# I'm having a lot of trouble with ray diagrams (lenses). what's the easiest way to understand them

#### mrh125

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i understand the whole convex lenses and concave mirrors have the same properties and are both converging (positive focal length) while concave lenses and convex mirrors are both diverging (negative focal length) and that real and inverted images are always associated with each other and usually virtual and upright ones are. I have a lot of trouble with diagrams and stuff like seeing what happens to the image at the focal point, beyond the focal point and determining whether or not the images are magnified or what? How is this easily done? is there something simple that I could memorize so i dont have to draw the things out?

#### Jepstein30

honestly, this is one of the few topics I recommend just memorizing the rules. It mostly shows up as a discrete with a very common answer structure.. so you can get away with just knowing the rules/tricks and not fully understanding ray diagrams. Of course, this can backfire if you get an optics passage but even then, you should be okay.

converging:
1) the object is located beyond the radius of curvature (R)- SIR (smaller, inverted, real)
2) the object is located at R- SSIR (same size, inverted, real)
3) the object is located between R and F- LIR (larger, inverted, real)
4) the object is located at F- no image is formed
5) the object is located between F and the mirror- LUV (larger, upright, virtual)

diverging:
always SUV (smaller, upright, virtual)

from there, you just need to know the sign conventions so you can figure out which side of the mirror/lens the object/image is on, etc. You can also relate the size relationship to the image location (larger = farther out, etc.).

That really should take care of 90% of optics questions and is a quick memorization. It's best to memorize as little as possible but here is a situation where I'd say go for it.

mrh125

#### BerkReviewTeach

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Jepstein hit the nail on the head. To see that list with pictures tossed in, assuming you have access to BR books, look at pages 247-250. It's that exact list with the ray diagrams (which are obsolete if you use the chart).

mrh125

#### Jepstein30

Jepstein hit the nail on the head. To see that list with pictures tossed in, assuming you have access to BR books, look at pages 247-250. It's that exact list with the ray diagrams (which are obsolete if you use the chart).
exactly where I picked it up from originally

makes it easy.

Diverging SUV
Converging SIR -> LUV