Physical Principles of Light Introduction.
Physical Principles of Light depends on Reflection, Refraction, Dispersion , Polarization etc. Light terms refers to an electromagnetic radiation that can be seen by naked eye and in physical sense of wider range ; light covers larger area of electromagnetic radiation ranges between microwave radiation to X-rays. Multicolored rainbows, blue skies, green forests, etc. can be enjoyed by those who have
eyes with which to see them.
By studying the branch of physics called optics, which deals with the behavior of light and other electromagnetic waves, we can reach deeper appreciation of the visible world. A knowledge of the properties of light allows us to understand the colors of the rainbow and designs of the optical devices such as telescopes, microscopes, cameras, eyeglasses and the human eyes. The same basic principles of light also lie at the heart of the some modern equipment like laser, optical fibres, holograms, optical computers and new techniques in medical imaging.
Until the time of Issac Newton (1642–1727), most scientists thought that light consisted of streams of particles (called corpuscles) emitted by light sources. Galileo and others tried to measure speed of light. Around 1665, it was evident that light has wave properties. In 1873, James Clark Maxwell predicted the existence of electromagnetic waves and calculated its speed of propagation. This development along with the work of Heinrich Hertz in 1887 showed conclusively that light is indeed an electromagnetic wave.
The wave picture of light does not reveal the whole story. Several effects associated with emission and absorption of light concludes a particle aspect, in that the energy carried by light waves is packed in discrete bundles called photons or quanta. These apparently contradictory wave and particle properties have been reconciled in 1930’s with development of quanta electrodynamics which is a comprehensive theory that includes both wave and particle properties. The propagation of light is best described by a wave model and the understanding of emission and absorption requires a particle approach.
The fundamental sources of all electromagnetic radiation are electric charges in accelerated motion. All bodies emit electromagnetic radiation as a result of thermal motion of their molecules. This radiation called thermal radiation is a mixture of different wavelengths. At sufficiently high temperatures, all matter emits enough visible light to become luminous. Thus, hot matter in any form is a source of light.
Familiar examples are: incandescent lamp, flame of a candle, coils in an electric heater, etc. Light is also produced during electrical discharges through ionized gases. The bluish light of mercury arc lamp, the orange-yellow light of sodium vapor lamp and various colors of neon sign boards are common examples. A variation of the mercury arc lamp is a fluorescent lamp. This light source uses a material called a phosphor to convert the ultraviolet radiation from a mercury arc into a visible light. This conversion makes fluorescent lamps more efficient than the incandescent lamps in converting electrical energy into light.
A light source that has attained prominence in recent years is LASER. It is an acronym of Light Amplification of Stimulated Emission of Radiation. In most light sources, light is emitted independently by different atoms within the source. In a laser, by contrast, atoms are induced to emit light in a cooperative, coherent fashion. The result is a very narrow beam of radiation that can be enormously intense and that is monochromatic, i.e. having single frequency than light from any other source. Laser now a days is used by physiotherapists for treatment purposes.
Reflection and refraction.
The ray model of light explains two of the most important aspects of light propagation are Reflection and refraction. In a homogeneous medium, light travels along a straight path. When a light wave strikes a smooth interface separating two transparent materials (such as air and glass or water and glass), the wave is generally partly reflected and partly refracted (transmitted) into the second material . The phenomenon of change in path of light as it goes from one medium to another is called refraction.
Laws of reflection and refraction.
1. The incident, reflected and refracted rays and the normal to the surface all lie in the same plane . The plane of the three rays is perpendicular to the plane of the boundary surface between the two materials.
2. The angle of reflection is equal to the angle of incidence for all wavelengths and any pair of materials. This relation, together with the observation that the incident and reflected rays and the normal all lie in the same plane is called the law of reflection.
3. For monochromatic light and for a given pair of materials, a and b, on opposite sides of the interface, the ratio of the sines of the angles where both angles are measured from the normal to the surface, is equal to the inverse ratio of the two indexes of refraction:
This experimental result, together with the observation that the incident and refracted rays and the normal all lie in the same plane, is called the law of refraction or Snell’s law, after the Dutch Scientist Willebrord Snell.
Characteristic of the image formed by a plane mirror.
1. Image is as far as behind the mirror, as the object is in front of the mirror.
2. The size of the image is same as that of the object.
3. The image formed is virtual in nature.
4. The image formed is erect in nature.
5. The image formed is laterally inverted. The lateral inversion means that the right side of the object appears as the left side of the image and vice versa.
The portion of a reflecting surface, which forms part of a sphere is called a spherical mirror.
The spherical mirrors are of two types.
1. Concave spherical mirror: A spherical mirror whose reflecting surface is toward the centre of the sphere of which mirror forms a part is called concave spherical mirror.
2. Convex spherical mirror: A spherical mirror whose reflecting surface is away from the centre of the sphere of which mirror forms a part is called convex spherical mirror.
3. Pole: The centre of spherical mirror is called its pole.
4. Principal axis: The line joining the pole and the centre of curvature of the mirror is called the principal axis of the mirror.
5. Centre of curvature: The centre of sphere of which mirror forms a part is called the centre of curvature of the mirror.
6. Radius of curvature: The radius of sphere of which mirror forms a part is called the radius of curvature of the mirror.
7. Aperture: The diameter of the mirror is called aperture of the mirror.
8. Principal focus: The point at which a narrow beam of light incident on the mirror parallel to its principal axis after reflection from the mirror meets or appears to come from is called principal focus of the mirror.
9. Focal length: The distance between the pole and the principal focus of the mirror is called the focal length of the mirror.
Applications of plane or curved mirrors .
1. Concave mirrors are used for dressing up or used as make up mirrors. It is because a person keeps his body or face between pole and focus of the concave mirror, a highly magnified image of his body or face is formed.
2. Concave mirrors are used by dental surgeons for examining dental cavities.
3. Concave mirrors are used by ophthalmologists for examining the eye.
4. Concave mirrors are used as reflectors in cinema projectors, magic lanterns, etc.
5. Concave mirrors are used to make reflecting type astronomical telescope of large aperture.
6. Concave parabolic mirrors are used in search lights.
7. Convex mirrors are used in vehicles as drivers mirror. The driver of the vehicle can get a clear and much wider field of view of the objects behind him.
8. Convex mirrors are used as a safety feature at sharp turns or dangerous corners of the road. These are also used to prevent shop lifting activities in the market.
Dispersion .
Ordinary white light is a superposition of waves with wavelengths extending throughout the visible spectrum. The speed of light in vacuum is the same for all wavelengths, but the speed in a material substance is different for different wavelengths. Therefore, the index of refraction of a material depends on wavelength. The dependence of wave speed and index of refraction on wavelength is called dispersion.
The phenomenon of splitting up of white light into its constituent colors is called dispersion of light. If a beam of white light is made to fall on one face of a prism, the light emerging from the other face of the prism consists of seven colors namely violet, indigo, blue, green, yellow, orange and red. The deviation suffered by the violet color is maximum, while that by the red is minimum. The band of seven colors produced at the screen is called spectrum .
Scattering of light.
The sky is blue. Sunsets are red. Skylight is partially polarized; that’s why the sky looks darker from some angles than from others when it is viewed through polarized sunglasses. It turns out that one phenomenon is responsible for all of these effects. When you look at the daytime sky, the light you see is sunlight that has been absorbed and then reradiated in a variety of directions. This process is called scattering.
When light falls on particles of large size such as dust and water droplets, it does not get scattered. However, when light travels through the atmosphere, it gets scattered from the air molecules. The blue light (light of smaller wavelength) is scattered more than red light (light of longer wavelength), when the light travels through the atmosphere. Sir CV Raman was awarded Nobel prize for his work on elastic scattering of light by molecules. It is popularly known as Raman’s effect.
Wavefront .
According to wave theory of light, a source of light sends out disturbances in all directions. In a homogenous medium, the disturbances reaches all those particles of the medium in phase with each other and therefore at any instance, all such particles must be vibrating in phase with each other. The locus of all the particles of the medium, which at any instant are vibrating in the same phase is called the wavefront.
Depending upon the shape of the source of light, wavefront can be of following types.
Spherical wavefront: A spherical wavefront is produced by a point source of light.
Cylindrical wavefront: When the source of light is linear in shape (such as a slit), a cylindrical wavefront is produced.
Plane wavefront: A small part of a spherical or a cylindrical wavefront originating from a distant source will appear plane and hence called a plane wavefront .
Huygens’ principle.
Huygens’ principle is a geometrical construction which is used to determine the new position of a wavefront at a later time from its given position at any instant. In other words, Huygens’ principle gives a method to know as to how light spreads out in the medium.
Huygens’ principle is based upon the following assumptions.
1. Each point on the given or primary wavefront acts as a source of secondary wavelets, sending out disturbances in all directions in a similar manner as the original source of light does.
2. The new position of the wavefront at any instant (called secondary wavefront) is the envelope of the secondary wavelets at that instant.
Interference of light .
When a source of light emits energy, the distribution of energy is uniform in the medium, but when two sources of light lie close to each other and emit light of same wavelength and preferably of same amplitude, then due to superposition of waves from the two sources, the distribution of light energy no longer remains uniform.
The phenomenon of non-uniform distribution of energy in the medium due to superposition of two light waves is called interference of light. At some points in the medium, the intensity of light is maximum (constructive interference), while at some other points, the intensity is minimum (destructive interference). Thomas Young (1801) demonstrated the interference of light experimentally. His experiment led to the conclusion that light has a wave nature.
Diffraction .
The phenomenon of bending of light round the sharp corners and spreading into the regions of the geometrical shadow is called diffraction. The light waves are diffracted only when the size of the obstacle is comparable to the wavelength of the light. All types of wave motion exhibit diffraction effect. Sound waves or radiowaves shows diffraction effect in day-to-day life.
Polarization.
In general, waves are of two types:
1. Longitudinal waves: The waves in which particles oscillate along the direction of propagation of the waves are called longitudinal waves.
2. Transverse waves: The waves in which direction of oscillation of particles is perpendicular to the direction of propagation of the waves are called transverse waves.
Both types of waves exhibit the phenomenon of reflection, refraction, diffraction and interference but polarization of the waves is only exhibited by the transverse waves. Polarization is characteristic of all transverse waves. This is the only phenomenon where two types of waves essentially differ from one another.
When a wave has only y-displacements, we say that it is linearly polarized in y-direction; a wave in z-displacements is linearly polarized in the z-direction. The phenomenon due to which the vibrations of light are restricted in a particular plane is called the polarization of light. For mechanical waves we can build a polarizing filter, or a polarizer that permits only waves with a certain polarization direction to pass. Commonly used polarizers are tourmaline crystal or Nicol prism.
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