# Homework Help: A question on geometric optics

1. Aug 17, 2015

### roborahull

a concave mirror of 5cm radius of curvature whose circular ring has a radius of 4cm is blackened except for a narrow strip round the edge. A beam of light parallel to the principal axis falls on the mirror. Find the distance between the centre of curvature of mirror and the point at which light is focused.

2. Aug 17, 2015

### DEvens

Greetings robarahull. Welcome to the forum.

To get help with homework questions you need to follow the rules. You need to fill in the template for asking questions. And you need to show that you have done some of the work trying to solve your homework yourself.

Because you are new I will give you a tiny bit of help anyway. What angle will the light make with the surface of the mirror at the non-black radius? And so where will it reflect? So where will it be focussed?

3. Aug 17, 2015

### Wminus

In the paraxial case, what difference does it make for a mirror to have its centre blackened? Will the intensity of the reflected light change? Will the focus point change (hint: optical path length)?