Experimental details...

• A filter is placed between the white light source to produce monochromatic (single wavelength) light.
• Monochromatic light is incoherent so a narrow single slit is placed between the source and the double slits. This ensures the light arriving at the double slits is in phase.
• The experiment should be performed in a darkened room so as to be able to detect the interference pattern.
• The colour of the fringes depends upon the monochomatic light used.
• The slit separation, s and width, b are usually given on prepared slits.
• The fringe pattern is viewed through a travelling microscope with a vernier scale. The distance between the centres of a large number of fringes is measured, so that the average fringe width can be calculated. Subtract one vernier scale reading from the other then divide by the number of fringes.