Question

The focal length of the combination of convex lens of power 1d and a concave lens is -1.5d is

Answer 1

Total power of the combination of the lenses is :-

1 + (-1.5)= -0.5 D

So according to the question, we know that power =1/focal length in meters.

So focal length is 1/power=1/-0.5= -2 meters.

But focal length can't be negative, so the focal length is 2 meters and the combination of the lenses acts like a concave lens.

Answer 2

It is given  

Power of Convex lens (P₁) = 1 d

Power of Concave lens (P₂) = -1.5d

To Find:-

Focal length of the combination (F).

Solution:-

As we know that

Power is defined as the reciprocal of its focal length.

Firstly we calculate the power of combination of lens

→ P = P¹ + P₂

Substitute the value we get

→ P = 1 + (-1.5)

→ P = 1-1.5

→ P = -0.5D

Now Calculating the Focal length of combination of lens

→ P = 1/F

Substitute the value we get

→ P = 1/F

→ -0.5 = 1/F

→ -1/0.5 = F

→ F = -2cm

Thus, The focal length of the combination of lens is -2cm or 2/100 = 0.02m