- A source of light and screen are separated by a distance of 50 cm . A sharp image of the source is produced by lens when placed at 2 positionsbetween the source and the screen . If the distance of the separations of the lens between 2 positions is 20 cm , find the focal length of the lens .
where,
v=distance of image
u=distance of object
f=focal length of the lens
Now let the distance between point B and the screen is x,
Now applying the lens formula when the lens is kept at both the points A & B separately,
For point B: using proper sign conventions
u = -(50-x)
v = x
For point A:
u = -(30-x)
v = (20+x)
Solving two above mentioned equations we can get the value of focal length of the lens.