What is meant by power of a lens ? You have three lenses A, B, C of power +10 D, +5 D and -10 D respectively. State the nature and focal length of each lens. Explain which of the three lenses will from a virtual and magnified image of an object placed at 15 cm from the lens. Draw the ray diagram in support of your answer.

What is meant by power of a lens ? You have three lenses A, B, C of po

1.	What is meant by power of a lens ? You have three lenses A, B, C of power +10 D, +5 D and -10 D respectively. State the nature and focal length of each lens. Explain which of the three lenses will from a virtual and magnified image of an object placed at 15 cm from the lens. Draw the ray diagram in support of your answer.
Answer» <P> Solution :The degree of convergence or divergence of a LENS is called the power of a lens [P]. Power of a lens is defined as the RECIPROCAL of focal length of a lens. Power of lens [P] `=(l)/("Focal length of the lens in METRE (f)")` `P=(l)/(f)` Focal length in m `[f] = (f)=(l)/(P)` `{:(,"Lens A","Lens B","Lens C"),("Power",+ 10 D,+5 D,-10D),("Focal length",1//10=+0.1,1//5=+0.2,1//10=-0.1),(,m=+10 cm,m=+20cm,m=-10 cm),("Nature of lens","Convex lens","Convex lens","Concave lens"):}` Concave lens cannot form magnified virtual image. Lens B will form a virtual and magnified image of an object placed at 15 cm from the lens. Object distance `u=-15` cm which is less than focal length of the lens = 20 cm. In other words, the object is placed between the optic centre and focus of the convex lens. The image is formed on the same side as the object.