How Mirrors Work: Reflection, Image Formation, and Types

Flat Mirror Physics

A flat (plane) mirror produces a virtual image that appears to be located behind the mirror surface at the same distance as the object is in front. The image is the same size as the object, upright, and laterally inverted (left and right are swapped). This lateral inversion is why text appears backward in a mirror, and why your reflection raises its left hand when you raise your right.

The image is called virtual because light rays do not actually pass through the image location. Instead, reflected rays diverge from the mirror surface, and your brain traces them backward in straight lines to their apparent origin point behind the mirror. You cannot project a virtual image onto a screen because no light converges at the image point. Every flat mirror image shares these properties regardless of the mirror size or the observer position.

Modern mirrors consist of a glass substrate with a thin metallic coating on one surface. Bathroom mirrors typically have the coating on the back surface, protected by paint. This produces a faint secondary reflection from the front glass surface, creating a slight ghost image. First-surface mirrors, used in scientific instruments and telescopes, have the coating on the front, eliminating double reflections but leaving the delicate coating exposed to potential damage.

Aluminum is the most common mirror coating, reflecting about 90% of visible light. Silver reflects about 95% but tarnishes quickly in air. Gold reflects over 98% of infrared radiation and is used for infrared telescopes and thermal applications. Dielectric coatings (alternating thin layers of different materials) can achieve reflectivities exceeding 99.9% at specific wavelengths, essential for laser cavity mirrors where even tiny losses accumulate over millions of reflections.

Concave Mirrors

A concave mirror has an inwardly curved reflecting surface, like the inside of a spoon. Parallel rays hitting a concave mirror converge to a focal point located halfway between the mirror surface and its center of curvature. The focal length equals half the radius of curvature: f = R/2. This focusing ability makes concave mirrors useful wherever concentrated light or magnified images are needed.

The behavior of a concave mirror depends entirely on where the object sits relative to the focal point and center of curvature. Objects beyond the center of curvature produce small, inverted, real images between the focal point and center. Objects at the center produce same-size, inverted, real images at the center. Objects between the center and focal point produce large, inverted, real images beyond the center. Objects at the focal point produce no image (reflected rays are parallel). Objects between the focal point and mirror produce large, upright, virtual images behind the mirror.

Reflecting telescopes use large concave mirrors as their primary light-gathering elements. The parabolic shape (a refinement of spherical) ensures all parallel rays from a distant star focus perfectly to a single point regardless of where they hit the mirror. Isaac Newton built the first practical reflecting telescope in 1668 using a spherical primary mirror with a flat secondary mirror to redirect light to an eyepiece on the side. Modern astronomical reflectors range from 0.2 meters for amateur instruments to the 39-meter Extremely Large Telescope under construction in Chile.

Concave mirrors also serve as solar concentrators. Parabolic dish collectors focus sunlight to produce temperatures exceeding 1000 degrees Celsius at the focal point, sufficient to generate steam for electrical power or to drive industrial chemical processes. On a smaller scale, concave mirrors in flashlights and car headlights collect light from a bulb at the focal point and project it as a roughly parallel beam, illuminating distant objects efficiently.

Convex Mirrors

A convex mirror curves outward, like the back of a spoon. Parallel rays hitting a convex mirror diverge after reflection, appearing to come from a virtual focal point behind the mirror. This divergence means convex mirrors always produce virtual, upright, diminished images regardless of object position. The image is always smaller than the object and located between the focal point and the mirror surface.

The primary advantage of convex mirrors is their wide field of view. Because the curved surface faces outward, it collects light from a much larger angular range than a flat mirror of the same size. Vehicle side mirrors are convex for this reason, showing a wide view of adjacent lanes and blind spots. The tradeoff is that objects appear smaller and further away than they actually are, which is why passenger-side mirrors carry the warning about objects being closer than they appear.

Security mirrors in retail stores and at blind corners use convex surfaces to provide panoramic views of large areas from a single vantage point. The distortion (fisheye effect) increases toward the mirror edges but remains acceptable for surveillance purposes. Traffic mirrors at dangerous intersections serve the same function, allowing drivers to see around corners or over obstacles.

Spherical vs. Parabolic Shapes

Most curved mirrors have either spherical or parabolic cross-sections. Spherical mirrors are easier and cheaper to manufacture because any section of a sphere can be ground uniformly. However, spherical mirrors suffer from spherical aberration: rays far from the center focus at a different point than rays near the center, producing a blurred focal region rather than a sharp point.

Parabolic mirrors eliminate spherical aberration entirely for objects at infinity. Every parallel ray, regardless of distance from the axis, reflects to the exact same focal point. This makes parabolas ideal for astronomical telescopes, satellite dishes, and solar concentrators where incoming radiation is essentially parallel. The mathematical property that guarantees this perfect focusing is that every point on a parabola is equidistant from the focus and a line called the directrix.

Manufacturing parabolic surfaces requires more precision than spherical ones. For telescope mirrors, the glass blank is first ground to a sphere and then carefully figured (selectively polished) to achieve the parabolic shape. Modern optical testing uses interferometry to measure surface accuracy to fractions of a wavelength of light, ensuring the final mirror focuses properly across its entire aperture.

Mirror Applications in Science and Technology

Laser systems use mirrors extensively, both for directing beams and within the laser cavity itself. A typical laser has two mirrors facing each other with the gain medium between them. One mirror reflects nearly 100% while the other (output coupler) transmits a small percentage. Light bounces back and forth between the mirrors, being amplified on each pass, with a fraction escaping through the output coupler as the laser beam.

Interferometers split a beam of light using a partially reflecting mirror and recombine the two halves after they travel different paths. The resulting interference pattern reveals path length differences smaller than a wavelength of light. LIGO, the gravitational wave detector, uses mirrors separated by 4 kilometers to detect distance changes of 10^-18 meters, less than one-thousandth the diameter of a proton. This extraordinary sensitivity depends on nearly perfect mirror surfaces and extreme vibration isolation.

Adaptive optics systems in ground-based telescopes use deformable mirrors to correct atmospheric distortion in real time. A flexible mirror with hundreds of actuators on its back surface changes shape thousands of times per second to counteract the twinkling caused by turbulent air. Wavefront sensors measure the distortion from a guide star, and a computer calculates the mirror shape needed to cancel it, producing images nearly as sharp as space-based telescopes.