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what are direction and the wave theory of Light?

7 years ago


Answers : (1)


When light passes through a narrow slit, the light beams not only flare out far beyond the geometrical shadow of the slit, they also give rise to a series of alternating light and dark bands that resemble interference fringes.

Although diffraction was already known at the time of Huygens and Newton, neither of them believed that it provided evidence that light must be a wave. Newton in particular believed that light traveled as stream of particles.

A strong proponent of the wave theory of light was the French engineer Augustin Fresnel. Fresnel explained diffraction based on the wave theory, which was not widely accepted even after Thomas young’s experiments on double slit interference. In 1819, Fresnel submitted a paper on his theory of diffraction in a competition sponsored by the French Academy of Science. Once of the member of the Academy, Simeon Denis Poisson and ridiculed Fresnel’s diffraction theory led to the “absurd” prediction that the shadow of an opaque object should have a bright spot at its centre.

Above figure shows that generalized detraction situation. The curve surface on the left represent wavefronts of the incident light. The light falls on the diffracting object B, which we show in figure. Figure as an opaque barrier containing an aperture of arbitrary shape.

In the figure C is a screen or photographic film that receives the light that passes through or around the diffracting object.

We can calculate the pattern of light intensity on screen C by subdividing the wavefront into elementary areas dA, each of which becomes a source of an expanding Huygens wavelet. The light intensity at an arbitrary point p is found by superimposing the wave disturbances caused by the wavelets reaching p from all these elementary sources.

The wave disturbances reaching p differ in amplitude and in phase because (1) the elementary sources are varying distances from P, (2) the light leaves the elementary sources at various angles to the normal to the wave diffraction occurs when coherent wavefronts of light fall on opaque barrier B, which contains an aperture of arbitrary shape. The diffraction pattern can be viewed on screen C, front, and (3) somesources are blocked by barrier B, while others are not. Diffraction calculations, which are simple in principle, may become difficult in practice. The calculation must be repeated for every point on screen C at which we wish to know the light intensity. We followed exactly this program in calculation the double slit intensity pattern in above section. The calculation there was simple because we assumed only two elementary sources, the two narrow slits.

7 years ago

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