Introduction to SAT II Physics

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:

1. 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.


2. 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.


3. 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.