optical contact[′äp·tə·kəl ′kän‚takt]
Surfaces of transparent bodies are said to be in optical contact when the distance between them is of the order of the action range of the molecular forces. Bringing the surfaces together to this distance is called setting them in optical contact. If bodies with equal indexes of refraction are placed in optical contact, light passes through their interface, the optical contact surface, without changing direction. In addition, the reflection coefficient of the optical contact surface is extremely low—from 10-4 to less than 10-7. Clean, well-polished surfaces generally can be brought easily into optical contact, but they then can no longer be moved apart without damage.
Optical contact is also often said to take place when the surfaces of transparent bodies are brought together in such a way that the reflection coefficient of each surface becomes a function of the distance d between the surfaces and decreases rapidly with decreasing d. This phenomenon is particularly evident in the case of total internal reflection, when the reflection coefficient varies as a function of d from 1 to an imperceptibly small value. This effect is used for the intensity modulation of light and for crude spectral separation of the longwave and shortwave parts of radiation.