# Mirror/Lens crap

This forum made possible through the generous support of SDN members, donors, and sponsors. Thank you.

#### laczlacylaci

##### Full Member
7+ Year Member
! is image
concave mirror=converging mirror --!--)----
inverted, smaller image, behind mirror

convex mirror=diverging mirror --!--(-----
upright, smaller image, in front

concave lens=diverging lens=image is smaller, upright, behind mirror

convex lens=converging lens=image is smaller, inverted, in front of mirror

Why the f did they switch around the names. this is so confusing, any mnemonics?

It seems like the image is always smaller?
converging always produces in front of mirror/lens
diverging always produces behind mirror/lens.

This is one of the most confusing MCAT topics IMO. Here's my approach that hopefully makes sense and requires the least amount of memorization.

1. Forget ray diagrams and rules like "converging always produces ___". The items below will take care of that.

2. Know the thin-lens equation: 1 / i + 1/o = 1/f and magnification equation: m = - i / o. Note: "o" is always a positive number and we usually are given f, and solve for i.

3. Conceptually, light bounces off of mirrors and reflects back, while light passes through lenses to the other side.

4. Concave mirrors and convex lenses are converging. For these f is positive.

5. Convex mirrors and concave lenses are diverging. For these f is negative.

Using #2, #4, and #5, the equation should always work out to figure out what side the image gets put on and whether it's inverted or upright. Let me know if this makes sense!

1 user
Yes it does! When do we create larger images?
+f=real =converging
-f=virtual=diverging
correct?

Members don't see this ad :)
larger or smaller images depend on the magnification term m = - i / o. Larger would be when m > 0 and smaller is when m < 0.

+f = converging. This means image on the same side for mirrors and on the opposite side of lenses (real).

-f = diverging. This means images on the opposite site for mirrors and on the same side for lenses (virtual).

1 user
From a conceptual standpoint for naming converging and diverging - imagine what happens to parallel light rays as they approach a mirror or lens. If the rays strike the surface and move away from each other, this is a diverging lens/mirror. If the light rays move closer to one another - this is a converging lens/mirror.