. What is meant by power of a lens? Define its S.I unit

You have two lenses A and B of focal lengths +10 cm and -10 cm respectively, State the nature and power of each lens. Which of the two lenses will form   a virtual and magnified image of an object placed 8 cm from the lens?  Draw a ray diagram to justify your answer.                                                                                                                 


Best Answer

Ans.  Power of a lens  and its S.I. unit.


The power of convex lens is positive therefore, lens A is a convex lens

Lens B: FB = -10 cm =


The power of a concave lens is negative therefore, lens B is a concave lens.

When an object is placed at 8 cm (i.e. between the optical centre and principal focus) only convex lens will form the virtual and magnified image.

Therefore, lens A will form a virtual and magnified image of the object placed 8 cm from it.  When the object is placed between   the optical centre and the focus: (i.e., between 0 to B' and F') the image formed is behind the object (on   the    same    side), virtual,    erect    and magnified.


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