Question

A bulb is located on a wall. Its image is to be obtained on a parallel wall with the help of convex lens. If the distance between parallel walls is 'd' then required focal length of lens placed in between the walls is : -
(1) Only d/4
(2) Only d/2
(3) More than d/4 but less than d/2
(4) Less than or equal to d/4

Answer 1

I think the answer should be only d/4

By using lens formula,

Image distance = d/2
Object distance = d/2

So focal length should be d/4

But the answer given is option 4

Answer 2

try drawing the image formation before you start using formulas

Answer 3

are you assuming that focal length is always d/2?

Answer 4

Nopes

Answer 5

Focal length has to be calculated.

Answer 6

d/2 is the distance of bulb from lens and distance of other wall from the lens. It's given in the question.

Answer 7

Yes, that should be the diagram and we have to calculate the focal length of the lens.

Answer 8

I have calculated it to be d/4 only but the answer says option 4. Can you explain how?

Answer 9

I am not assuming anything. I am using lens formula which states that

\(\sf \large \frac{1}{v}-\frac{1}{u}=\frac{1}{f}\),
where, v = image distance
u= object distance
f=focal length

Answer 10

yeah that's a simplification without accounting the refractive index of the lens

Answer 11

Any info about refractive index or change in shape is not given, so we have to assume it as constant.

Answer 12

or the medium in which light travels from air to end

Answer 13

from air to lens*

Answer 14

Option 4 is only possible if the lens is not kept strictly in middle of the two walla.

Answer 15

*walls

Answer 16

@iambatman