Reflection of Light in Curved Mirrors
Senior 2 · Physics
Reflection of Light in Curved Mirrors
Lesson 4 of 4
18 min
Curved mirrors are some of the most useful objects in daily life. The mirror on a moto-taxi, the reflector in a torch, the round mirror at a sharp bend on a mountain road, the dentist's little mirror and the reflector of a solar cooker are all chosen because of the kind of image they form. In an exam you will be asked not only which mirror is used, but why.
| Use | Where the object or source is | Why it works |
|---|---|---|
| Shaving and make-up mirrors | Face between F and P | Gives a virtual, upright, magnified image, so small details are easy to see |
| Dentists' mirrors | Tooth close to the small mirror, inside F | Gives an upright, magnified view of the back of the teeth |
| Torches, car headlamps and searchlights | Bulb at F | Light from F is reflected into a strong parallel beam that does not spread. A parabolic reflector is used so that all the light becomes parallel |
| Solar cookers and solar concentrators | The Sun is at infinity; the pot is at F | Parallel sunlight is concentrated onto the pot at the focus, making it very hot |
| Reflecting telescopes and satellite dishes | Distant star or satellite (at infinity) | A large mirror or dish collects weak parallel light or radio waves and brings them to a focus where the detector sits |
Worked example. Mugisha builds a solar cooker from a concave reflector of radius of curvature 1.2 m. Where should he put the pot? Sunlight arrives as parallel rays, which meet at F. f = R/2 = 1.2 ÷ 2 = 0.60 m in front of the pole.
| Use | Why it works |
|---|---|
| Rear-view and side mirrors on cars, buses and motos | Always gives an upright image, and its wide field of view shows much more of the road behind than a flat mirror of the same size |
| Road safety mirrors at bends and junctions | Drivers on winding hill roads can see traffic coming round a blind corner |
| Security mirrors in shops, banks and supermarkets | One mirror lets a guard watch a whole room |
There is a price for the wide view: the image is diminished, so vehicles in a convex mirror look further away than they are. That is why some mirrors are printed with the warning "Objects in mirror are closer than they appear". A driver or rider must allow for this before overtaking or changing lanes.
A clinic needs (a) a mirror to examine patients' teeth, (b) a mirror to watch the corridor from the reception desk, and (c) a reflector for a strong examination lamp.
Tip
Match the need to the image: need to see bigger → concave, object inside F. Need to see more (wide view) → convex. Need a parallel beam → concave, source at F. Need to concentrate sunlight or signals → concave, receiver at F.
Why is a convex mirror, not a flat mirror, used as a side mirror on a moto?
Solution: A convex mirror gives a wider field of view, so the rider sees more of the road behind, and the image is always upright.
A torch gives a weak, spreading beam. The bulb is found to be between F and C of the concave reflector. How should it be moved, and why?
Solution: Move the bulb towards the mirror, to F. Only light from the principal focus is reflected as a parallel beam; from beyond F the rays converge and then spread out again.
A solar-cooker reflector has a focal length of 0.45 m. What is its radius of curvature, and where should the pot go?
Solution: R = 2f = 0.90 m. The pot goes at the focus, 0.45 m in front of the pole.
Why does a shaving mirror stop giving a useful upright image if you hold it too far from your face?
Solution: Once your face is further from the mirror than F, the image becomes real and inverted (and, beyond C, diminished). An upright, magnified image only forms when the face is between F and the mirror.
Remember
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