For large magnifying power of a microscope; f o < f e should be small.
Resolving Power
The ability of an optical instrument to produce separate and clear images of two near by objects, is called its resolving power.
Limit of Resolution
The minimum distance between two near by objects which can be just resolved by the instrument, is called its limit of resolution( d).
Resolving power of a microscope = 1 / d = 2 μ sin θ / λ
where, d = limit of resolution, λ = wavelength of light used. μ = refractive index of the medium between the objects and objective lens and θ = half of the cone angle.
Resolving power of a telescope = 1 / dθ = d / 1.22 λ
where, dθ = limit of resolution, A = wavelength of light used and d = diameter of aperture of objective
Aberration of Lenses
The image formed by the lens suffer from following two rnaiIl drawbacks
( i) Spherical Aberration Aberration of the lens due to which the rays passes through the lens are not focussed at a single and the image of a point object placed on the axis is blurred. called spherical aberration.
It can be reduced by using
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� � � � lens of large focal lengths� plano-convex lenses� crossed lenses� combining convex and concave lens�
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( ii) Chromatic Aberration Image of a white object formed by lens is usually coloured and blurred. This defect of the image produced by lens is called chromatic aberration.