Mammoth Memory

Convex lenses and the lens equation

We can use the lens equation that we have previously seen to find the principal focal point.

In an exam, the lens equation may be used in a whole host of questions.

As we know, the lens equation is:

Lens equation




      `F` `=` Principal focal length
  `do` `=` Object distance from centre of convex lens
  `di` `=` Image distance from centre of convex lens


NOTE: This works for lenses that are thin but the thicker a lens gets the less accurate the equation becomes. But for general purposes and basic exams, this equation is normally used.


You will need to be aware of what is positive and what is negative.


Convex single lens

Principal focal length behind a convex lens

Object distance in front of a convex lens

Image distance behind a convex lens

The image distance is negative if its in front of a convex lens


If you use the lens equation for convex lenses ALL NUMBERS are +ve except for one number, `di`, which is negative if the image is on the object side.

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