Presentation on: INTRODUCTION TO INTERFERENCE SUBMITTED BY: HIMANI BSC-II NM 1180

COHERENT SOURCES Two light waves are said to be coherent if they emit continuous light waves of…….. Same wavelength or frequency. Same phase or constant phase difference. Two independent sources of light can never be coherent because it is practically impossible that they may emit light waves of same phase or constant phase difference. Hence two coherent sources are made by taking one source and its image which act as second source OR two images of same source.

INTERFERENCE OF LIGHT Consider two coherent sources of light sending out light waves simultaneously in a medium .At a certain position crest due to waves from one source falls on the crest of other. Or trough of one wave falls on the trough of other OR vice versa …..interference takes place. The phenomenon of redistribution of light energy in the medium due to superposition of light waves from two coherent sources of light is called interference of light. Only redistribution of energy takes place…however the total energy remains constant.

CONDITION FOR INTERFERENCE Two sources of light must be coherent. Sources must be narrow……A broad source is equivalent to a number of narrow sources placed side by side. Each pair will produce its own interference pattern and different patterns will overlap…. Distance between sources and screen should b large…This increases the fringe width and hence fringes become more prominent…. Distance between sources should be small…This also increases fringe width….

CONDITION FOR INTERFERENCE Amplitude of light waves from two sources should preferably be equal…This increases contrast between maxima and minima…. Sources should b preferably monochromatic…If sources are white we get a central white fringe surrounded by a few colored fringe and their general elimination occurs due to overlapping of different colors….

TYPES OF INTEREFENCES CONSTRUCTIVE INTERFERENCE: Such interference are formed when crest due to one wave falls on the crest due to another…..or…..trough due to one wave falls on the trough due to another…. At this place energy becomes maximum and a MAXIMA is produced…..

TYPES OF INTERFERRENCE DESTRUCTIVE INTERFERENCE: Such interference patterns are observed when crest due to one wave falls on trough of another…or….vice versa… At this place energy becomes minimum and a MINIMA is produced…

CLASSIFICATION OF INTERFERENCE The production of two interfering beams of light from a same source can b studied under 2 main headings.. Division of wavefront: In this case wavefront is divided into parts by utilizing the phenomenon of reflection, refraction or diffraction, in such a manner that the two parts must form coherent sources…. As represented by Young’s experiment Division of amplitude: In this case we get two or more beams from a single wave by partial reflection or refraction.These waves are then made to interfere because they satisfy the requirements of coherence…. As represented by Newton’s ring

YOUNG’S DOUBLE SLIT EXPERIMENT

In the early 1800's (1801 to 1805), Thomas Young conducted an experiment. He allowed light to pass through a slit in a barrier so it expanded out in wave fronts from that slit as a light source. That light, in turn, passed through a pair of slits in another barrier (carefully placed the right distance from the original slit). Each slit, in turn, diffracted the light as if they were also individual sources of light. The light impacted an observation screen.

When a single slit was open, it merely impacted the observation screen with greater intensity at the center and then faded as you moved away from the center. There are two possible results of this experiment: Particle interpretation: If light exists as particles, the intensity of both slits will be the sum of the intensity from the individual slits. Wave interpretation: If light exists as waves, the light waves will have interference under the principle of superposition, creating bands of light (constructive interference) and dark (destructive interference).

IMPACT OF YOUNG’S DOUBLE SLIT EXPERIMENT At the time, this seemed to conclusively prove that light traveled in waves, causing a revitalization in Huygen's wave theory of light, which included an invisible medium, ether, through which the waves propagated. Several experiments throughout the 1800s, most notably the famed Michelson-Morley experiment, attempted to detect the ether or its effects directly. They all failed and a century later Einstein's work in the photoelectric effect and relativity resulted in the ether no longer being necessary to explain the behavior of light. Again a particle theory of light took dominance.

EXPRESSIONS RELATED TO YOUNG’S EXPERIMENT The first bright fringe is observed at P. Angle PMO is θ N is a point on BP such that NP = AP Since P is the first bright fringe BN = λ For small values of θ, AN cuts MP at almost 900 giving angle MAQ = θ and hence angle ΒΑΝ = θ Again providing θ is very small, sin θ = tan θ = θ in radians

From triangle BAN: θ = λ / d and from triangle PMO: θ = Δx / D So λ / d = Δx / D Therefore

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