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

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 (see the sketch). 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.

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Key Concepts

Wedge-Shaped Mirror Geometry

The mirror’s wedge shape introduces a slight angular difference between its front and back surfaces. This geometry allows for selective redirection of light: when tilted to the nighttime (or anti-glare) position, the glare from bright headlights is reflected upward (away from the driver’s line of sight), while light from farther back is still directed into the driver’s eyes, preserving the mirror’s functionality.

Law of Reflection

This principle states that the angle at which light strikes a surface (the angle of incidence) is equal to the angle at which it is reflected (the angle of reflection). It is fundamental to understanding how light behaves when it encounters the mirror’s surfaces, determining the paths of both the glare from headlights and the desired image of objects behind the vehicle.

Multiple Reflective Surfaces

A car mirror typically features an uncoated front surface and a silvered back surface, each contributing to the overall reflection. The main image is formed by light reflecting off the silvered back surface; however, the uncoated front surface also reflects a small fraction of light. The interplay between these two reflective surfaces, combined with the mirror’s wedge shape, is key to achieving a mirror that functions well under both daytime and nighttime conditions.

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