Web optics is based on wave theory of light put forward by Huyghen. According to wave theory, light is a form of energy which travels through a medium in the form of transverse wave.
A wave front
It is defined as the locus of all the particle of a medium, which are vibrating in the same phase. When source of light is a point source, the wave front is spherical. When source is linear, the wave front is cylindrical. At very large distance from the source, a portion of spherical or cylindrical wave front appears to be plane. A wave front travels parallel to itself and perpendicular to the rays.
Huyghen’s principle of geometrical contraction of a wave front at any instant says:
1) Every point on given wave front (called primary wave front) acts as a fresh source of new disturbance, called secondary wavelets.
2) The secondary wavelets travel in all the directions with the speed of light in the medium.
3) A surface touching these secondary wavelets tangentially in the forward direction at any instant gives the new (secondary) wave front at that instant. Using Huyghen’s principle, we can prove the law of reflection and law of reflection on the basis of wave theory.
Nature of light
Nature of light has been a subject of controversy. Newton was the first to give corpuscular theory of light, according to which, light consist of tiny corpuscles of light. This theory could explain rectilinear propagation of light and reflection of light. Newton’s corpuscular theory failed as it could not explain why light travels faster in a rarer medium than a denser medium. Maxwell suggested electromagnetic theory of light. The electromagnetic waves are transverse waves which require no material medium for propagation. In these waves, the electronic field vector and magnetic field vector vibrate in same phase, along mutually perpendicular directions, and both are perpendicular to the direction of velocity of light.
It says that when two or more wave motions travelling through a medium superimpose one another they lose their individual identity. A new wave is found in which resultant displacement at any instant is equal to vector sum of the displacements due to individual waves at that instant.
Interface of light
Interface of light is the phenomenon of redistribution of light energy on account of superimposition of light waves from two coherent sources at the point where resultant intensity is maximum, interference is said to be constructive. At the points, where the resultant intensity is minimum, interference is said to be destructive.
Thomas Young’s double slit experiment
It was the first to be demonstrating the phenomenon of interference of light. Using two slits illuminated by monochromatic light source, he obtained bright and dark bands of equal width place alternately. These where called interference fringes. All bright fringes have same intensity and all dark fringes are perfectly dark.
Coherent-sources are those which emit continuous light waves of same amplitude, same wavelength/frequency in same phase or heaving a constant phase difference. Two independent sources can never be coherent. Coherent source are usually the object-image sources i.e. they are produced from a single source of light. Coherent sources are an essential requirement for interference of light. They should be strong, with least back ground, lying close to each other.