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A car headlamp has a concave mirror, but how can you see the light outside if it converges the light inside it?
- Rikesh (age 13)
The headlamp's concave mirror is open on one end, and the light bulb's filament is placed at or near the focus. Parallel ight rays, if they were coming into the headlamp from outside (say, the driver is driving his car in the direction of the setting sun), would converge near the filament of the light bulb.
At night, the process works in reverse. Light rays which originate at the light bulb follow the reverse paths, travelling away from the the bulb, hitting the mirror, and being reflected into approximately parallel paths. You don't want the rays to be exactly parallel, or the headlamps will act as narrow spotlights, and you usually want to illuminate the road to the sides and a little up and down as well. Most car headlamps have diffusers on the side to spread some of the light around for better visibility.
(republished on 07/29/06)
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