Wave Optics
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Some Points
NATURE OF LIGHT
Newton's corpuscular theory of light
This theory was given by Newton.
* Characteristics of the theory
(i) Extremely minute, very light and elastic particles are being constantly emitted by all luminous bodies (light
sources) in all directions which are known as corpuscles.
(ii) These corpuscles travel with the speed of light..
(iii) When these corpuscles strike the retina of our eye then they produce the sensation of vision.
(iv) The velocity of these corpuscles in vacuum is 3 × 108 m/s.
(v) The different colours of light are due to different size of these corpuscles.
(vi) The rest mass of these corpuscles is zero.
(vii) The velocity of these corpuscles in an isotropic medium is same in all directions but it changes with the
change of medium.
(viii) These corpuscles travel in straight lines.
(ix) These corpuscles are invisible.
The phenomena explained by this theory
(i) Reflection and refraction of light.
(ii) Rectilinear propagation of light.
(iii) Existence of energy in light.
The phenomena not explained by this theory
(i) Interference, diffraction, polarisation, double refraction and total internal reflection.
(ii) Velocity of light being greater in rarer medium than that in a denser medium.
(iii) Photoelectric effect
Huygen's Wave theory of light
This theory was enunciated by Huygen in a hypothetical medium known as luminiferrous ether.
Ether is that imaginary medium which prevails in all space and is isotropic, perfectly elastic and massless.
The velocity of light in a medium is constant but changes with change of medium.
This theory is valid for all types of waves.
(i) The locus of all ether particles vibrating in same phase is known as wavefront.
(ii) Light travels in the medium in the form of wavefront.
(iii) When light travels in a medium then the particles of medium start vibrating and consequently a disturbance
is created in the medium.
(iv) Every point on the wavefront becomes the source of secondary wavelets. It emits secondary wavelets in
all directions which travel with the speed of light
(v) The tangent plane to these secondary wavelets represents the new position of wave front
The phenomena explained by this theory
(i) Reflection, refraction, interference, diffraction.
(ii) Rectilinear propagation of light.
(iii) Velocity of light in rarer medium being greater than that in denser medium.
Phenomena not explained by this theory
(i) Photoelectric effect
WAVEFRONT, VARIOUS TYPES OF WAVEFRONT
* Wavefront
The locus of all the particles vibrating in the same phase is known as wavefront.
* Types of wavefront
The shape of wavefront depends upon the shape of the light source from the wavefront originates. On this
basis there are three types of wavefronts.
(a) The phase difference between various particles on the wavefront is zero.
(b) These wavefronts travel with the speed of light in all directions in an isotropic medium.
(c) A point source of light always gives rise to a spherical wavefront in an isotropic medium.
(d) In anisotropic medium it travels with different velocities in different directions.
(e) Normal to the wavefront represents a ray of light.
(f) It always travels in the forward direction in the medium
INTERFERENCE OF LIGHT
When two light waves of same frequency with constant phase difference superimpose over each other, then the
resultant amplitude (or intensity) in the region of superimposition is different from the amplitude (or intensity) of
individual waves.
This modification in intensity in the region of superposition is called interference.
(a) Constructive interference
When resultant intensity is greater than the sum of two individual wave intensities [I > (I1 + I2)], then the
interference is said to be constructive.
(b) Destructive interference
When the resultant intensity is less than the sum of two individual wave intensities [I < (I1 + I2)], then the
interference is said to destructive.
There is no violation of the law of conservation of energy in interference. Here, the energy from the points
of minimum energy is shifted to the points of maximum energy
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