Thomas Young first demonstrated the interference of light in 1801. His experiment gave strong support to the wave theory of light.

This experiment shows interference fringes created when a coherent light source (in this case a ruby laser) is shone through double slits.

 Change the values of s and D to see the effect on the appearance of the fringes.

Laser light produces an intense beam of monochromatic (single frequency) light. All the waves across the beam are in phase. The beam illuminates the slits, which are narrow to ensure adequate diffraction.

The diffracted beams from the two slits overlap causing the waves in the beam to superimpose.  The interference pattern due to the superposition of the waves appears on the screen as alternate dark and bright bands, called fringes. The bright fringes are caused by constructive interference and the dark fringes by destructive interference.