The thin lens equation is stated as follows: where
-
do is the distance (measured along the axis) from the object to the center of the lens
-
di is the distance (measured along the axis) from the image to the center of the lens
-
f is the focal length of the lens
The expression 1/f in called the power of a lens. It is measured in Diopters, where 1 D = 1 m-1.
When using this equation, signs are very important:
|
do |
positive
|
when the object is placed "in front of the lens"
|
|
|
|
|
|
di |
positive
|
when real images are formed (inverted, "behind the lens")
|
|
di |
negative
|
when virtual images are formed (upright, "in front of the lens")
|
|
|
|
|
|
f
|
positive
|
when the lens is converging |
|
f
|
negative
|
when the lens is diverging |
Remember that do, di, and f must be measured in the same unit - usually meters is preferred.
The following formula is used to calculate the magnification of an image:
If a problem states that a real image is formed that is twice as large as an object, then you would use the relationship di = +2do in the thin lens equation. If a problem states that a virtual image is formed that is twice as large as the object, then you would use the relationship that di = −2do.
|