Ans. Linear (or Transverse) magnification: “The ratio of the size (linear dimensions) of the image to those of the object”. M = I / O = q / p Angular magnification (or Magnifying power): “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”; M = β / α In linear magnification, we take ratio of linear dimensions, but in angular magnification we take ratio of the angles. Magnifier: An ordinary convex lens held close to the eye is served as magnifying glass or simple microscope. The image formed is erect, virtual and magnified.
Q.2 Explain the difference between angular magnification and resolving power of an optical instrument. What limits the magnification of an optical instrument?
Ans. Angular magnification (or Magnifying power): “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”; M = β / α Resolving power (αmin): “The ability of an instrument to reveal the minor details of the object under examination”; αmin = 1.22 λ / D, where λ = wavelength of light & D = lens diameter Angular magnification means how large a magnification is the image formed. And resolving power reveal the minor details of the object. Limits: Due to chromatic and spherical aberrations, the magnification of the optical instruments is limited.
Q.3 Why would it be advantageous to use blue light with a compound microscope?
Ans. Blue light increases the resolving power and more details of an object can be seen. As blue light produce less diffraction due to short λ.
Q.4 One can buy a cheap microscope for use by the children. The image seen in such a microscope have coloured edges. Why is this so?
Ans. Due to chromatic aberration, we see coloured edges in cheap microscope. It is due to non-focusing of light of different colours. These colours arise due to dispersion.
Q.5 Describe with the help of diagrams, how (a) a single biconvex lens can be used as a magnifying glass. (b) biconvex lenses can be arranged to form a microscope.
Ans.(A)
Q.6 If a person were looking through a telescope at the full moon, how would the appearance of the moon be changed by covering half of the objective lens.
Ans. The intensity of the image become half and there will be no change of shape. Less transmitted light due to half-covered objective, still he will see full image of the moon.
Q.7 A magnifying glass gives a five times enlarged image at a distance of 25 cm from the lens. Find, by ray diagram, the focal length of the lens.
Ans. [ M = 1 + d/f or f = d / M – 1 = 25/5-1 = 6.2 cm]
Q.8 Identify the correct answer.
i) The resolving power of a compound microscope depends on;
a) The refractive index of the medium in which the object is placed.
b) The diameter of the objective lens.
c) The angle subtended by the objective lens at the object.
d) The position of an observer’s eye with regard to the eye lens.
ii) The resolving power of an astronomical telescope depends on:
a) The focal length of the objective lens.
b) The least distance of distinct vision of the observer.
c) The focal length of the eye lens.
d) The diameter of the objective lens.
Ans. i) Correct answer is (d) The diameter of the objective lens. αmin = 1.22 λ / D,
Q.9 Draw sketches showing the different light paths through a single-mode and multimode fibre. Why is the single-mode fibre preferred in telecommunications?
Ans. Single-mode fibre is preferred in modern telecommunications because they are digital and use monochromatic laser light. here the transmission is free from dispersion, and can carry 14 TV channels & 1400 phone calls at the same time.
Q.10 How the light signal is transmitted through the optical fibre?
Ans. By total internal reflection on continuous refraction light signals is transmitted through the optical fibre. A transmitter converts electrical signal into light signal and at the receiving end these are converted back to electrical signals. The most common method of transmission is digital modulation, in which the laser is flashed on and off at extremely fast rate. The communication is represented by code of 1s and 0s. The receiver is programmed to decode 1s and 0s.
Q.11 How the power is lost in optical fibre through dispersion? Explain.
Ans. Power is lost due to scattering and absorption of light signals during travel through the optical fibre. The information received can be faulty and distorted due to dispersion, i.e. spreading of light signals into component wavelengths. Due to impurities in the glass and multiple reflections along the fibre is occurred.
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