The Lens (and Mirror) Formula
Rules to follow when using the Lens/Mirror Equation:
1. do is always positive
2. di is positive if the image is real
di is negative if the image is virtual.
3. ho is always positive
4. hi is positive if the image is upright.
hi is negative if the image is inverted.
5. f is positive if refracted light rays can pass through the focus (convex
lens)
f is negative if extensions of refracted light pass through the focus (concave lens)
In General: + means real/upright
- means virtual/inverted.
When M + means virtual and upright
M - means real and inverted
Lens Problems
1) Complete the following table for lenses. These problems required
calculations and should be solved on another piece of paper.
Lens
Convex
f (cm) do (cm)
20
25
Concave -20
di (cm)
M
25
Real/Virtual
Real
Orientation
Virtual
20
20
15
-10
10
-1
-0.5
Real
Inverted
2) Find the image distance (di) of a candle 5.0 cm tall situated at the
following positions in front of a converging lens of focal length 15 cm
(include image characteristics) using the lens equation.
a. 30 cm →
b. 25 cm →
c. 15 cm →
d. 10 cm →
3) A lens
a.
b.
c.
has a magnification of +2.0. If the focal length is +10 cm find:
The image position
The object position
The type of lens
4) A magnifying glass consists of a convex (converging) lens with the object
placed in front of the focal point. What are the properties (size, attitude,
type) of the image formed by a magnifying glass?