The first equation, called the mirror equation, is

where so is the object’s distance from the mirror, si is the image’s

distance from the mirror, and f is the focal length of the mirror.

The value of so is always positive for a real object, but si can be positive or

negative. The sign of si tells us whether the image is real or virtual: If si is positive,

the image is real; and if si is negative, the image is virtual.

The second equation is called the magnification equation

This gives the magnification; the height of the image, hi, is |m| times the height of the

object, ho. If m is positive, then the image is upright relative to the object; if m is