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

Lesson 7: Mirror formula & magnification

# Mirror formula

Let's explore the mirror formula (1/f = 1/v+1/u) and see how to locate images without drawing any ray diagrams. Created by Mahesh Shenoy.

## Want to join the conversation?

• In the previous video, Mirror formula derivation, it was said that this formula (1/f=1/v+1/u) was only for one kind of mirror and one kind of image, but in this video, it was said that it is used for all the mirrors. I am not sure which one is correct.
• If it turns out that the mirror produces a virtual image will the formula tell us that v equals an undefined value?
(1 vote)
• No. It means that whatever image is formed due to reflection is formed BEHIND the mirror. Hence, it would be virtual, erect and behind the mirror. V's value would be POSITIVE.

In a convex mirror, in both existing cases, it forms a virtual, erect and diminished image. Only U's value is negative, while V and F are positive.

Even in the last case of a concave mirror, when object is placed between the focal point and pole, it produces a virtual, erect and enlarged image behind the mirror.
• as tolkein said, "one formula to rule them all"
• are all formulas sign sensitive, and how do we know that a formula is sign sensitive
(1 vote)
• As far as I know, there are majorly 4 formulae in light: the mirror formula, lens formula and magnification.

All three are sign sensitive as in a mirror and lens, each side has a different denotation.

Even the power of lens formula is sign sensitive, as it is calculated using focal length of a lens.

The magnification also is composed of V and U, or height of image and object, all sign sensitive.

So, any formula having image height, object height, U, V and F, which are all sign sensitive, would automatically become a sign sensitive one.