Eye Ring of Compound Microscope

Entrance pupil: When a diaphragm with a small hole in it is placed over the objective, it is found that the final image is less bright than before but the size of the image remains the same. Therefore, the area of the objective limits the amount of light entering the microscope and determines the brightness of the image. On the other hand it does not affect the size of the image since the size depends upon the focal length. The lens thus acts as a stop to the light from the object.

The clear area of the objective is called the entrance pupil.

Exit pupil (eye ring): If the rays leaving all the points of an object AB are traced, it is found that all rays after passing through the eyepiece pass through a small circular area. This is called the exit pupil or eye ring. This is the best position where the eye should be placed to collect as much light that passes through the objective as possible. Actually R is the image of the objective through the eye-piece and all the emergent rays pass through R.

Therefore, the observer should ensure while using a microscope that his eye coincides with the eye ring or the exit pupil. When the eye of the observer coincides with the ring the whole of the image will be seen clearly.

While constructing the microscope, a circular hole slightly greater than the exit pupil is placed behind the eyepiece so that the two coincide. The eye is placed behind the hole to view the image.

Suppose the objective is at a distance of 20 cm from the eyepiece and the focal length of the eyepiece is 2 cm.



Or, v = 2.22 cm

i.e. the exit pupil is at a distance of 2.22 cm from the eyepiece. Therefore, a circular hole is fixed at a distance of 2.22 cm from the eyepiece, so that the observer’s eye has the best position when it is placed close to it.

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