Question

You are using a converging lens to look at a splinter in your finger. The lens has a 9.0 cm focal length, and you place the splinter 6.5 cm from the lens. How far from the lens is the image? What is the magnification?

Answer 1

Sign convention for Lens is:

focal length, f

+ for converging lens

- for diverging lens

object distance, do

+ if object is to the left of lens (real object)

- if object is to the right of the lens (virtual object)

image distance, di

+ if image is to the right of the lens for real object (real image)

- if image is to the left of the lens for real object (virtual image)

magnification, m

+ for an image that is upright with respect to an object

- for an image that is inverted with respect to the object

a)

Here:

do = +6.5 cm

f = +9.0 cm

Use:

1/f = 1/do + 1/di

1/9.0 = 1/6.5 + 1/di

1/di = 1/9.0 - 1/6.5

1/di = 0.1111 - 0.1538

1/di = -4.274*10^-2

di = -23.4 cm

Answer: -23.4 cm

b)

m = -di/do

= -(-23.4)/(6.5)

= 3.6

Answer: 3.6