# Lens and mirror combinations

1. Sep 8, 2009

### stpmmaths

1. The problem statement, all variables and given/known data

Question B only

2. Relevant equations

3. The attempt at a solution

1) Why we use f=15.0cm? Shouldn't it be f=infinity considering plane mirror have an infinity focal length?
2) Normally I do combination(lens, mirrors), firstly I will consider the front lens/mirror by finding the 1st image formed. Then the 1st image will be the object of the subsequent lens/mirror. Am I using the right method to do?

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2. Sep 9, 2009

### stpmmaths

Any help from the web?

3. Sep 9, 2009

### rl.bhat

Rays from the object first refract through the lens, reflect from the mirror and then again refract though the lens to from I2. During the second refraction, the object is the vertual image I1 formed by the mirror. After refection from the mirror the rays appear to be converging at a distance equal to the distance of I1 behind the mirror. Hence u is negative.