A converging lens having focal length 50cm is kept in contact with a diverging lens having focal length 20cm. The power of combination of the two lenses is:

Correct option is D

Correct option : (d), -3D

The total power  P _total of a combination of lenses in contact is the sum of power of the individual lenses.

P _total = P ₁ + P ₂ 

• ₁P  is the power of the converging lens,
  
• _2​P  is the power of the diverging lens.

The Power of Each Lens

The power P of a lens is given  by the formula.

P = $$\frac{1}{f}$$ 

f is the focal length of the lens in meters.

Power of the converging lens (with focal length f ₁ = 50cm = 0.50m

P₁ = $$\frac{1}{f_1}$$ = $$\frac{1}{0.50}$$ = 2D

Power of the converging lens (with focal length f ₁ = 20cm = -0.20m

P ₂ = $$\frac{1}{f_2}$$ = $$\frac{1}{-0.20}$$ = - 5D

Now, add the powers of the lenses.

P _total = P ₁ + P ₂ = 2D + (-5D) - -3D

The total power of the combination of the two lenses is -3 diopters.