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10.OPTICAL INSTRUMENTS eLearn.Punjab

The maximum size of an object as seen by naked eye is obtained when the object is placed at the
least distance of distinct vision. For lesser distance, the image formed look blurred and the details
of the object are not visible.

The magnifying power or angular magnification is defined as the ratio of the angles subtended
by the image as seen through the optical device to that subtended by the object at the unaided

eye.

The optical resolution of a microscope or a telescope tells us how close together the two point
sources of light can be so that they are still seen as two separate sources. If two point sources are
too close, they will appear as one because the optical instrument makes a point source look like a
small disc or spot of light with circular diffraction fringes.

Although the magnification can be made as large as one desires by choosing appropriate focal
lengths, but the magnification alone is of no use unless we can see the details of the object distinctly.

The resolving power of an instrument is its ability to reveal the minor details of the object
under examination.

Resolving power is expressed as the reciprocal of minimum angle which two point sources subtends
at the instrument so that their images are seen as two distinct spots of light rather than one. Raleigh
showed that for light of wavelength through a lens of diameter D, the resolving power is given by

=R =1 D Tidbits
amin 1.22l
If you find it difficult to read small
Where amin =1.22 l ........ (10.1) print, make a pinhole in a piece of
D paper and hold it in front of your eye
close to the page. You will see the
print clearly.

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v: 1.1