# Lenses - Thin-Lens Equation and Magnification Equation

1. Mar 5, 2008

### missmia

[SOLVED] Lenses - Thin-Lens Equation and Magnification Equation

I can't figure out what I am doing wrong, your help would be greatly appreciated :)

Here is the Homework Problem

A camera is supplied with two interchangeable lenses, whose focal lengths are 35.0 and 150.0 mm. A woman whose height is 1.80 m stands 9.00 m in front of the camera. What is the height (including sign) of her image on the film, as produced by each of the following.
(a) the 35.0 mm lens
(b) the 150.0 mm lens

ATTEMPTED SOLUTION:

$$\frac{1}{d_{i}}$$ = $$\frac{1}{.0035m}$$ - $$\frac{1}{9.0m}$$

which gives $$d_{i}$$ = .003501m

then, I used the magnification equation: m = - $$\left[\frac{.003501m}{9.0m}\right]$$ = -3.89 x $$10^{-4}$$x

then, I used $$h_{i}$$ = m$$h_{o}$$ = -3.89 x $$10^{-4}$$ x 1.80m = -7.00 x $$10^{-4}$$

2. Mar 5, 2008

### Kurdt

Staff Emeritus
Be careful with the units. remember 35mm is 3.5cm which is 0.035m.

3. Mar 5, 2008

### missmia

oh gosh! I can't believe I didn't notice that. Thanks again :)