Question

A convex lens of focal length 10 cm is placed in contact with a concave lens of focal length 20 cm. The focal length of this combination of lenses will be:
(a) +10 cm
(b) +20 cm
(c) −10 cm
(d) −20 cm

Answer 1

 +20 cm
We know that:
Power (P) = `1/"focal length"` 

Therefore 

Power of convex lens `p_1=1/(10cm)=1/(0.1m)=+10D` 

Power of convave lens `P_2=1/(-20cm)=-1/(0.2m)=-5D` 

We know that power of lens adds up when lenses comes in contact. Therefore, we have:
Net power (P) = P₁ + P₂
Net power (P) = 10 D-5 D = 5 D 

Net focal length = `1/"power"=1/5=0.2m=20cm`