Lens Optics

question about lens optics?

a thin convex lens is placed 25 cm from a pinhole that acts as a point source of light. A plane mirror placed 20 cm behind the lens reflects the light back through the lens, and the reflected light forms a sharp spot beside the pinhole.
What is the focal length of the lens (in cm)?

this is a prelab question so i am not sure how to do this. can anyone give me some hints that will point me in the right direction?

The focal length is 25 cm. After checking the ray tracing (and doing a solving a fairly long algebra) I found that f = 25cm. The value of 20 cm is irrelevant. That is the answer.

You have to solve it using the following equations

1/f = 1/so1 + 1/si1 => first pass

and

1/f = 1/so2 + 1/si2 => second pass after reflection from the mirror

where so1 = 25, si2=so1 and so2 = si1 + D (D=20). Solving for so1 in the first equation we get

si1 = f so1 / (so1 - f)

and substituting all of the above in the second equation

1/f = 1/(f so1 / (so1 - f)+D) + 1/so1)

at this moment, all the variables are known except for f. Solving for f we find that f = so1. D cancels out at some point.

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Physics: optics of lenses and mirrors (1)

Optics Question/Lens/focal length?

You want to take a full-length photo of your friend who is 1.80 m tall, using a 35.0 mm camera having a 50.0 mm lens. The image dimensions of 35.0 mm film are 24.0 mm times36.mm and you want to make this a vertical photo in which your friend's image completely fills the image area.

How far should your friend stand from the lens?
How far is the lens from the film?

Get him to stand 2.5m away

focal length/ frame height = subject distance/ subject height

50/36 = h/1800
h = (50 * 1800)/36

The film will be 50mm away from the lens. The 50mm in the 50mm Lens nomenclature refers to its focal length.