Young obtained a monochromatic source of light by passing light through a colour filter. He placed a single slit in front of the monochromatic light source, so that light diffracting from the single slit arrived at a double slit placed in front,
and passed through the double slit. 
Light diffracted at each slit of the double slit overlaps and interferes. An interference pattern of alternating light and dark regions (fringes) can be seen on a screen. It is possible to derive a formula to calculate the wavelength λ of the light used from the dimensions of the experiment. 
This equation applies if the distance between the slits is much smaller than the screen to the double slits.
and passed through the double slit. Light diffracted at each slit of the double slit overlaps and interferes. An interference pattern of alternating light and dark regions (fringes) can be seen on a screen. It is possible to derive a formula to calculate the wavelength λ of the light used from the dimensions of the experiment. This equation applies if the distance between the slits is much smaller than the screen to the double slits.