What is Optics ?
The branch of physics which deals with the study of light and the phenomena associated with the generation, transmission and the detection of light is called Optics.
Lets divide this topic in two parts for easy understanding.
(a) Geometrical Optics
(b) Optical instruments
(a) Geometrical Optics: We know that light always travels in straight line and gets deflected from its path when an obstacle comes in its path, but again the path is a straight line so, the study of these straight line paths and angular deflections are termed as geometrical optics. The operation of mirrors, lenses, prisms etc. falls in this category.
(b) Optical instruments: Sometimes it is not possible to see micro objects or distant objects with our naked eye, so we make use of lenses, prisms, or mirrors to see better than as with our naked eye. Therefore, the arrangements of lenses, prisms, or mirrors which help us to see better than what can see with our naked eye is called optical instrument.
Light: It is that agency which produces in us the sensation of sight. It itself is not visible but it helps us in seeing things. A ray is a thin beam of light that travels in a straight line.
Dual Nature of Light: Various theories were put forward to explain the nature of light and different phenomena connected with its behaviour. Some of these theories are:
1. Newton's Corpuscular(particle) theory
2. Huygen's Wave theory
3. Maxwell's Electromagnetic theory
4. Planck's Quantum theory
From the above discussion it is clear that none of theories could explain all phenomena related to light because light has dual nature. In order to explain the phenomena like reflection, refraction, interference, diffraction and polarisation, we make use of wave theory.
Light has dual nature; sometimes it behaves like a wave and sometimes like particles or photons. In order to explain different phenomena associated with light, let us now discuss some important phenomena's related with geometrical optics.
Reflection of Light: When a ray of light traveling in a medium falls on a polished surface and is sent back to the same medium, the phenomena is called Reflection of light.
For example, if a ray IO is incident on a plane mirror M at o, it goes along OR after suffering reflection from the mirror. If we draw ON perpendicular at the point of incidence 'O'. Then this is called Normal ray. IO is called incident ray.
OR is called Reflected ray and ON is called Normal.
OR is called Reflected ray and ON is called Normal.
Laws Of Reflection:
First Law: This law states that the incident ray, the reflected ray and the normal at the point of incident of light all meet at a point and lie in the same plane.
In the above fig. OI, OR and ON all are meeting at O and lie in the same plane of paper.Furthermore, the angle of reflection r is equal to the angle of incidence i. Both angles are measured with respect to the normal to the mirror.
The angles are measured relative to the perpendicular to the surface at the point where the ray strikes the surface.
Second Law: The angle of incidence is equal to the angle of reflection.
By law : θr = θi.
i.e., <i = <r
Angle of incidence (<i):The angle made by incident ray with the normal is called angle of incidence.
Angle of reflection (<r):The angle made by reflected ray with the normal is called angle of reflection.
Reflecting Surfaces: The surfaces which reflect light are called reflecting surfaces.
Points to remember:
Light rays are not real. They are merely used to show the path that light travels.
Courtesy:R.A.Banwat
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