When light travels from an optically rarer medium to an optically denser medium the velocity decreases because of?

1. Theory of Light:

(i) Corpuscular Theory (Newton) 1575: It says that light source emits material particles (corpuscles) of small size with little mass-moving with the velocity of light.

(ii) Wave Theory (Huygens) 1678: Light moves in the form of waves through a hypothetical medium, called ether. The great success of this theory is that it can explain satisfactorily interference, diffraction, polarisation which could not be explained by earlier theory.

(iii) Electromagnetic Theory (Maxwell) 1873: Light consists of electromagnetic waves and can move in vacuum.

(iv) Quantum theory (Max Planck) 1901: Tight consists of particles, catted photons.

2. Huygen's wave theory:

According to Huygens principle, each point of the wavefront is the source of a secondary disturbance and the wavelets referred to as secondary wavelets emanating from these points spread out in all directions with the speed of the wave. If we draw a common tangent to all these spheres, we obtain the new position of the wavefront.

3. Wave front and ray:

(i) In a propagating wave, the locus of points, which oscillate in phase is called a wavefront. Thus, a wavefront is defined as a surface of constant phase.

(ii) The wavefronts of a point source will be spherical in shape and that of a distance source will be planar in shape.

(iii) Ray is the direction, in which light energy is transmitted. In a homogenous, Isotropic medium, ray is perpendicular to the wave front.

4. Success of wave theory:

It is experimentally found that the formula, as derived in wave theory, is correct showing that velocity of light in air is higher than that in water.

5. Application of wave theory:

The phenomenon of refraction of light could not be explained by the corpuscular model of light proposed by Newton, but it was successfully explained by the wave nature of light proposed by Huygens.

6. Interference:

(i) Superposition of waves: When there are two coherent sources of light of wavelength (λ), if the path difference at a point from the two sources is nλ, where nis an integer, then constructive interference occurs, but when it is nλ+λ2, destructive interference occurs.

(ii) Intensity is proportional to the amplitude, so at constructive interference, the amplitude doubles and intensity becomes four times.

(iii) If the phase difference of sources of amplitude (a) and intensity (I) at a point is, ϕ then the amplitude of the resultant wave is 2a cos⁡ϕ2 and the resultant intensity is 4I cos2⁡ϕ2.

7. Coherent sources:

Coherent source of light are those sources which emit a light wave having the same frequency, wavelength and in the same phase, or they have a constant phase difference.

8. Young's double slit experiment:

(i) In Young’s Double slit experiment, the path difference between two sources is approximately given as xdD, where D is the screen distance, d is the slit separation and x is the distance of the point on the screen from the centre.

(ii) At x=nλDd, intensity is maximum (bright fringe) and at x=n+12λDd, intensity is minimum (dark fringe).

9. Fringe width:

Fringe width is the distance between two successive maxima or minima, It is, w=λDd

10. Conditions for interference:

(i) Coherent sources of light are needed.

(ii) Amplitudes and intensities must be nearly equal to produce sufficient contrast between maxima and minima.

(iii) The source must be small enough that it can be considered a point source of light.

11. Diffraction:

It is the phenomena of bending of light around obstacle whose dimension is comparable to the wavelength of light.

11 (a). Faundhofer diffraction by a single slit:

(i) In single slit diffraction, the brightest spot is seen at θ=0. At θ=n+12λa, subsequent maxima are seen, and at θ=nλa, minima are seen. Where 2a is the slit width.

(ii) When a lens of focal length (f) is placed in front of the slit, the separation of the bright fringes on the screen will be fλa.

(iii) The radius of the bright spot formed by a lens is given by, r0=1.22λf2a, where f is the focal length of the lens and 2a is the diameter of the lens.

12. Revolving Power:

(i) The angular separation of two faraway objects to be viewed clearly by a lens is given by, Δθ=0.61λa

(ii) The resolution of a microscope is given by, dmin=1.22λa2nsin⁡β, where β is the angle made by the principal axis and the tip of the lens with object, and n is the refractive index of the medium. nsin⁡β is called numerical aperture.

(iii) The Fresnel distance is the distance from a lens beyond which the diffraction effects of the lens are comparable to the size of the lens. It is given by the formula, Z=a2λ, where a is the radius of the lens.

13. Polarization:

(i) Plane polarized wave is the wave in which the disturbances at all the points occur in the same plane.

(ii) A polaroid is a material which allows disturbances in only one direction called pass axis, thus producing a plane polarized wave. It also reduces the intensity of the unpolarized wave by half.

(iii) According to Malus' law, if a polarized light of intensity (I0) falls on a polaroid whose pass axis makes an angle of θ with the plane of incident light, then the intensity of the emitted light will be, I=I0cos2⁡θ

(iv) The angle of incidence at which the reflected wave will be totally polarized is called as the Brewster’s angle and it is given by, tan iB=μ. In this situation, the reflected and the refracted waves will be perpendicular to each other.

14. Electromagnetic spectrum:

(i) The spectrum of electromagnetic waves stretches, in principle, over an infinite range of wavelengths. Different regions are known by different names.

(ii) Electromagnetic waves in order of increasing wavelength are given as: γ-rays, X- rays, ultraviolet rays, visible rays, infrared rays, microwaves and radio waves.

(iii) Infrared waves are also known as heat waves.

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Wave light travels from an optically rarer medium to an optically denser medium its velocity decrease because of change in (A) frequency (B) wavelength (C) amplitude (D) phase

When light travels from an optically rarer medium to an optically denser medium, the speed decreases because of change in ______