Question

When viewing through a compound microscope, our eyes should be positioned not on the eyepiece but a short distance away from it for best viewing. Why?

Answer 1

Practically visualize the scenario. Compound microscope eyepiece is made of a convex lens which has a large focal length. In order to obtain an image of large focal length on the eye-ring, you need to place the eye exactly where you can catch the reflected rays off the object mirror. Using this logic, answer the question.

Complete step by step solution:
While viewing through the ocular lens of the compound microscope, the point of eye contact should be at a distance from the lens because, closer we keep the eyes to the ocular lens, the path of light rights would be prematurely objected by our eye lens before intersecting to form an image and also reduce the field of view. This reduces the quality of the image obtained.
If the position of our eye ball is exactly on the eye ring near the ocular lens, the area of our eye ball will be much greater than that of the eye ring which will then obtain all the refracted rays from the objective lens, thus obtaining a clear view of the test piece.
The microscope is built in such a way that the focal length of the ocular lens can be adjusted based upon the distance of separation maintained between the objective lens and eyepiece. Based on our eyeball positioning, we can adjust the distance which can give us the clear image.

Note: The ocular lens is made of convex lens or the converging lens of large focal length. This is so because it will converge all the refracted rays from the objective lens and provide an accurate image up to micron level magnification, which is also due to their large focal length.