A parallel beam of light falls on a convex mirror of focal length 16 cm, along its principal axis.
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A parallel beam of light falls on a convex mirror of focal length , along its principal axis.
- Describe the reflected beam. Where does it seem to come from?
- How far behind the mirror is its centre of curvature?
Need a reminder? The key ideaShow
A concave mirror reflects from the inside of a sphere, and a convex mirror from the outside. The focus is half way between the pole and the centre of curvature : . A concave mirror has a real focus in front; a convex mirror has a virtual focus behind.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. (a) The beam. Each ray parallel to the axis reflects as if it came from the focus behind the mirror. So the beam spreads out. Traced back with dashed lines, the rays meet at , behind the mirror.
Step 2. (b) The centre. , behind the mirror.
Answer: (a) a diverging beam that seems to come from behind the mirror; (b) behind it.
Check. This is why a convex mirror is called a diverging mirror, with a virtual focus.
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