Optics

About OPTICS

Optics is the branch of physics which deals with the study of light. It is mainly divided into three parts :

• Geometrical optics or ray optics : It deals with the reflection and refraction of light.
• Wave or physical optics : It concerned with nature of light and deals with interference, diffraction, polarisation.
• Quantum optics : It deals with the interaction of light with the atomic entities of matter such as photo electric effect, atomic excitation etc.

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Light is produced by one of two methods:

• Incandescence is the emission of light from "hot" matter (T ≳ 800 K).

• Luminescence is the emission of light when bound electrons fall to lower energy levels.

SOURCES OF LIGHT :

Those body which emit light in all directions are called sources of light. The sources can be point one or extended one. The sources of light are of two types :

(i) Luminous sources : Those objects which by itself emit light are called luminous sources.

e.g. Sun and stars (natural luminous sources), electric lamps, candles and lanterns (artificial luminous sources).

(ii) Non-luminous sources : Those objects which do not emit light but become visible only when light from luminous objects falls on them. They are called non-luminous sources.

e.g. Moon, planets (natural non- luminous sources), wood, table (artificial non-luminous sources) etc.

MEDIUM OF LIGHT :

Substance through which light propagates or tends to propagate is called a medium of light. According to the medium of light objects are divided into three parts :

(i) Transparent object : Bodies that allow light to pass through them i.e. transmit light through them, are called transparent bodies.

e.g. Glass, water, air etc.

(ii)Translucent object : Bodies that can transmit only a part of light through them are called translucent objects.

e.g. Froasted or ground glass, greased paper, paraffin wax etc.

(iii) Opaque object : Bodies that do not allow light to pass through them at all are said to be opaque objects.

e.g. Chair, desk etc.

RECTILINEAR PROPAGATION OF LIGHT:

Light travels in a straight line. In vacuum or air, light travels with the velocity of 3 × 108m/s.

SPEED OF LIGHT :

• The speed of light depends upon the medium through which it travels.

• The speed of light in a vacuum is a universal constant in all reference frames.

• All electromagnetic waves propagate at the speed of light in a vacuum.

• The speed of light in a medium is always slower than the speed of light in a vacuum. (The difference is usually negligible when the medium is air.)

• The speed of anything with mass is always less than the speed of light in a vacuum. (The speed of light in a vacuum is the universal speed limit.)

• The speed of light in a vacuum is fixed at 299,792,458 m/s by the current definition of the meter.

THE AMPLITUDE OF A LIGHT WAVE IS RELATED TO ITS INTENSITY.

• Intensity is the absolute measure of a light wave's power density.

• Brightness is the relative intensity as perceived by the average human eye.

THE FREQUENCY OF A LIGHT WAVE IS RELATED TO ITS COLOR:

• Color is such a complex topic that it has its own section in this book.

• Monochromatic light can be described by only one frequency.

• Laser light is very nearly monochromatic.

• There are six simple, named colors in English (and many other languages) each associated with a band of monochromatic light. In order of increasing frequency they are red, orange, yellow, green, blue and violet.

• Polychromatic light is composed of multiple frequencies.

• Every light source is essentially polychromatic.

• White light is very polychromatic.

• A graph of relative intensity vs. frequency is called a spectrum (plural: spectra).Although frequently associated with light, the term can be applied to many phenomena.

• A continuous spectrum is one in which every frequency is present within some range.

• Blackbody radiators emit a continuous spectrum.

• A discrete spectrum is one in which only a set of well defined and isolated frequencies are present.(A discrete spectrum is a finite collection of monochromatic light waves.)

• The excited electrons in a gas emits a discrete spectrum.

• The wavelength of a light wave is inversely proportional to its frequency:Frequency is respresented by (v) and is inversely proportional to its wavelength represented by ()

V ∝ 1/λ

v = c λ

(c = velocity of light)

• Light is often described by its wavelength in a vacuum.
• Light ranges in wavelength from 400 nm on the violet end to 700 nm on the red end of the visible spectrum.

• Wavelengths slightly shorter than 400 nm are said to be ultraviolet. (They are "beyond violet" in terms of frequency.)

• Wavelengths slightly longer than 700 nm are said to be infrared. (They are "below red" in terms of frequency.)

PHASE DIFFERENCES BETWEEN LIGHT WAVES CAN PRODUCE VISIBLE INTERFERENCE EFFECTS:

(There are several sections in this book on interference phenomena and light.)

BEHAVIOUR OF LIGHT AT THE INTERFACE OF TWO MEDIA:

When light travelling in one medium falls on the surface of a second medium, the following three effects may occur:

• A part of the incident light is turned back into the first medium. This phenomenon is called reflection of light.
• A part of the incident light is transmitted into the second medium along a changed direction. This phenomenon is called refraction of light.
• The remaining third part of light energy is absorbed by the second medium. This phenomenon is called absorption of light. 

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