Car mirrors are uncoated on the front surface and silvered on the back surface.

Question

Car mirrors are uncoated on the front surface and silvered on the back surface. When the mirror is properly adjusted, light from behind reflects from the silvered surface into the driver's eyes. Good. But this is not so good at nighttime with the glare of headlights behind. This problem is solved by the wedge shape of the mirror. When the mirror is tilted slightly upward to the "nighttime" position, glare is directed upward toward the ceiling, away from the driver's eyes. Yet the driver can still see cars behind in the mirror. Explain.

(Conceptual Physics 11th Edition - Chapter 28, Exercise 10)

Explanation / Answer

The key is that the mirror is "uncoated." When an E-M wave encounters a change in index, there is a reflection. For the change in index from air to glass at the approximate wavelength range of visible light, the reflection is about 4%.

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