A plane wave travels from left to right according to Huygens's principle. Let's suppose that an obstruction is placed in the path of the light, such as a piece of cardboard with a narrow opening, that partially obstructs the wavefront so that
just one small part would pass through. This portion of the plane wave would emit wavelets that would move forward through the opening without interference from any wavelets above and betow, which are obstructed. What would the wave do on the right side of the obstruction?
