Answer:
The objective collects a fan of rays from each object point and images the ray bundle at the front focal plane of the eyepiece. The conventional rules of ray tracing apply to the image formation. In the absence of aberration, geometric rays form a point image of each object point. In the presence of aberrations, each object point is represented by an indistinct point. The eyepiece is designed to image the rays to a focal point at a convenient distance for viewing the image. In this system, the brightness of the image is determined by the sizes of the apertures of the lenses and by the aperture of the pupil of the eye. The focal length and resulting magnification of the objective should be chosen to attain the desired resolution of the object at a size convenient for viewing through the eyepiece. Image formation in the microscope is complicated by diffraction and interference that take place in the imaging system and by the requirement to use a light source that is imaged in the focal plane.
