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Light Waves Each of Amplitude "a" and Frequency "ω", Emanating from Two Coherent Light Sources Superpose at a Point. - Physics

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Light waves each of amplitude "a" and frequency "ω", emanating from two coherent light sources superpose at a point. If the displacements due to these waves are given by y1 = a cos ωt and y2 = a cos(ωt + ϕ) where ϕ is the phase difference between the two, obtain the expression for the resultant intensity at the point.

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Let the displacement of the waves from the sources S1 and S2 at point P on the screen at any time t be given by:

y1 = a cos ωt


y2 = a cos (ωt + Φ)

where, Φ is the constant phase difference between the two waves

By the superposition principle, the resultant displacement at point P is given by:

y = y1 + y2

y = a cos ωt + a cos (ωt + Φ)


`y=2acos(omegat+phi/2)cos(phi/2)" ...(i)"`

Let 2 `acos(phi/2)=A ...(ii)"`

Then, equation (i) becomes:


Now, we have:

`A^2=4a^2cos^2(phi/2)" ..(iii)"`

The intensity of light is directly proportional to the square of the amplitude of the wave. The intensity of light at point P on the screen is given by:

`I=4a^2cos^2(phi/2)" ...(iv)"`

Concept: Principle of Superposition of Waves
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