Combining Lenses and Mirrors: Finding the Focal Length and Image Formation

[stpmmaths]

Homework Statement

Question B only

Homework Equations

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?

 

[stpmmaths]

Any help from the web?

 

[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.