In numericals, we need to calculate some things like

  • Object distance
  • Image distance
  • Object height
  • Image height
  • Focal length

To calculate them correctly, we need to find whether it is positive or negative

The sign convention mentioned below helps us in it

Sign Convention for Lens - Teachoo.jpg

We can see the sign convention in both Lens

 

Some points to note

  • Focal length of Convex Lens is positive
  • Focal length of Concave Lens is negative
  • Since object is always in front of the lens, object distance is always negative
  • If a real image is formed, image is formed on right side, so image distance is positive
  • If a virtual image is formed, image is formed on left side, so image distance is negative
  • Since object is always above the principal axis, object height will be positive
  • If image is above the principal axis, image height will be positive. It means that image is erect
  • If image is below the principal axis, image height will be negative. It means that image is inverted
  1. Class 10
  2. Chapter 10 Class 10 - Light - Reflection and Refraction

About the Author

Davneet Singh's photo - Teacher, Computer Engineer, Marketer
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.