Question

Why must both objective and eyepiece of compound microscope have short focal length?

Answer 1

The focal lengths must me small for magnification to become large. In the diagram below, you can see that the first image size B’A” of object AB depends on how close the object can get to the objective. This distance to the objective ( a.k.a. working distance) needs to be close to but larger than the objective focal length to get an image focused far from the lens and proportionately magnified. You may have noticed that a high magnification objective is usually a tiny lens that almost touches the slide (and can sometimes break a slide if moved too close), while low magnification objectives are larger and stand farther from the object.

In the second phase of magnification, an eyepiece magnifies the first image and the amount of magnification is greater with a shorter focal length. In general the eyepiece focal length will be close to the low magnification objective focal length, but much bigger than the med or high magnification objectives. The initial magnification of microscope objectives are something like 4X, 10X, and 40x (the actual focal lengths can vary since since the magnification is the ratio of the tube length and working distance), which is compounded by the magnification of the eyepiece (typically, about 25 mm focal length) of 10X to give typical magnifications of 40X, 100X, and 400X. If an eyepiece of longer focal length was used, say 50mm or 5X, then the overall magnifications would be smaller at 20X, 50X, and 200X with the same objectives.

Answer 2

$hey \: mate$

✔IT HAS SHORT FOCAL LENGTH.

BECAUSE:

⚫LESS THE FOCAL LENGTH,MORE WILL BE MAGNIFICATION.