Light is a form of energy that enables us to see objects around us. When light from an illuminated object falls on the eye, an image is formed on the retina from where a message is transmitted to the brain through optic nerves. Asensation of sight follows.
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The branch of Physics that deals with the nature, sources, properties and effects of light is called Optics.
Optics may be broadly divided into two branches namely
physical optics, which is the study of the wave-like nature of light and interactions between light and matter.
Geometrical optics which is the study of simple properties of light and optical instruments by assuming that light travels in straight lines. The ray concept is^ used in this study.
Some of the terms we generally use in the study of geometrical optics are :
o Transparent bodies are those, which allow a major portion of the incident light to pass through them.
o Opaque bodies are those, which do not allow a major portion of the incident light to pass through them.
o Translucent bodies are those, which allow only a fraction of the incident light to pass through them.
o Ray of light is a straight line, which represents the direction of propagation of light.
o Beam of light is a collection of rays of light. It may be convergent, divergent or parallel.
Convergent beam is one in which the rays meet or tend to meet at a point.
Divergent beam is one in which the rays diverge or appear to diverge from a point. (Fig. 1.2).
Parallel beam is one in which the rays are parallel to one another. (Fig. 1.3).
Image : Rays of light coming from a point source after reflection, refraction or both converge to a point or appear to diverge from a point. This point is known as the image of the point source.
Ifthe rays after reflection or refraction converge to a point, the imageis real. Real images are always inverted and can be caught on a screen. If the rays appear to diverge from a point, the virtual.
Virtual images are always erect. They can only be| seen, but cannot be caught on a screen.
Optical medium is one, which allows the incident light to pass through it. It may or may not comprise of matter.
Anoptical medium, which comprises of matter, is homogeneous if the density is the same at all points in it; otherwise it is heterogeneous.The medium is isotropic if the speed of light in it is the same in alldirections.
Example : glass is isotropic while quartz is anisotropic. Anisotropic media are generally crystalline. In geometrical optics, only homogeneous, isotropic media are considered.
Reflection is the phenomenon in which light travelling in one medium, incident onthe surfjace of another returns to the first medium obeying the following laws :
1. The incident ray, the reflected ray and the normal to the surface at the point of incidence, all lie in the same plane.
2. The angle of incidence is equal to the angle of reflection.
The following are some of the consequences of reflection, which are used, in our later discussions.
a) The image formed in a plane mirror is as far behind the mirror as the object is in front of it.
b) Whena plane mirror is turned through by an angle 6, the incident ray beingfixed in direction, the reflected ray turns through an angle 29.