How does an image form in a mirror?
A mirror image forms when light from objects reflects from a smooth surface in organized directions. In a plane mirror, rays reaching your eyes appear to originate from points behind the surface when traced backward. These points make up a virtual image. No real object or illuminated screen has to exist behind the mirror.
Mechanism and everyday examples
1. Why can a shiny surface show an image? Some light reaching a surface can be absorbed, some transmitted, and some reflected. A mirror's reflective coating sends a substantial proportion back. When the surface is sufficiently smooth compared with visible wavelengths, the arrangement of neighboring rays is preserved. A rough wall also reflects light, but scatters it into many directions, preventing it from showing a clear image of your face despite being visible itself. 2. What does the reflection law say? Angles of incidence and reflection are measured relative to a line perpendicular to the surface, called the normal. These angles are equal for specular reflection. Each object point emits or scatters light in many directions, but only appropriate rays reflect into your eye. The eye's lens brings them onto the retina, and the brain interprets their incoming directions. The mirror need not bring every reflected ray to a single physical point. 3. Why does the image appear behind it? If an object stands one meter in front of a plane mirror, its image appears approximately one meter behind it. The directions you observe correspond to rays from that symmetrical position. Light has not actually traveled from behind the mirror. The term virtual describes this geometrical property rather than suggesting that the image is imaginary. You can photograph it, but cannot directly project it onto a screen placed at its apparent position behind the mirror.
Observation, applications and misconceptions
4. Why do curved mirrors look different? Normals on a concave mirror can redirect rays toward one another. Depending on object position, it may produce a magnified virtual image or a real image that can be projected onto a screen. A convex mirror shows a wider field while making images smaller. Vehicle side mirrors and magnifying grooming mirrors exploit these geometrical differences. No single magnification rule applies to every mirror without specifying curvature and viewing position. 5. Where can a second faint image come from? An ordinary mirror often has its metallic coating behind a glass sheet. The front glass surface also produces a weak reflection, sometimes visible as a faint ghost when viewed obliquely. Front-surface mirrors used in precision optical arrangements can reduce this secondary reflection. Scratches, dirt, and deviations from the intended curvature can also affect contrast and image accuracy, especially when the reflection is being used for measurement. 6. Why does a mirror not generate light? A normal mirror redirects incoming light; it cannot independently illuminate your face in a dark room. Less illumination means fewer visible details. The positions of surrounding light sources, the field of view, and surface cleanliness all change what you see. Curved mirrors can concentrate sunlight enough to create substantial heat, so such arrangements must not be directed at eyes or combustible materials. Their focusing action is a transfer of incoming energy, not new energy production. An image need not be the same size as the mirror; image size is compared with the object. Seeing only part of a large object in a small plane mirror does not mean that its image has shrunk. The mirror has limited the field of view.