Question

Focal Length

a) Is there a trend in image distances vis-a-vis focal lengths as the object gets larger?

b) How do these distances compare with your essential focal lengths from your projections of distant light source images compared to lens abberations?

c) What happens to the thin lens formula when object distance is very large? Write out the formula and explain.

Answer 1

a)No,

since the image distance is calculated using lens eqn

1/f = 1/i + 1/o

i = o x f/(o - f)

it depends on the focal length and object distance only. Not on the height of the object.

But magnification depends on the height of the object.

M = h'/h = -i/o

b)Using the lens eqn

1/f = 1/i + 1/o

i = o x f/(o - f) ; o = (i x f)/(i - f)

Both the image and the object distance vary directly with the focal length of the lens.

c)when o = infinity, the lend eqn becomes

1/f = 1/i + 1/infinity

1/f = 1/i + 0

f = i

In that case the image if formed right at the focal point of the lens.