Refraction, Dispersion, Reflection and Diffraction
The change of angle, which occurs at the interface when light traveling through a vacuum or air enters a medium such as glass or water, is called refraction. The amount of refraction that takes place is expressed by a quantity called the index of refraction or refractive index. If the angle of incidence in the figure is i and the angle of refraction is r, these quantities are related by:
Index of refraction n =sini/sinr n is independent of the angle of incidence. Strictly speaking, it is called the index of refraction of the medium on the side of the transmission with respect to the side of incidence. If sunlight is passed through a prism, it is split up into a continuous spectrum, commonly thought of as being made up of seven colours: violet, indigo, blue, green yellow, orange and red. This is caused by the difference between the angles of refraction of light of different wavelengths. Different types of optical glass are made for lenses; some have a high index of refraction while others have a low index of refraction
The splitting of light of different colors due to the different indices of refraction at different wavelengths when the light enters a lens or prism is called dispersion. The wave of monochromatic visual light varies from about 400nm (lnm=1 nanometer=1/ 1,000,000,000 meter) for violet to about 700nm for red. This difference between wavelengths is what gives us our sense of color. There are different types of optical glass, some producing high dispersion and others producing low dispersion. The relative refraction of the many different wavelengths between blue and red is called partial dispersion. In lenses made of' normal optical glass, this partial dispersion has a certain definite trend. However, it is possible to have glass of low dispersion and large partial dispersion, or high dispersion and small partial dispersion; such glass is called abnormal optical glass.
In reflection, in contrast to refraction, when light strikes a medium such as glass it comes off of the interface in a completely new direction. If the interface is smooth relative to the wavelength of the light, the angle of incidence equals the angle of reflection, but if the roughness of the interface is on the same scale as the wavelength of the light or smaller, the reflected light is scattered in many directions. To keep the amount of light reflected by a lens to a minimum and maximize the amount of light that passes through, it is important for the front surface of the lens to be coated. Measures such as anti reflective paint, electrostatic powder, etc. are used to prevent fogging due to reflection off of the metallic surface of the lens barrel.
The amount of light passing through a photographic lens is adjusted by a diaphragm. The process by which light gets in behind the edge of the diaphragm is called diffraction. This causes the image on the film to be lower in contrast and resolution and to lose sharpness. The diffraction effect increases as the f stop value becomes larger. The occurrence of diffraction depends not only on the diameter of the diaphragm opening, but also on the wavelength of the light, the focal length of the lens and the aperture ratio.