Huygens’s Principle
Huygens’s construction:
Huygens’s construction is a geometrical method which is used to determine the position and nature of given wavefront at a later instant. Huygen’s construction is based on the Huygen’s principle
Huygen’s Principle:
Each and every point on a wavefront acts as a secondary source of light. They will emit secondary waves in all possible directions with the same velocity in the same medium. The tangential surface which touches all the secondary wavelets, at later instant is position of the new wavefront at that instant.
Huygen’s Construction of a spherical wavefront and plane Wavefront
Huygen’s construction of a spherical wavefront:
Consider a point source S of light, placed at a point in vacuum or in air medium. The wavefront originating from the point source of light at finite distance is spherical wavefront. Let PQR be the initial position of the spherical wave front. According to Huygen’s principle, each and every point on the wavefront PQR acts as secondary source of light and emits secondary waves in all possible direction with the same velocity in the same medium. Let ‘v’ be the velocity of light in air medium. Hence to determine the position of new wavefront after time‘t’, draw spheres assuming each and every point of wavefront PQR as its centre and vt as its radius. The tangential surface P`Q`R` which touches all spheres in position of the new wavefront after time ‘t’. From figure it appears that there is a back wavefront, but the waves moving in backward direction does not exist.
Huygen’s construction of a plane wavefront:
Consider a point source ‘S’ of light placed at a point in vacuum or in air medium. The wavefront originating from the point source of light at very large distance is plane wavefront.
Let PQR be the initial position of the wavefront. According to huygen’s principle each and every point of the wavefront PQR acts as secondary source of light and emits secondary waves in all possible direction with the same velocity in the same medium. Let ‘v’ be the velocity of light in air medium. To determine the position of new wavefront after time ‘t’ draw spheres assuming each and every point of wavefront PQR as its centre and vt as its radius. The tangential surface P`Q`R` which touches all spheres is the position of new plane wavefront after time t. From figure it appears that there is a back wavefront but the waves moving in backward direction does not exist.
Sunday, August 1, 2010
Newton’s corpuscular theory
Newton’s corpuscular theory:
The corpuscular theory of light was proposed by Newton in 1675.
The main features of Newton’s corpuscular theory of light are as follows.
1. According to Newton, light consists of extremely small weightless and elastic particles. These particles are called as corpuscles.
2. These particles are emitted from a source of light in all directions.
3. These particles travel in a straight line with constant velocity in the same medium.
4. When these particles enter our eyes, we get a sensation of light.
5. The different colors of light are due to different sizes and masses of the particles.
6. These particles are repelled by the reflecting surfaces. On the basis of this, reflection of light can be explained.
7. These particles are attracted by the refracting surfaces. On the basis of this, refraction of light can be explained.
8. When these particles travel from one medium to other there will be change in direction and velocity of particles.
Drawback’s of Newton’s corpuscular theory:
1. As light consists of small particles, emission of particles from a source causes reduction in the mass of source of light. It has been experimentally proved that there is no reduction in the mass of the source of light.
2. If light is incident on a transparent material like glass or water, some part of light is reflected and some part of light is refracted. This simultaneous phenomenon of reflection and refraction cannot be explained on the basis of corpuscular theory.
3. According to Newton different colors of light are due to the different sizes and masses of the particles. But this fact was not experimentally proved.
4. The application of Newton’s theory to refraction concludes that velocity of light in air medium is less than velocity of light in denser medium but experimentally it was found to be wrong.
5. Several phenomena of light such as double refraction, interference, diffraction and polarization of light cannot be explained on the basis of corpuscular theory.
The corpuscular theory of light was proposed by Newton in 1675.
The main features of Newton’s corpuscular theory of light are as follows.
1. According to Newton, light consists of extremely small weightless and elastic particles. These particles are called as corpuscles.
2. These particles are emitted from a source of light in all directions.
3. These particles travel in a straight line with constant velocity in the same medium.
4. When these particles enter our eyes, we get a sensation of light.
5. The different colors of light are due to different sizes and masses of the particles.
6. These particles are repelled by the reflecting surfaces. On the basis of this, reflection of light can be explained.
7. These particles are attracted by the refracting surfaces. On the basis of this, refraction of light can be explained.
8. When these particles travel from one medium to other there will be change in direction and velocity of particles.
Drawback’s of Newton’s corpuscular theory:
1. As light consists of small particles, emission of particles from a source causes reduction in the mass of source of light. It has been experimentally proved that there is no reduction in the mass of the source of light.
2. If light is incident on a transparent material like glass or water, some part of light is reflected and some part of light is refracted. This simultaneous phenomenon of reflection and refraction cannot be explained on the basis of corpuscular theory.
3. According to Newton different colors of light are due to the different sizes and masses of the particles. But this fact was not experimentally proved.
4. The application of Newton’s theory to refraction concludes that velocity of light in air medium is less than velocity of light in denser medium but experimentally it was found to be wrong.
5. Several phenomena of light such as double refraction, interference, diffraction and polarization of light cannot be explained on the basis of corpuscular theory.
Wave front and wave normal
Wavefront and Wave Normal
Concept of wavefront :
Consider a point source ‘S’ of light placed at a point in air or vacuum. The point source of light will emit the light waves in all directions with same velocity in the same medium. Let ‘v’ be the velocity of light in air medium. In time ‘t’ all the waves covers a distance equal to ‘vt’. In the other words in time ‘t’ all the waves will reach on a spherical surface of radius vt and centre S. The spherical surface so formed is called wavefront.
It should be noted that all points on the spherical surface should be in the same state of vibration.
Definition of wavefront :
“The locus of all the points of the medium to which all the waves will reach simultaneously, so that all the points are in the same state of vibration is called wavefront”
Types of wavefronts:
There are three types of wavefronts.
1. Spherical wavefront.
2. Plane wavefront.
3. Cylindrical wavefront.
1. Spherical wavefront : A wavefront originating from the point source of light at finite distance is called spherical wavefront.
2. Plane wavefront : As time passes the spherical surface of spherical wavefront is so large that a small part of it can be considered as a plane wavefront.
A wavefront originating from the point source of light at infinite distance i.e. at very large distance is called as plane wavefront. The plane wavefront can be obtained by keeping point source at the focus of the convex lens.
The light from the sun reaches the earth in the form of plane wavefronts.
3. Cylindrical wavefront : A wavefront originating from a linear source of light is called cylindrical wavefront.
Wave normal :
A normal or perpendicular drawn to the surface of a wavefront at any point, in the direction of propagation of light, is called a wavenormal.
The wavenormal gives the direction of propagation of light.
Concept of wavefront :
Consider a point source ‘S’ of light placed at a point in air or vacuum. The point source of light will emit the light waves in all directions with same velocity in the same medium. Let ‘v’ be the velocity of light in air medium. In time ‘t’ all the waves covers a distance equal to ‘vt’. In the other words in time ‘t’ all the waves will reach on a spherical surface of radius vt and centre S. The spherical surface so formed is called wavefront.
It should be noted that all points on the spherical surface should be in the same state of vibration.
Definition of wavefront :
“The locus of all the points of the medium to which all the waves will reach simultaneously, so that all the points are in the same state of vibration is called wavefront”
Types of wavefronts:
There are three types of wavefronts.
1. Spherical wavefront.
2. Plane wavefront.
3. Cylindrical wavefront.
1. Spherical wavefront : A wavefront originating from the point source of light at finite distance is called spherical wavefront.
2. Plane wavefront : As time passes the spherical surface of spherical wavefront is so large that a small part of it can be considered as a plane wavefront.
A wavefront originating from the point source of light at infinite distance i.e. at very large distance is called as plane wavefront. The plane wavefront can be obtained by keeping point source at the focus of the convex lens.
The light from the sun reaches the earth in the form of plane wavefronts.
3. Cylindrical wavefront : A wavefront originating from a linear source of light is called cylindrical wavefront.
Wave normal :
A normal or perpendicular drawn to the surface of a wavefront at any point, in the direction of propagation of light, is called a wavenormal.
The wavenormal gives the direction of propagation of light.
Wave theory of light
Wave theory of Light
The wave theory of light was proposed by Huygen in 1678.
The main features of Huygen’s wave theory of light are as follows:
1. According to Huygen, light is propagated in the form of waves.
2. The waves are emitted from a source of light and travel with a very high velocity.
3. The waves can propagate in all possible directions with a constant velocity in a homogeneous medium.
4. When these waves enter our eyes, we get the sensation of light.
5. The different colors of light are due to the different wavelengths of the light waves.
6. To explain propagation of light through vacuum, Huygen assumed that there exist a hypothetical medium called as ether and light can propagate through ether.
Merit’s of Huygen’s wave theory:
1. On the basis of wave theory, phenomenon of reflection and refraction can be satisfactorily explained.
2. The application of wave theory to refraction concludes that the velocity of light in a rarer medium is greater than the velocity of light in a denser medium.
3. Several phenomena of light such as double refraction, interference diffraction of light can be satisfactorily explained on the basis of wave theory.
Demerit’s of Huygen’s wave theory:
1. Rectilinear propagation of light cannot be explained on the basis of the wave theory of light.
2. Several phenomena of modern physics such as photoelectric effect, Compton effect cannot be explained on the basis of the wave theory of light.
3. For the propagation of light through vacuum, Huygen assumed presence of ether medium. But such medium does not exist.
4. Huygens wave theory initially could not explain polarization light.
The wave theory of light was proposed by Huygen in 1678.
The main features of Huygen’s wave theory of light are as follows:
1. According to Huygen, light is propagated in the form of waves.
2. The waves are emitted from a source of light and travel with a very high velocity.
3. The waves can propagate in all possible directions with a constant velocity in a homogeneous medium.
4. When these waves enter our eyes, we get the sensation of light.
5. The different colors of light are due to the different wavelengths of the light waves.
6. To explain propagation of light through vacuum, Huygen assumed that there exist a hypothetical medium called as ether and light can propagate through ether.
Merit’s of Huygen’s wave theory:
1. On the basis of wave theory, phenomenon of reflection and refraction can be satisfactorily explained.
2. The application of wave theory to refraction concludes that the velocity of light in a rarer medium is greater than the velocity of light in a denser medium.
3. Several phenomena of light such as double refraction, interference diffraction of light can be satisfactorily explained on the basis of wave theory.
Demerit’s of Huygen’s wave theory:
1. Rectilinear propagation of light cannot be explained on the basis of the wave theory of light.
2. Several phenomena of modern physics such as photoelectric effect, Compton effect cannot be explained on the basis of the wave theory of light.
3. For the propagation of light through vacuum, Huygen assumed presence of ether medium. But such medium does not exist.
4. Huygens wave theory initially could not explain polarization light.
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