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## Class 10 Physics (India)

### Course: Class 10 Physics (India)>Unit 1

Lesson 13: Lens formula & magnification

# Lens formula

Let's see how to use lens formula (1/v-1/u= 1/f) to locate images without having to draw ray diagrams.  Created by Mahesh Shenoy.

## Want to join the conversation?

• why is the mirror formula and the lens formula totally different and just opposite of each other's formula and even the magnification formula. pls clarify
• Yo,
check out
https://youtu.be/PgQJCm_sXVY
mirror formula derivation
https://youtu.be/x9F0AQCGkbc
lens formula derivation
After you see those 2 videos (by khan academy)
I am sure you would understand,
But the main reason (spoiler to those videos) to the difference is that
when we derive the formula,
we shall obtain the following
Concave mirror - along with sign convention
`v = -ve, f = -ve, u = -ve`
1/-f = 1/-u + 1/-v
-(1/f) = -(1/v + 1/u)
1/f = 1/u + 1/v

Convex lens - along with sign convention
`v = +ve, f = +ve, u = -ve`
1/f = 1/v + 1/u
1/f = 1/v + 1/-u
1/f = 1/v - 1/u

We apply sign convention to make the equation obtained by similarity of triangles to make it general

as the signs for f and v are opposite with respect to concave mirror and convex lens the difference arises
Now try out for the magnification formula as well
Hope this helps,
If I'm wrong do let me now
Ciao for now
• But in our school,they don't teach us with a minus before the 1/u, it's basically just like the mirror formula. I'm kinda confused.
(1 vote)
• I am little confused while using sign convention for focal length of lenses .
Let say focal length is 6 cm, since we have f1, f2; which sign convention will be used for this focal length.(-ve or +ve).
(1 vote)
• look at where the image is forming
if the image is on the other side of the lens it is positive
(1 vote)
• *It has been taught to us before...*