How Do Mirrors Work?

A mirror does not reflect light by acting like a tiny wall that makes light particles bounce off it. Light is an electromagnetic wave, and its electric field drives charges in the material. The response of those charges produces new electromagnetic fields; together, the incoming and emitted fields form the reflected light you see.

The material and smoothness of the surface determine how much light is reflected and whether it forms a sharp image. The same basic physics explains ordinary glass reflections, metallic mirrors, and why a rough surface scatters light.

How the reflected wave forms

Light’s electric field pushes charged particles. In a metal, some electrons can move relatively freely through the material. When the field of incoming light reaches the surface, these electrons oscillate in response. Their motion generates electromagnetic radiation of the same frequency as the incident light.

At the surface, the incoming wave and the fields produced by the moving charges combine. The combined field must satisfy the electromagnetic boundary conditions at the boundary between the two materials. For a good metal at visible-light frequencies, the result is usually a strong wave traveling back into the air: reflection. Some energy may instead be absorbed in the metal or transmitted through it.

This is a wave interaction, not a sequence of independent impacts. The reflected light has the same frequency as the incident light when the mirror is stationary, though its amplitude and phase can change. The phase change depends on the material, wavelength, and angle; it is not always exactly a half-cycle.

Why the angle of reflection equals the angle of incidence

For a flat surface, the reflected wave must fit the boundary consistently across the whole surface. That requirement makes the reflected direction symmetric to the incoming direction about the surface normal—the imaginary line perpendicular to the surface.

If $\theta_i$ is the angle between the incoming ray and the normal, and $\theta_r$ is the angle between the reflected ray and the normal, then

$$\theta_r = \theta_i.$$

Both angles are measured from the normal, not from the mirror face. The law applies to each small part of a flat mirror. A curved mirror has a different normal at each point, so the reflected rays can converge or diverge.

How a mirror forms an image

A flat mirror reflects light from each point on an object in an orderly direction. Your eyes and brain trace the reflected rays backward in straight lines. Those backward extensions appear to meet at a point behind the mirror, so you perceive a virtual image there. The light does not actually travel from that apparent point.

For a plane mirror, the image appears as far behind the mirror as the object is in front. It has the same size as the object, though the familiar left-right reversal is more precisely a reversal of front and back relative to the mirror plane. A warped or rough mirror changes the directions of reflected rays and distorts or blurs the image.

A simple ray example

Suppose a small lamp is 2 m in front of a flat mirror. Rays from the lamp reflect from the surface into your eyes. If you extend those reflected rays backward, they meet at a virtual point 2 m behind the mirror. The lamp’s image therefore appears 4 m from the real lamp when measured straight through the mirror, even though no light from the lamp crosses that 4 m gap behind the surface.

Why metals make effective mirrors

Metals often reflect visible light well because their mobile electrons can respond to the changing electric field of the light wave. The response creates a reflected field and also determines how much energy is absorbed. The balance varies with the metal and the wavelength, which is why different metals can give mirrors different colors or brightness.

Not all mirrors are simply polished metal. A household mirror commonly uses a thin reflective metal coating on the back of a glass sheet, with protective layers behind it. Light passes through the glass, reflects from the coating, and passes through the glass again. Some light also reflects at the front glass surface, which can create a faint secondary image. A front-surface mirror places the reflective coating on the side facing the light, reducing that extra reflection.

Even transparent materials reflect some light. At a boundary between air and glass, the electromagnetic response produces a reflected wave as well as a transmitted wave. A dielectric coating can be designed to reflect selected wavelengths, while antireflection coatings are designed to reduce reflection over a chosen range.

Why surface smoothness matters

A mirror must be smooth enough that light from neighboring points reflects in a coordinated way. If surface bumps are large enough to alter the reflected wave’s phase and direction substantially, the light spreads into many directions rather than forming a clear image. This is diffuse scattering, the same reason a wall does not show a sharp reflection even though it reflects some light.

A shiny surface can therefore reflect plenty of light without being a good image-forming mirror. A polished metal object may look bright, but small irregularities blur its reflection. For a sharp image, the surface also needs the right overall shape: a flat mirror should be flat, while a curved mirror must have a controlled curvature.

Common misconceptions

• “The image is behind the mirror.” The image is an apparent location found by tracing reflected rays backward; it is not a source of light behind the mirror.
• “All reflected light bounces off at exactly the same angle.” This describes an ideal smooth surface. Rough surfaces scatter light in many directions.
• “A mirror reflects all the light.” Real mirrors absorb or transmit some light. Their reflectivity depends on wavelength, angle, coating, and material.
• “Only metals reflect light.” Glass and other nonmetals reflect light at their surfaces too, though often less strongly.