Final answer:
The incident light ray must contact the center of the semicircular dish to ensure precise focusing and the maintenance of a clear optical path, which is critical for minimizing optical aberrations and achieving sharp image formation in optical instruments.
Step-by-step explanation:
It is important that the incident light ray contacts the semicircular dish at the center because the precision in alignment ensures that light rays are accurately focused and maintain a precise path through the optical system. In optical instruments, such as the Cassegrain telescope, the light-gathering concave mirror is designed with a central hole through which light is directed towards the eyepiece.
The force of gravity can cause large lenses to sag if not properly supported, which would distort the path of the light rays. In the context of a semicircular dish used in experiments or instruments, the central contact point is critical for minimizing optical aberrations like spherical aberration or coma, both of which would affect the sharpness and clarity of the resulting image.