Specular reflection

Specular reflection, also called regular reflection, is like the mirror-like reflection of waves, such as light, from a smooth surface. When light hits a surface and reflects, it follows a special rule called the law of reflection. This law says that the light bounces off at the same angle it hits the surface, but on the opposite side of an imaginary line called the surface normal.

The earliest known description of this reflection behavior was recorded by Hero of Alexandria around AD 10–70. Later, Alhazen gave a full explanation of the law of reflection, showing that the incoming light, the reflected light, and the surface normal all lie in the same plane.

Specular reflection is different from diffuse reflection, where light scatters in many directions instead of reflecting in a single, clear path like a mirror.

Law of reflection

When light hits a surface, it can bounce back in a special way called specular reflection. This is like looking in a mirror—the light reflects at the same angle it hits the surface.

The law of reflection says that the angle at which the light hits the surface (called the angle of incidence) is exactly the same as the angle at which it bounces back (called the angle of reflection). Both angles are measured from an imaginary line called the surface normal, which is perpendicular to the surface. This makes the light ray, the surface normal, and the reflected ray all lie in the same flat plane.

If the light hits the surface straight on, it bounces straight back the way it came. This special bouncing of light helps us see clear, sharp images in mirrors and shiny surfaces.

See also: Snell's law § Vector form

Reflectivity

Reflectivity is the way we measure how much light bounces off a surface compared to how much hits it. It changes depending on the color of the light and the type of material. Scientists can study how materials absorb light by looking at how they reflect light instead.

To measure how well a surface reflects light, special tools called reflectometers are used. These tools shine light on the surface and measure how much bounces back. Sometimes, simpler tools called glossmeters are used to check how shiny a surface looks.

Consequences

When light moves through a material and hits a surface with a material that lets light pass more easily, some of the light bounces back. If the light hits at a steep enough angle, called the critical angle, all of the light reflects back. This is known as total internal reflection.

When light hits a surface between two materials, the reflected light can act in special ways. At a certain angle, called Brewster's angle, the reflected light aligns in one direction, which is called being linearly polarized.

A flat mirror creates an image with some special features. The image looks the same distance behind the mirror as the object is in front. It is the same size as the object and appears upright, but it is reversed. This image is called virtual, meaning it seems to be behind the mirror and cannot be shown on a screen. Mirrors can make things look reversed in different ways depending on how you look at them. For example, a right shoe in a mirror may look like a left shoe.

Examples

A classic example of specular reflection is a mirror, which is designed to reflect light in a clear, mirror-like way.

Specular reflection isn't just for visible light. It can also happen with radiowaves bouncing off the ionosphere, and with radio- or microwave radar signals reflecting off objects in the sky. Scientists use this reflection with x-rays to study very thin layers of materials. Sound waves can also reflect in a similar way, as can atoms under special conditions.