Question

The focal length of david's lens is 60 cm. if rebecca stands in front of david at a distance of do and david perceives the position of rebecca at di, what does do equal if the magnification is -0.40?

Answer 1

if rebecca stands in front of david at a distance of do and david perceives the position of rebecca at di, di will be +84 cm

What is focal length ?

How strongly light converges or diverges depends on an optical system's focal length, which is the inverse of optical power. A system with a positive focus length is said to converge light, whereas one with a negative focal length is said to diverge light.

focal length = +60 cm

magnification m = -0.40

focal length being positive an magnification negative.

given lens is a convex lens.

for a lens

m = di/do and 1/f = (1/di) - (1/do)di

= -0.4do1/f = (1/-0.4do) - 1/do do

= -210 cmdi = -0.4 * -210

di = +84 cm

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