1. Reflection: When light strikes a surface, it bounces back in a process called reflection. The angle of incidence, or the angle at which the light strikes the surface, is equal to the angle of reflection. This principle is essential in designing and building mirrors, which use reflection to form images.
2. Refraction: When light passes through a medium with a different refractive index, such as air to water or vice versa, it changes direction(i.e., it bends) in a process called refraction. The amount of bending depends on the angle of incidence and the refractive index of the medium. This principle is used in lenses, which are designed to refract light to form images. The human eye also uses refraction to focus light onto the retina, allowing us to see.
3. Interference: When two or more light waves meet, they can interact with each other and produce a new wave pattern(called interference pattern). This is known as interference. This principle is used in many optical instruments, including interferometers, which are used to measure small changes in length or distance.
4. Diffraction: When light passes through a narrow opening or around an obstacle, it bends and spreads out(in the regions of geometric shadow) in a process called diffraction. This principle is essential in the design of diffraction gratings, which are used in optical spectroscopy to measure the properties of light.
5. Polarization: Light waves can vibrate in many directions, but when they vibrate in only one direction, they are said to be polarized. When light is polarized, the electric and magnetic fields oscillate in a specific direction. Polarization can be used to filter light waves, which is useful in many applications, including photography, 3D movies, and liquid crystal displays.

These principles are essential in understanding the behavior of light in different environments and in designing and building optical instruments, such as lenses, mirrors, and telescopes.

Models of Light

(Optics)

Light is the chameleon of the physical world. In some phenomenon(reflection, refraction, photoelectric effect) it acts like particles traveling in a straight line, but in some other phenomenon(interference, diffraction, and polarization) light shows wavelike behavior like sound waves or water waves. Some times light exhibits behavior that is neither wave-like nor particle-like but has characteristics of both(reflection, refraction).

Hence it will be better to develop three models of light. Each model successfully explains the behavior of light within a certain domain—that is, within a certain range of physical situations. A brief summary of all three models is:

1. The Ray Model :- There are two equally well-known “facts” about light. First, that light travels with enormous speed(presently accepted value in vacuum is c = 2.99792458 × 10⁸ m s^–1 ≅ 3 × 10⁸ m s^–1) and second, that it travels in a straight line. These straight-line paths are called light rays. The properties of prisms, mirrors, and lenses are best understood in terms of light rays. Unfortunately, it’s difficult to reconcile “light travels in straight lines” with “light is an electromagnetic wave.”  The answer is that the wavelength of light is very small compared to the size of ordinary objects that we encounter commonly (generally of the order of a few cm or larger). In this situation, a light wave can be considered to travel along a straight line. Using this model, we explain the phenomena of reflection, refraction and dispersion of light.
2. The Wave Model :- Under many circumstances light exhibits the same behavior as sound or water waves. Lasers and electro-optical devices are best described by the wave model of light. In this model, light is considered as an electromagnetic radiation that consists of oscillating electric and magnetic fields, which propagate through space at a constant speed. The wave model of light can be used to explain a variety of phenomena, such as reflection, refraction, diffraction, interference and polarization.
3. The Photon Model/the particle model of light :- In the quantum mechanics(the branch of physics that studies the behavior of matter and energy at the smallest scales, such as atoms, molecules, and subatomic particles), light behaves like neither a wave nor a particle. Instead, light consists of photons that have both wave-like and particle-like properties. So we can say that photon model of light deals with dual nature of light. The photon model of light can be used to explain several phenomena, such as the photoelectric effect, where electrons are ejected from a metal surface when light is shone on it, and the Compton effect, where the energy and wavelength of a photon changes when it collides with a particle.

In class 12^th, the study of optics is divided into two parts :-

1. Ray Optics / Geometrical Optics and
2. Wave Optics / Physical Optics