Reflection

Reflection is the abrupt change in direction of a wave front at an interface between two dissimilar media so that the wave front returns into the medium from which it originated. Examples are the reflection of light, of sound and of water waves.

Reflection of light may be specular (i.e., mirror-like) or diffuse (i.e., not retaining the image, only the energy) according to the nature of the interface. Depending on the nature of the interface, i.e., dielectric-conductor or dielectric-dielectric, the phase of the reflected wave may or may not be inverted.

Specular (mirror-like) reflection

A mirror provides the commonest model for specular light reflection and consists of a glass sheet with a metalised coating where the reflection actually occurs. Reflection can also occur from the surface of other media, including transparent ones (water, glass). In the diagram above, a light ray PO hits a mirror (vertical line) at point O, and bounces off it as ray OQ. By projecting an imaginary line through point O perpendicular to the mirror, known as the normal, we can measure the angle of incidence, θ_i and the angle of reflection, θ_r. The law of reflection states simply that θ_i = θ_r, i.e. the angle of incidence equals the angle of reflection.

In fact, reflection of light may occur whenever light travels from a medium of a given refractive index into a medium with a different refractive index. In the most general case, a certain fraction of the light is reflected from the interface, and the remainder is refracted. Solving Maxwell's equations for a light ray striking a boundary allows the derivation of the Fresnel equations, which can be used to predict how much of the light is reflected, and how much is refracted in a given situation. Total internal reflection of light from a denser medium occurs if the angle of incidence is above the critical angle.

Other types of reflection

Diffuse reflection

Light bounces off in all directions due to the microscopic irregularities of the interface; this is an omnipresent phenomenon, applicable for all non-shiny objects that are not black.

Retroreflection

Light bounces back in the same direction as it came from.

Neutron reflection

Materials that reflect neutrons, for example beryllium, are used in nuclear reactors and nuclear weapons.

See also:

• Reflectivity.
• Ripple tank for a picture and description of water waves reflecting off a boundary.