# Physics mirror problem

• whitehorsey

#### whitehorsey

1. Light from a distant star is collected by a concave mirror. How far from the mirror is the image of the star if the radius of curvature is 150 cm?

2. 1/f = 1/do + 1/di

3. 1/f = 1/do + 1/di
f = 150/2 = 75 cm
di = dof/do - f
= do(75)/do- 75

I don't know what do is or if there is one. If there is one, can you tell me what it is. Thank You!

The object distance is at infinity (or pretty close!) so the light arrives parallel

mgb_phys said:
The object distance is at infinity (or pretty close!) so the light arrives parallel

um.. what do u mean by the light is parallel? oh and i know that the answer is 75 cm but i don't know how to get it.

One of the rules of a concave mirror is that the focal length is half the radius of curvature.

Rules of optics
Parallel light in goes out through the focus
Light in through the focus goes out parallel

draw a sketch with the focal point, the centre of curvature an the light ray

oh ok thank you!