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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---teachoo2.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
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CA Maninder Singh

CA Maninder Singh is a Chartered Accountant for the past 12 years and a teacher from the past 16 years. He teaches Science, Accounts and English at Teachoo