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.

Explanation

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.