Wednesday, January 28, 2015

Working of Compound microscope

Compound microscope is useful to produce a better magnification than a simple microscope. Because of the design of this microscope it is able to produce better magnification without any aberrations.

The device consists of two convergent lenses arranged coaxially. The lens that is facing the object is called objective and the other lens that is close to the eye is called eye lens.

The focal length of the eye lens is more than that of the focal lens of the field lens. The distance between the two lenses can be varied with the help of the experimental arrangement.

An object is placed at a distance from the field lens who is within double the focal length but greater than the focal length of the field lens. Because of this arrangement the corresponding image of the object is inverted, real and enlarged. The location of the image is on the other side of the field lens. This image acts like a object for the eye lens. By varying the position of the eye lens, the final image of the compound microscope position can be varied. The final image is a inverted, magnified and virtual image.

The total magnification of the compound microscope is the product of magnification produced by both the lenses.

Depending on the position of the eye lens the final magnification very.

If the eye lens is adjusted in such a way that the intermediate image is within its focal length, the final images formed at a finite distance. By adjusting the lens properly we can see that the final images formed at the least distance of distant vision. This position is called strained eye position and in this position we are going to get the best magnification.

By adjusting the eye lens in such a way that the intermediate image is at the principal focus of the eye lens, the final image can be adjusted to form at infinity. This kind of arrangement is called relaxed eye position or normal adjustment. In this case magnification is less.

Depending on our requirement we can choose any of these positions and get the corresponding magnifications.

Some of the mathematical equations are explained as shown in the diagram.

At near point magnification is high and it far point of image magnification is low. Magnifying power is represented as negative value just because the final image is inverted. The magnification of a compound microscope may vary anywhere in between 20 to 40 for a low powered compound microscope. A well-designed compound microscope can produce a magnification even up to 500 times of the size of the image.

Problem and solution

A microscope consists of two convex lenses of focal lengths 2 cm and 5 cm placed 20 cm apart. Where must be the object has to be placed from the field lens so that the final image is at a distance of 25 cm from the eye?

We cannot apply a direct formula to solve this problem. We need to analyze the situation and identify each value separately as shown in the attachment paper. While we are solving this problem, we have to apply the lens formula with the proper sign convention.

Let us go through the problem and ask for any of the further clarifications, if  required.

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