The Skinny on Lenses
+si = −m = inverted image
= real image
−si = +m = upright image
= virtual image
Converging lenses produce
both real and virtual
images, depending on the
focal length and object’s
distance = +f
Diverging lenses only produce
virtual images = −f
(Perhaps this looks
familiar: It’s essentially
the same as when we
discussed mirrors earlier.)
Questions 27-30
An object of height 10 cm is placed 40 cm in front of a converging
lens with a focal length of 20 cm.
- Where’s the image?
- Is it real or virtual?
- Is it upright or inverted?
- What’s the height of the image?
Here’s How to Crack It
- With so = 40 cm and f = 20 cm, the lens equation gives us
