Young’s Double Slit Experiment

The source emits monochromatic light.(comprising waves of one frequency only)

The single slit ensures that the light waves incident on the double slits are coherent.

  • Coherent sources must have same frequency and constant phase difference. Constant phase difference means that the two sources always differ in phase by the same amount.

$x \approx \frac{\lambda D}{a}$, X is only valid for values of D >> a and when $\theta$ is very small. This equation cannot be used for diffraction grating!



$a \, sin \, \theta = n \lambda$

Factors affecting appearance of fringes due to double slit interference of light:

  • If the single slit together with the light source is moved closer to the double slits, the bright fringes produced are brighter as the intensity of the light passing through the double slit is greater. The fringe separation remains the same.
  • If one of the slit is closed, a different pattern is obtained. The central maximum is now much brighter and broader compared to the other maxima at the sides. This is the diffraction pattern for single slit.
  • If white light is used, the central fringe is white and the fringes on either side are coloured. Recall that $\lambda_{red} \, > \, \lambda_{blue}$, so $x_{red} \, > \, x_{blue}$. Blue is the colour nearer to the central fringe and red is farther away.
  • If the space between the double slit and screen is filled with a transparent medium of refractive index higher than that of air or vacuum, the fringes produced on screen will be narrower. This is because the wavelength is smaller in a medium where the refractive index is higher than that of air or vacuum.


The following video can help you understand: (I recommend that you stop at 1 min 50 seconds of the video unless you understand wave-particle duality)


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