Question

an
is placed 30 cms from a concave
lens
the focal length of the convey lens is 12 cm
the position of the image.​

Answer 1

Explanation:

An object is placed 30 cm in front of convex lens of focal length 10 cm :

Given :  u1=−30 cm        f1=10 cm

From lens formula,   v11−u11=f11

∴    v11−−301=101       ⟹v=15 cm

Thus image is formed at a distance of 15 cm behind the lens.

Now a concave lens of focal length f2 is placed in contact with convex lens. so the screen has to be shifted by 45 cm further away.

Now the new image distance   v2=15+45=60 cm

Let the focal length of combination of lens be F.

Using lens formula,  v21−u11=F1

∴    601−−301=F1             ⟹F=20 cm

Focal length of combination of lenses     F1=f11+f21       

∴  201=101+f21            

⟹f2=−20 cm    (minus sign comes as the focal length of concave lens is negative)