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## Class 10 Physics (India)

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

Lesson 12: Lens formula derivation (Bonus)

# Thin lens formula

Let's derive the thin lens formula which connects the object distance to the image distance. Created by Mahesh Shenoy.

## Want to join the conversation?

• how at can we say that two triangles with three equal angles are congruent to each other? as far as i know there is no congruency property for all three angles.
(1 vote) • @ Do the negatives cancel out? I thought you multiply both sides with (-1), so you'd also multiply the big M with (-1), getting a negative magnification. I thought it was kinda neat you could see from the sign in the equation that the image is flipped. • Yo,
I'm not quite sure if this is the answer to your question.(I have taken your question to be why isn't M multiplied with -1)
The negative of the general formula is not the case here as,
`M = v/u = Hi/Ho`
like how `F = m*a` (force = mass * acceleration)
we won't multiply M with -1 as M is just signifying the value if you know what I mean.
Hope this helps!
• If there would have been a concave lens instead of the convex lens during derivation, the last part (where he uses sign convention on 1/f= 1/v + 1/u) the formula would have come out different (Since the object distance and the image distance both would have been negative in that scenario, the formula would have been 1/f = -1/v - 1/u). So, does the lens formula not work for concave lenses? • in why cant we directly write it as 1/f = 1/v + 1/u as a similar thing of taking "u" as negative can happen in convex mirrors, yet we don't put the negative sign in the formulae of mirrors, so why here in lenses?
(1 vote) • I have a question from the Thin Lenses quiz that is after this video (see playlist on left).

Here is the question and solution from the quiz [spoiler]: https://i.imgur.com/odGu9Oj.png

In the solution, the second blue arrow is on the focal point of the eyepiece. Yet, the eyepiece is a converging lens and any image on the focal point will not form an image (see figure 1).

Figure 1-https://i.imgur.com/0t39bZU.png

Should the second blue arrow be towards the right of the eyepiece focal point so that a magnified image is provided? (see figure 2).

Figure 2 - https://i.imgur.com/LDU0cAB.png

I have attempted to correct this problem in figure 3.

Figure 3 - https://i.imgur.com/O5iOGe4.png

Is this correct or am I missing something?
(1 vote) • In this video, we take the focal length to be on the right side of the lense, but we could've taken the focal length on the left side as there are two foci. When it comes to derivations and formulas, do we take the focus to be in the direction of the rays?
(1 vote) • so .. wouldn't the "+" side always be opposite to whichever side the object is placed
(1 vote) • Is there a better way to explain where the minus comes from at ish? The sign convention argument comes across like a last minute rabbit in the hat to make it work. 