What is Object Distance

Distance of Object from Optical Centre of the lens is called Object Distance

It is denoted by u

What is Image Distance

Distance of Image from Optical Centre of the lens is called Object Distance

It is denoted by v.

What is Focal length

We have already studied that

Distance of Principal Focus from the optical centre of the lens is Called Focal Length

It is denoted by f


What is lens Formula

This formula denotes relationship between

  • Object Distance (u)
  • Image Distance (v)
  • Focal length (f)

Lens Formula and Magnification - Teachoo.jpg


Note : - We follow the sign convention for lenses
In the image, the red text is negative, and blue text is positive


  • If Virtual image is formed,v is negative
  • If Real Image is formed, v is positive
  • Object Distance (u) is always negative as it is on left side of the lens
  • Focal Length of Convex Lens is Positive
  • Focal Length of Concave Lens is Negative


In addition to the mirror formula,

we have the magnification formula


What is the magnification formula?


It is Ratio of Height of Image to the Height of Object

Hence it gives extent image is magnified compared to image of object


Suppose Height of object is h

Height of Image is h'

Magnification is m




Magnification = Height of Image/ Height of Object

m = h'/h



We also have another formula for magnification in lenses


Magnification = v/u


v is image distance

u is object distance


Note: -  If magnification (m) is positive, It means image formed is virtual and erect
If magnification (m) is negative, It means image formed is real and inverted

  1. Class 10
  2. Chapter 10 Class 10 - Light - Reflection and Refraction

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Davneet Singh
Davneet Singh is a graduate from Indian Institute of Technology, Kanpur. He has been teaching from the past 9 years. He provides courses for Maths and Science at Teachoo.