Focusing mirrors are mirrors cut from a parabola of reflecting material; the parabola is fabricated in such a way that distant rays will be bent through a single point i.e. the focus of the mirror. In fact, there are two types of curved mirrors; converging mirrors made from parabolas that are concave in shape , and diverging mirrors that are made from parabolas that are convex. In either case, the focal length of the mirror is half the radius of the sphere from which it is cut.
From the diagram below, you can see that in the case of a converging (concave) mirror, parallel rays are focused down to an image at the focal point (this is the point of such a mirror). In this type of mirror the rays reflected by the mirror actually pass through F and it is therefore a real focus.
Converging (concave) mirror
By contrast, parallel rays appear to come from a focal point behind the mirror in the case of a diverging mirror. i.e. the focus is virtual.
Diverging (convex) mirror
There is a simple set of rules to follow when finding the position of an image in curved mirrors:
1. Rays parallel to principal axis are reflected through the principal focus
2. Rays through the principal focus are reflected parallel to the principal axis
3. Rays passing through the centre of curvature are reflected back along their own path
These rules are not mysterious but smply a result of how the mirrors are fabricated.
In the diagram above, note that the object is close to the converging mirror, but outside of the focal length. Using the first 2 rules above, the intersection of the reflected rays gives the position of the image. You can see the image is inverted and diminished.
Image tracing in a diverging mirror
More quantiatively, for any object a distance u from the mirror of focal length f, the location v of the image can be found from the ‘mirror’ equation
1/u + 1/v = 1/f
Note that there are only 2 variables in this equation since f is fixed for a given mirror. Typically, one uses the formula to find the location of the image of an object a given distance from the lens. One can also calculate the height of the image; this is because the magnification m of the mirror is given by the equation
m = -v/u
Note: in using both the above formulae, we use the convention that any distance that is real object is taken as a positive.
Actually, focusing mirrors are cut from parabolic surfaces, not spherical ones – I forgot this. See comment below by Norman.
1. An astronomer is observing a distant star with a reflecting telescope: use the mirror formula above to calculate where the photographic plate should be positioned. What kind of magnification can one expect?
2. If an object 5 cm high is placed 40 cm in front of a converging mirror of focal length 20 cm, calculate the position and height of the image. Is the image real or virtual?