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10.OPTICAL INSTRUMENTS eLearn.Punjab

SUMMARY

• Least distance of distinct vision is the minimum distance from the eye at which an object
appears to be distinct.

• Magnification is the ratio of the size of the image to the size of the object, which equals to the
ratio of the distance of the image to the distance of the object from the lens or mirror.

• Magnifying power or angular magnification is the angle subtended by the image as seen
through the optical device to that subtended by the object at the unaided eye.

• Resolving power is the ability of an instrument to reveal the minor details of theobject under
examination.

• Simple microscope is in fact a convex lens used to help the eye to see small objects distinctly.
The magnifying power of a simple microscope is given by

M = d =1+ d
pf

• Compound microscope consists of two convex lenses, an objective lens of very short focal
length and an eye piece of moderate focal length. The magnifying power of a compound
microscope is given by

M = q  d
p 1 + 
 fe 

• Telescope is an optical instrument used to see distant object. The magnifying power of the

telescope is given by

M = fo
fe

• Spectrometer is an optical device used to study spectra from different sources of light.
• Index of refraction is the ratio of speed of light in vacuum to the speed of light in thematerial.
• Critical angle is the angle of incidence in the denser medium for which the angle ofrefraction

in the rarer medium is equal to 90°.
• When the angle of incidence becomes greater than the critical angle of that material, the

incident ray is reflected in the same material, which is called total internal reflection.

Cladding is a. layer, of lower refractive index (less density) over the central core ofhigh refractive

index (high density).

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