13 April 2010

Optics and Spectroscopy

UNIT – I
1. When the media on the two sides of a lens system are the same

a) Principal points coincide with focal points
b) Principal points coincide with nodal points
c) Nodal points coincide with focal points
d) None
2. The point at which image is formed due to a lens by marginal rays is
different from that due to paraxial rays. The defeft in the image thus produced is known as
a) circle of least confusion b) chromatic aberration
c) spherical aberration d) coma
3. For minimum spherical aberration, two lenses of focal lengths f1 and
f2 are to be placed at a distance equal to
a) f1+f2 b) f1-f2 c) f1/f2 d) (f1+f2)/2
4. An achromatic doublet is made of two glasses of
a) crown b) flint c) crown and flint d) none
5. Two thin lenses of focal lengths f1 and f2 separated by a distance “d”
form an achromatic combination when “d” is equal to
a) (f1-f2)/2 b) (f1+f2)/2 c) f1+f2 d) f1-f2
6. For achromatic combination of two lenses in contact, the lenses must
be
a) Both convex b) Both concave
c) One convex and the other concave d) None of the above
7. The inability of rays of different colours of light starting from a
distant source of white light, to converge to a common point after passing through a convex lens is referred to as
a) spherical aberration b) optical illusion
c) chromatic aberration d) astigmatism
8. A direct vision spectroscope is based upon

a) deviation without dispersion b) dispersion without deviation
c) neither deviation nor dispersion d) all the above
9. The deviations in the size, shape, position and colour in the actual
images produced by a lens in comparison to the object are called
___________ produced by a lens.
a) aberrations b) dispersion c) deviation d) none
10. The line joining the centres of curvature of the twosurfaces is called
a) principal section b) principal axis c) principal mode d) none
11. The power for convex lens is
a) negative b) square value c) positive d) none
12. The failure of a lens to form a point image of a point object on the
axis is called
a) spherical aberration b) cubic aberration
c) elliptical aberration d) none
13. A spherical lens which is free from the defects of spherical aberration
and coma is called
a) aplanatic point b) edge point c) focal point d) none
14. Direct Vision Spectroscope
a) can be used to make any measurements on spectra
b) can not be used to make any measurements on spectra
c) can be used to measure IR spectrum only
d) none of the above
15. An achromatic telescope objective of 1.5m focal length consists of two
thin lenses in contact with each other and their dispersive powers are 0.05 and 0.075 respectively. What is their focal lengths.
a) f2=-0.75m b) f2=-0.95m c) f2=-1.75m d) f2=-2.75m
Answers:
1) b 2) c 3) b 4) c 5) b 6) c 7) c
8) b 9) a 10) b 11) c 12) a 13) a 14) b 15) a
UNIT – II
1. Two sources of light are said to be coherent if the waves produced by
them have the same
a) wavelength b) amplitude
c) wavelength and a constant phase difference
d) amplitude and the same wavelength
2. Two separate sources giving out light of the same frequency do not
produce interference because
a) The amplitude of the waves from the sources are not equal
b) The two sources are not close to each other
c) The waves are not traveling in the same direction
d) The phase difference between the waves given out by the two
sources is always changing
3. A laser is coherent source because it contains
a) many wavelengths
b) uncoordinated wave of a particular wavelength
c) coordinated wave of many wavelengths
d) coordinated waves of a particular wavelength
4. To demonstrate the phenomenon of interference, we require
a) two sources which emit radiation of the same frequency
b) two sources which emit radiation of nearly the same frequency
c) two sources which emit radiation of the same frequency and
have definite phase relationship
d) two sources which emit radiation of different wavelengths
5. For the formation of colours of thin films, the source of light should
be
a) broad and monochromatic b) broad and white
c) narrow and monochromatic d) narrow and white
6. When white light is used, in interference phenomena, we get
a) dark and bright fringed b) coloured fringes
c) no fringes d) all are true
7. Which of the following phenomena produces the cours in soap bubble
a) interference b) diffraction c) polarization d) dispersion
8. When a thin film is seen in a monochromatic light
a) even then it shows many colours
b) it shows only bright and dark fringes
c) it shows no fringes d) none of these
9. Colours of thin films are due to
a) Dispersion of light b) Interference of light
c) Diffraction of light d) Absorption of light
10. When a light wave is reflected from the surface of an optically denser

medium, it suffers a phase change of
a) π/2 b) 2/π c) 2π d) π
11. If we consider two consecutive bright fringes, the fringe width b will
be the
a) different b) constant c) square multiple d) same
12. In a Michaelson’s interferometer
a) movable mirror is half silvered
b) stationary mirror is half silvered
c) rear side of the plane glass plate is half silvered
d) rear side of the compensating plate is half silvered
13. When the movable mirror in Michaelson’s interferometer is moved
through a distance λ/2, the number of fringes crossing the field of
view is
a) 2 b) 1 c) 10 d) 0
14. When the two mirrors of Michealson’s interferometer are perfectly
perpendicular to each other __________ fringes will be observed
a) Straight line b) Inclined c) Circular d) Elliptical
15. When the movable mirror of a Michelson interferometer is moved
by 0.0589mm, a shift of 200 fringes is observed. What is the
wavelength of light used?
a) 589nm b) 989nm c) 689nm d) 389nm
16. The fringe width (b) is ___________
a) λD/d b) λd/D c) D/λd d) d/λD
17. Condition for sustained interference of light waves is given below.
Which one is wrong statement?
a) The two interfering sources must be coherent.
b) The two sources must be narrow
c) The separation between the two sources must be large
d) The two interfering sources should emit light of the same
frequency or wavelength
18. In Michelson’s interferometer, Light from the source S is rendered
parallel by a lens L and falls on the glass plate G, at an angle of _____
a) 25° b) 38° c) 90° d) 45°
Answers:
1) c 2) d 3) d 4) c 5) b 6) b 7) a 8) b 9) b
10) d 11) d 12) c 13) b 14) c 15) a 16) a 17) c 18) d
UNIT – III
1. The bending of light rays about corners of an obstacle is called
a) Dispersion b) Diffraction c) Deviation d) Refraction
2. In Fraunhofer diffraction, the source and the screen are _____ from
the slit causing diffraction
a) at equal finite distance b) at unequal finite distance
c) effectively at infinite distance d) none
3. In Fresnel’s diffraction at a straight edge, the incident wave front is
a) plane b) spherical c) cylindrical d) none
4. The ability of an optical instrument to show the images of two nearby
point objects as separate is called
a) magnifying power b) dispersive power
c) resolving power d) none of these
5. Two closely spaced objects are said to be resolved when
a) principal maximum of one falls upon the principal maximum of
another
b) principal maximum of one falls upon the first minimum of
another
c) first minimum of one falls on the first minimum of another
d) none
6. The resolving power of a telescope is
a) limited by the diameter of its objective
b) limited by the kind of the glass used
c) independent of the wavelength of light used
d) dependent on the diameter of its eyepiece
7. Resolving power of a diffraction grating depends on
a) the order of spectrum only b) the number of lines on the grating c) both (a) and (b) d) none
8. The correct formula for resolving power of grating is given by
a) N×n b) N2/n c) n/N d) (N×n)2
9. The resolving power of a prism is
a) directly proportional to the rate of change of refractive index
with wavelength
b) inversely proportional to the rate of change of refractive index
with wavelength
c) inversely proportional to thickness of prism
d) independent of thickness of prism
10. The minimum intensity points are __________ in diffraction pattern
a) Perfectly dark b) not perfectly dark
c) Partially coloured d) none
11. Diffraction fringes are
a) not of the same width b) the same width
c) colourful one d) none of these
12. In diffraction, _______ can be observed
a) all fringes b) only a few fringes
c) all the same width fringes d) all the different width fringes
13. In diffraction, the interaction occurs between the secondary wavelets
originating from different points of the exposed parts of the _________
a) different wave front b) multi level wave front
c) same wave front d) none of these
14. A diffraction grating has 0.15m of surface ruled with 6×105 lines/m.
What is its R.P. in the first order?
a) 9×105 b) 9×107 c) 9×106 d) 9×104
Answers:
1) b 2) c 3) c 4) c 5) b 6) a 7) c 8) a 9) a 10) b
11) a 12) b 13) c 14) d


UNIT – IV
1. The discovery of Polarization of light confirmed that, Light waves are
a) neither waves are nor particles b) longitudinal in character
c) transverse in nature d) all of the above
2. Plane of polarization and plane of vibration are
a) one and the same b) Coplanar
c) Perpendicular to each other d) none
3. Plane polarized light can be produced by
a) simple reflection b) nicol prism
c) pile of plates d) all of the above
4. Polaroid sun glasses decrease glare on a sunny day because they
a) refract the light b) completely absorb the light
b) have a special colour d) block a portion of light
5. A calcite crystal is placed over a dot on a piece paper and rotated. On
seeing through the crystal, one observes
a) one stationary dot b) two rotating dots c) two stationary dots d) one stationary and the other rotating about the stationary dot
6. Nicol prism is based on the action of
a) refraction b) double refraction c) scattering d) none
7. Nicol prism is used for
a) production of plane polarized light only
b) detection of plane polarized light only
c) production and detection of polarized light
d) none of the above
8. In a nicol prism, the extraordinary ray
a) passes straight through b) undergoes total internal reflection
c) is bent twice as much as the ordinary ray
d) is at right angles to the ordinary ray
9. Along the optic axis, the velocity of O-wave is
a) greater than the velocity of E-wave
b) less than the velocity of E-wave
c) the same as the velocity of E-wave
d) none
10. In doubly refracting crystal no and ne are the refractive indices of the
crystal for O–rays and E-rays. Then along the optic axis of the crystal
a) no > ne b) no = ne c) no <>e d) all the above
11. If Vo and Ve are the velocities of ordinary and extra-ordinary rays
along the optic axis of a doubly refracting crystal
a) Vo >Ve b) Vo= Ve c) Vo <>e d) none
12. Quarter wave plate introduces a path-difference
a) l/2 b) l/4 c) l d) 2l
13. The phase difference introduced by a quarter wave plate between the
ordinary and the extra ordinary rays is
a) 2π b) π c) π/4 d) π/2
14. When the principal planes of the polarizer and analyzer are
perpendicular ( or when the two nicols are crossed) if plane polarized light is examined, the intensity of light will be
a) zero b) maximum c) minimum d) uniform
15. In elliptically polarized light
a) amplitude of vibrations changes in direction only
b) amplitude of vibrations changes in magnitude only
c) amplitude of vibrations changes in magnitude and direction both
d) none of the above
16. If the intensity of light examined by an analyzer remains uniform, the
light is
a) plane polarized b) partially polarized
c) elliptically polarized d) ordinary or circularly polarized
17. Two Nicols are being used as polarizer and analyzer respectively.
When the analyzer is rotated from o to 2π, the transmitted intensity
remains same. What conclusion can be drawn about the light’s
polarization charactertics?
a) it could be unpolarised or circulary polarized or a mixture of the two
b) elliptically polarized
c) plane polarized
d) mixture of plane polarized and circularly polarized light
18. The term optical activity refers to
a) degree of polarization produced when ordinary light is passed
b) decrease in polarization when plane polarized light is passed
c) rotation of the plane of polarization when plane polarized light is
passed
d) none of the above
19. For a given thickness of the optically active substance, the angle of
rotation is approximately
a) proportional to the wavelength of light
b) inversely proportional to the wavelength
c) directly proportional to square of the wavelength
d) inversely proportionally to square of the wavelength
20. Instrument used to measure optical rotation is called
a) Optometer b) Photometer c) Polarimeter d) Rotometer
Answers:
1) c 2) b 3) d 4) d 5) d 6) b 7) c 8) a 9) c 10) b 11) b
12) b 13) d 14) a 15) c 16) d 17) a 18) c 19) d 20) c
UNIT – V
1. Infrared radiations are detected by
a) heating effect b) fluorescence c) phosphorescence d) none
2. The source of ultraviolet radiation used in the laboratory is
a) Nernst glower b) Globar c) mercury vapour lamp d) none
3. Raman frequency is found to be dependent on

a) incident frequency
b) angle between the direction of incidence and scattering
c) scattering substance
d) none of the above
4. The apparent change in frequency of the wave due to relative motion
between the source and the observer is known as
a) Raman effect b) Doppler effect c) Faraday effect d) none
5. Numerical aperture of the optical fibre is
a) NA = sinim b) NA=tan im c) NA=cos im d) none
6. Who was the discoverer of IR radiations?
a) Faraday b) Newton c) Einstein d) Herschel
7. The wavelengths of IR radiations range from ______ to _____.
a) 750nm to 1mm b) 350nm to 2mm c) 950nm to 1mm d) none
8. What is the natural source of infrared radiations?
a) Pluto b) Moon c) Saturn d) Sun
9. Incandescent solid bodies at temperatures of _____ to ____ emit
radiations throughout the infrared.
a) 1000°C to 2500°C b) 1000°C to 1500°C
c) 1500°C to 2500°C d) none
10. The lines having wavelengths greater than that of the incident
wavelengths are called
a) Stokes lines b) Anti-stokes c) Rayleigh lines d) none
Answers:
1) a 2) c 3) c 4) b 5) a 6) d 7) a 8) d 9) b 10) a