Wavelength of Light

Young’s double-slit experiment can be used to find the wavelength of light.

The distance between the two slits and between the barrier and the screen are kept constant.

At points $P_{1}$ and $P_{2}$ , there are bright bands on both sides. But, the distance $\bar{S_{2} P_{1}}$ is longer than $\bar{S_{1} P_{1}}$ by a wavelength $λ$ which makes the two waves in phase at $P_{1}$ .

At point $P_{0}$ , the phase angle between the two waves is zero. Therefore, a central bright band appears.

Large Triangle

The in this can be calculated using trigonometry.

$\tan θ$ $= \frac{x}{L}$

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The point $P_{1}$ is located at the center of the first bright fringe.

The path difference between the two wavefronts is:
$λ = \bar{S_{2} P_{1}} - \bar{S_{1} P_{1}}$

wavelength from double-slit investigation:

λ = \frac{x d}{L}

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INTERFERENCE