Complete the diagram showing reflected waves and state the function of the slits S1 and S2 in Young's double-slit experiment.

When straight (plane) waves encounter a concave reflector, they are reflected and converge towards a focal point. The reflected wavefronts will be circular, with their center at the focal point of the reflector.

The completed diagram should show:

• The incident straight waves approaching the concave reflector.
• The reflected waves as circular arcs, with their curvature increasing as they move away from the reflector, converging towards a point.

In Young's double-slit experiment, the slits $S_1$ and $S_2$ serve two main functions:

• They act as two coherent sources of light. The light from the single slit S diffracts and illuminates $S_1$ and $S_2$ equally, ensuring that the waves emerging from $S_1$ and $S_2$ have a constant phase relationship.
• They cause the light to diffract, allowing the waves to spread out and overlap, which is necessary for interference to occur on the screen.