How Do You Calculate the Focal Length of a Lens for a Projector?

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Homework Statement

a projector is used to project an image of an 18mm film onto a screen which is 3.50M high. the image fills the screen. the screen is exactly 9.7M from the centre of the lens.

a) what type of lens will be in the projector?
Convex
B) calculate the focal length of the lens needed using the above data.

C) how far from the lens should the film be positioned?

D) if a mirror was to be used instead of a lens, briefly decribe the type of mirror used.


i haven't done lenses and light type questions for over a term and a half and have nothing in my notes to help me.

Homework Equations

1/f = 1/v + 1/u
this is the only equation i see relavent
M=194.4444
this may help

The Attempt at a Solution

i have really got stuck at part B

 

[andrevdh]

The lateral magnification, m, of a lens is given by

m = \frac{h_i}{h_o} = -\frac{d_i}{d_o}