# Q10.5    In Young’s double-slit experiment using monochromatic light of wavelength $\lambda$ , the intensity of light at a point on the screen where path difference is $\lambda$ , is K units. What is the intensity of light at a point where path difference is $\frac{\lambda}{3}$?

Given, in youngs double-slit experiment.

the wavelength of monochromatic light = $\lambda$

The intensity of light when the path difference is $\lambda$ = K

Now,

As we know,

The phase difference $\phi$ is given by

$\phi =\frac{2\pi }{\lambda }(PathDiffernce)$

also

Total Intensity

$I=I_1+I_2+2\sqrt{I_1I_2}cos\phi$

Let  $I_1=I_2=I_0$

Now, when path difference is $\lambda$

the phase difference angle

$\phi=\frac{2\pi }{\lambda}*\lambda=2\pi$

so,

$I_0+I_0+2\sqrt{I_0I_0}cos2\pi=K$

$K=4I_0$

Now, when path difference is    $\frac{\lambda }{3}$

$\phi=\frac{2\pi }{\lambda}*\frac{\lambda}{3}=\frac{2\pi}{3}$

Intensity of light

$K'=I_0+I_0+2\sqrt{I_0I_0}cos\frac{2\pi}{3}$

$K'=I_0$

Now comparing intensity at both cases

$\frac{K'}{K}=\frac{I_0}{4I_0}=\frac{1}{4}$

$K'=\frac{K}{4}$

Hence intensity will reduce to one-fourth of initial when path difference changes from $\lambda$ to    $\frac{\lambda}{3}$.

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