Physics – Wave Theory of Light Additional Questions

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Brief Account of Wave Newton’s corpuscular Theory:

  • Every source of light emits a large number of tiny particles known as corpuscles in a medium surrounding the source.
  • These corpuscles are perfectly elastic, rigid and weightless.
  • The corpuscles travel in a straight line with very high speeds, which are different in different media.
  • Explanation of Sensation of Vision When corpuscles fall on the retina of the eye, they produce the sensation of vision.
  • The different colours of light are due to the different sizes of the Corpuscles.

Demerits of Newton’s Corpuscular Theory:

  • The corpuscular theory fails to explain partial reflection and refraction at the surface of transparent media.
  • It fails to explain the phenomenon like interference, polarization and diffraction of light.
  • The theory predicted that the velocity of light is more in the optically denser medium than that in rarer medium. Foucault’s experimental observation disproved it.
  • As Corpuscles were assumed as particles, there should be a continuous decrease in the mass of a source which emits light continuously for a long time. There was no experimental evidence for it.

Merits of Huygens’s Wave Theory of Light:

  • On the basis of wave theory, the phenomenon of reflection, refraction, diffraction, interference, and total internal reflection of light could be explained.
  • Wave theory correctly predicted that velocity of light in an optically denser medium is less than that in a rarer medium which is in agreement with experimental results.

Demerits of Huygens’s Wave Theory of Light:

  • Wave theory assumes the existence of luminiferous ether. However, experimental attempts to detect the presence of ether particle were unsuccessful.
  • Huygen’s theory could not explain the rectilinear propagation of light.
  • The phenomena of the photoelectric effect, Crompton effect in Modern Physics and polarization cannot be explained using wave theory.

Spherical Wavefront:

Wave Theory of Light Spherical Wave Front

  • The spherical wavefront can be obtained from a point source.
  • Consider a point source of light S placed in air.  The source will emit waves of light in all possible directions. If the velocity of light in air is c, then in time ‘t’, each wave will cover a distance ct. Therefore, at the end of the time interval ‘t’  the light emitted, by, the source will reach at all points as a sphere with centre S and radius equal to ct. All the points on the surface of this sphere are in the same phase.  Such a spherical surface is called spherical. wavefront.

Plane Wavefront:

Wave Theory of Light Plane Wave Front

  • At a very large distance from a point source of light, the spherical wavefront is so large that a small part of its surface can be considered as a plane surface i.e. a plane wavefront.
  • A plane wavefront is obtained only at an infinite distance from a point source.
  • Light emerging out of a convex lens when a point source is placed at its focus produces a plane wavefront.

Cylindrical Wavefront:

Wave Theory of Light Cylindrical Wave Front

  • If a source of light is linear i.e. in the form of a slit the wavefront originating from it has the shape of a cylinder.  Such a wavefront is called a cylindrical wavefront.

Laws  of Reflection:

  • The angle of incidence is equal to the angle of reflection
  • The incident ray and the reflected ray lie on either side of the normal at the point of incidence
  • The incident ray, reflected ray and the normal at the point of incidence lie in the same plane.

Laws  of Refraction:

  • The ratio of the sine of the angle of incidence to the sine of the angle of refraction is always constant and is equal to the refractive index of the medium.  This law is known as Snell’s law.
  • The incident ray and the refracted ray lie on the opposite side of the normal at the point of incidence.
  • The incident ray, refracted ray and the normal at the point of incidence lie in the same plane.

To Show Velocity of Light in Rarer Medium is Greater Than Velocity in Denser Medium:

  • From Huygens explanation diagram of refraction of light, we can see that

∠ i > ∠ r

∴ sin i > sin r

∴ sin i / sin r > 1

By Snell’s law sin i / sin r = μ = refractive index of medium

∴ μ > 1

Hence velocity of light in a rarer medium is greater than that in a denser medium

Proof of Brewster’s Law:

  • The tangent of the polarizing angle is equal to the refractive index of the material of the surface from which reflection is taking place.
  • The angle of incidence of ordinary light at which reflected light from a transparent medium is completely plane polarized is called polarizing angle.
  • Mathematically,     μ = tan ip.
  • This angle characteristic of the medium.

Polarized Light Partial polarization 01

ip = angle of incidence and r = angle of refraction

  • Proof: 
  • Let us consider the unpolarized monochromatic light incident in air at the polarizing angle ip on the plane surface XY of the transparent medium of refractive index μ. Experimentally Brewster proved that ∠ SQR = 90°.

Polarized Light Partial Brewsters Law

Use of Doppler Effect of Light to Measure Speed of Rotation of the Sun

  • A spectrum is an arrangement of electromagnetic radiation, which includes visible light, placed in order of wavelength. The spectrum of light tells us about the composition of an object such as a star, its temperature, its pressure, the abundance of elements in the star, its motion (velocity), etc.
  • we observe at the spectrum from different regions. If the lines are shifted towards the red end (longer wavelengths) relative to a spectrum at rest than that part of the Sun is moving away from us; a blue shift tells us that region of the Sun is approaching us. The extent of the shift tells us the velocity.
  • The east and west edges of the sun are photographed. Each photograph contains absorption lines due to elements like vapourized ion in the sun and oxygen in the earth’s atmosphere. Then the two photographs are put together such that the oxygen lines coincide. We observe a relative displacement in the iron lines w.r.t. each other. One line corresponds to the edge of the sun approaches the earth while another line corresponds to the other edge of the sun that recedes from the earth.
  • The difference in spectral lines is used to measure the speed of rotation of the sun. It is nearly 2 km/s.

Use of Doppler Effect of Light to Measure Plasma Temperature:

  • Plasma is phase constituting very hot gases where the temperature is of the order of millions of degree Celsius. Plasma state is obtained in thermonuclear fusion experiments.
  • In this state, the gas molecules are moving towards or away from the observer at very high speeds. Due to the Doppler effect, the wavelength observed of a particular spectral line is different from the actual wavelength.
  • one edge of the line corresponds to an apparently increased wavelength due to the molecules directly moving towards the observer while the other edge of the line corresponds to an apparently decreased wavelength due to the molecules directly moving away from the observer. The line is thus observed to be broadened. A diffraction grating is used to measure the breadth of line.
  • As we know the wavelength λ and speed of light c the velocity v can be obtained using the formula

Doppler Effect of Light

  • Where R is the molar gas constant, T is the temperature of the gas, M is the molecular mass of the gas.

Characteristics of Wavefront:

  • A  locus of all the points of a medium, to which light waves reach simultaneously so that all these points are in the same phase is called wavefront.
  • Wavefront travels with the speed of light in all directions in an isotropic medium.
  • They always travel in the forward direction.
  • The perpendicular line drawn at any point on the wavefront represents the direction of propagation of the wave at that point and is called the wave normal or ray of light.

Characteristics of Primary Source of Light:

  • It is a real source of light.
  • It generates life itself.
  • It sends primary waves in all the direction.
  • Generally, the primary source of light is in the air.

Characteristics of Secondary Source of Light:

  • It is a fictitious source of light
  • A secondary source of light is one that accepts incoming light and transforms it in some way, via reflection, refraction, diffusion, etc.
  • It sends out secondary waves only in forward direction.
  • The secondary source is situated on the wavefront.
Science > Physics > Interference of LightYou are Here
Physics Chemistry  Biology  Mathematics

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