Reflection (physics)
Reflection of VHF and higher frequencies is important for radio transmission and for radar.
Even hard X-rays and gamma rays can be reflected at shallow angles with special "grazing" mirrors.
Reflection of light is either specular (mirror-like) or diffuse (retaining the energy, but losing the image) depending on the nature of the interface.
In the most general case, a certain fraction of the light is reflected from the interface, and the remainder is refracted.
When light reflects off a material with higher refractive index than the medium in which is traveling, it undergoes a 180° phase shift.
Light waves incident on a material induce small oscillations of polarisation in the individual atoms (or oscillation of electrons, in metals), causing each particle to radiate a small secondary wave in all directions, like a dipole antenna.
Light–matter interaction in terms of photons is a topic of quantum electrodynamics, and is described in detail by Richard Feynman in his popular book QED: The Strange Theory of Light and Matter.
The light sent to our eyes by most of the objects we see is due to diffuse reflection from their surface, so that this is our primary mechanism of physical observation.
When flying over clouds illuminated by sunlight the region seen around the aircraft's shadow will appear brighter, and a similar effect may be seen from dew on grass.
A simple retroreflector can be made by placing three ordinary mirrors mutually perpendicular to one another (a corner reflector).
A surface can be made partially retroreflective by depositing a layer of tiny refractive spheres on it or by creating small pyramid like structures.
This is used to make traffic signs and automobile license plates reflect light mostly back in the direction from which it came.
In this application perfect retroreflection is not desired, since the light would then be directed back into the headlights of an oncoming car rather than to the driver's eyes.
In practice, these situations can only be approached but not achieved because the effects of any surface imperfections in the reflectors propagate and magnify, absorption gradually extinguishes the image, and any observing equipment (biological or technological) will interfere.
This leads into the field of architectural acoustics, because the nature of these reflections is critical to the auditory feel of a space.
Seismic waves produced by earthquakes or other sources (such as explosions) may be reflected by layers within the Earth.
Study of the deep reflections of waves generated by earthquakes has allowed seismologists to determine the layered structure of the Earth.
