Before light reaches the retina, it passes through the eye's lens, which focuses the light onto the retina. Interestingly, the image projected onto the retina is upside down. This occurs because when parallel rays of light strike a lens, they refract inward, and the rays eventually converge. After they converge, the rays pass through each other, flipping the image upside down. The brain corrects this image, so we see it right side up. The point where the light rays converge is called the focal point. Ideally, the focal point of light passing through the eye's lens will be on the eye's retina. When it is not, contact lenses or traditional glasses must be worn to correct vision.
The focal length of any lens is the distance between the center of the lens and the focal point. When light passes through a lens with a very short focal length, it only has to travel a short distance before it reaches its focal point. If the light hasn't yet been projected onto anything before reaching this point, the beams will pass through one another and invert.
Light passing through the lens of a microscope behaves in much the same way as light passing through the lens of an eye. A microscope will have at least two lenses. The objective lens is the one nearest the object being studied. Because the image of the object being studied needs to be magnified so much, objective lenses have a very short focal length. This means the light is unlikely to encounter resistance, most commonly in the form of another lens, before reaching its focal point. The light behaves in the same manner it would when passing through the lens of the eye, and the image inverts. This phenomenon is called microscopic inversion.
The eyepiece lens of a microscope is the one that can be most readily adjusted. Most microscopes will give you several options. The focal length of the eyepiece lens is much greater. This means that the light rays do not cross again before they reach the eye. As a result, the eyepiece lens magnifies the inverted image created by the objective lens without inverting it again. If the image were inverted again, it would be corrected, but as it is not, the microscope shows a greatly magnified but inverted image.