11th Class Physics Chapter 10 Optical instruments Short Questions Answer
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Magnification produced by the objective is
M1 = 5
Magnification produced by the eye-piece is
The total magnification ‘M’ produced by the objective and eye-piece of the compound microscope is equal to the product of M1 and M2. That is
Putting the values of M1 and M2, we get
M = L/f0 (1+ d/fe) ……………………… (1)
Where f0 = focal length of objective lens
fe= focal length of eye piece
L= Length of compound microscope
d= Least distance of distinat vision
It is clear from the above relation that smaller the focal length, greater will be the magnifying power (i.e. M α 1/f). therefore, in order to increase the magnifying power of microscope and objective of short focal length is used.
The purpose of an microscope or telescope is not only to magnify an object but to reveal (show) it in great detail. The amount of detail revealed by an optical instrument depends on what is called resolving power.
Mathematically it is expressed as
α min=1.22 λ/D
This equation shows that greater the diameter (aperture) of the convex lens and smaller value of the wave length of light, higher will be resolving power of the optical instrument.
Mathematically, it is expressed as
sinθ1/sinθ2 = n2/n1
or n1sinθ1= n2sinθ2
It can be expressed as
Magnifying power = Angle subtended by image/Angle subtended by the object
Or M= θl/θ0 = B/α
When a ray of light travel from a denser medium (glass) to a rarer medium (air) it bends away from the normal. The angle of incidence for which angle of refraction is 900 is called critical angle. It is denoted by θc
According to snell’s law
n1sinθ1 = n2sinθ2
θ1 = θc,θ2=900
n1sinθc = n2sin 900
or n1sinθc= n2 (sin 900= 1)
or sin θc = n2/n1
Power of a lens:-
The reciprocal of the focal length of a lens expressed in metres is called the power of a lens. It is denoted by D. That is
Unit of power:-
The unit of power of a len is dioptre.
Definition of dioptre:-
One dioptre is the power of a lens of one meter focal length.
D= 1/focal length of the lens in meter=1/f
The power of a convex lens is positive while that of concave is nagitive.
Magnifying power of a compound microscope is given by the formula
M = q/p ( 1+ d/fe)
The total length of the blue is given by
L = f0 + fe
putting the value, we have
24 = f0+fe ………………………… (2)
Magnifying power = M= f0/fe or 5 = fo/fe
Or fo= 5fe ………………………….(3)
putting the value of f0 from equ (3) in equ (2) we get
24 = 5fe+ fe
or 6fe = 24
fe = 4cm …………………(4)
Putting the value of fe in equ (2)
24= fo + 4
or f0 =24 -4= 20 cm
F0 = 20 cm
A lens which converges a beam of parallel rays to a point after refraction is called convex lens(or conversing lens)
A lens which is thicker in the center (middle) and thinner at the edges is called a convex lens or converging lens.
This lens produces a real image whose sign is positive.
Concave lens :-
A lens which diverges a beam of parallel rays after refraction is called concave lens(or diverging lens)
A lens which is thinner at the center and thicker at the edges is called a concave or diverging lens.
A concave lens produces a virtual image whose sign is taken as negative.
The refractive (u) of prism can be found by using the formula
u = sin(A+Dm)/2/ sin A/2
where ‘A’ is the angle of prism and Dm is the angle of minimum deviation.
It consists of three main parts.
(i) A collimator
(ii) A Turntable
(iii) A Telescope
(1) It is used to fined the wavelength of light.
(2) It is also used to fined the refractive index of transparent material (i.e. prism)
(3)It is used to study the spectra of different sources of light.
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