Osm Physics by Ashish

  Fresnel’s biprism is an optical device used to produce two coherent virtual sources from a single monochromatic source. It is used to demonstrate the interference of light and to determine the wavelength of monochromatic light . A Fresnel biprism consists of two thin prisms joined at their bases. It is equivalent to two thin prisms placed base-to-base. Structurally, the biprism acts as a single piece of optical glass with one obtuse angle (approx. 179°) and two small base angles (approx. 0.5°). When light from a narrow slit S S  falls on the biprism: The upper half deviates the light in one direction. The lower half deviates the light in the opposite direction. The rays appear to come from two virtual images, S 1 S_1 ​ and S 2 S_2 ​ , of the original source S S . Thus, S 1 S_1 ​ and S 2 S_2 ​ act as two coherent sources and produce interference fringes on the screen.

  There are two different methods of producing coherent sources: By division of the wavefront  Young's Double-Slit Experiment : A single source of monochromatic light illuminates a barrier with two closely spaced slits. The light emerging from these two slits acts as coherent sources. Fresnel's Biprism Method : A single wavefront is refracted through two adjacent, acute-angled prisms to produce two coherent virtual sources By division of the amplitude Newton's Rings: Light reflects back and forth between a spherical lens and a flat glass plate, dividing the amplitude to produce a coherent interference pattern.  Michelson's Interferometer: A beam splitter divides the amplitude of a light beam into two paths, which are then reflected by mirrors and recombined to create interference.

  Young’s Double Slit Experiment, Types of fringes According to Huygens’ principle , every point on a wavefront acts as a source of secondary wavelets. In Young’s experiment, light from a monochromatic source illuminates two narrow slits S 1 S_1 ​ and S 2 S_2 ​ .  These two slits behave as coherent sources because the waves emerging from them have: The same frequency A constant phase difference The same wavelength When the waves from S 1 S_1  and S 2 S_2 ​ overlap on the screen, they produce interference fringes consisting of alternate bright and dark bands. When two waves travel through the same medium simultaneously, the resultant displacement at any point is equal to the algebraic sum of the individual displacements produced by each wave. If the displacements due to two waves are: y 1 = a 1 sin ⁡ ω t and y 2 = a 2 sin ⁡ ( ω t + ϕ ) y_2=a_2\sin(\omega t+\phi) then, according to the principle of superposition: y = y 1 + y 2 \boxed{y=y_1+y_2} where y is ...

  Huygens' Principle (also called the Huygens–Fresnel Principle ) is a fundamental concept in wave optics proposed by the Dutch physicist Christiaan Huygens in 1678. It provides a geometric method for understanding wave propagation. The Principle Every point on a wavefront can be considered a source of secondary spherical wavelets (called secondary wavelets ). The new wavefront at a later time is the envelope of all these secondary wavelets. Key Concepts Primary Wavefront – The original advancing wave surface. Secondary Wavelets – Each point on the primary wavefront acts as a new point source, emitting tiny spherical waves in all directions. Envelope – The tangent surface (envelope) that touches all secondary wavelets at a given later instant forms the new wavefront .

Interference is a phenomenon in which two coherent waves are combined by adding their intensities or displacements with due consideration for their phase difference. The resultant wave may have a greater amplitude or a lower amplitude if the two waves are in phase or out of phase, respectively. Coherent sources of light If the phase difference the waves from two sou sources reaching any point in space does not change with time rather it remains constant then these sources of light are known as 'coherent sources of light'. This property of the waves is known as coherence .  A wave which appears to be a pure sine wave for an infinitely large period of time or in an infinitely extended space is said to be a perfectly coherent wave. Thus there are two different criteria of coherence. (i) Temporal coherence and (ii) Spatial coherence. (i) Temporal Coherence Definition: Temporal coherence refers to the correlation between the phase of a wave at a given instant of time.  It is a me...