Geometrical optics is useful as long as the objects with which the light interacts are much larger than the wavelength of the light. Otherwise, diffraction occurs. One thinks of light as traveling in straight lines as long as it is left alone. If an obstacle blocks some of the light rays headed for a screen, the light rays that are not blocked still reach the screen and make that part of the screen bright. Shadow , One can determine the location of the shadow by drawing straight lines from the point source of light to the edge of the obstacle and continuing them on the screen.
Light is travelling, say in air, in a straight line when it hits a different medium, say a mirror.
Its direction changes suddenly, the old ray stops and a new one starts. A central issue is to put together optical systems that deliver clear, undistorted images. Therefore, we need to discuss spherical mirrors. Whenever light goes from one medium to another, some of the light is reflected and some of it is transmitted. The transmitted beam usually does not continue to travel in exactly the same direction as the incident beam.