Download conceptual_wave_optics

Q.1
A.1
Why is interference not detected when the two coherent sources are far apart?
In interference the fringe width is inversely proportional to the distance between the two
coherent sources of light. Thus, if the sources are very close to each other the fringe width
may become so small that it becomes invisible.
Q.2
A.2
Why interference not detected when the two coherent sources are very close to each other?
The fringe width is inversely proportional to the distance between the sources thus if the
sources are very close the fringe width becomes very large and single interference fringe may
not be formed on screen.
Q.3
Light from two coherent sources reaches a point where the path difference for yellow light is
3/2. What will be the color of fringe at that point?
If monochromatic yellow light is used, then path difference of 3/2 results in destructive
interference and it will be dark fringe, but if white light is used colored fringes are formed but
yellow color will be missing.
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Q.4
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Q.5
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Q.6
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Q.7
A.7
INJ YDS experiment two independent bulbs of same power and material are used. What will
be observed on the screen?
Two independent sources of light can never be coherent as the phase difference between
them changes quickly and randomly, thus the position of maxima and minima also changes.
Thus, there will be general illumination of the screen.
What is the difference between colors seen on the oil film and colors when light passes
through prism?
Colors in the oil film in water arises because of the interference of light whereas in prism the
colors are due to the dispersion of light through the prism.
Two slits in YDS are illuminated by two different sodium vapor lamps emitting light of same
wavelength. Will interference be observed?
No, interference is not observed because two independent sodium lamps will never have
constant and zero phase difference, which is essential for sustained interference pattern.
The phase difference between light waves from two slits in YDS is  radian. Will the central
fringe be dark or bright?
As the waves travel same distance in moving from two slits to the center of the screen is
same. As there is initial phase difference if , thus the interference will be destructive and
dark fringe will be formed on the screen.
Q.8
A.8
Which phenomenon establishes the wave nature of light?
The phenomenon like interference, polarization and diffraction, which can’t have any
explanation using particle nature, establishes the wave nature of light.
Q.9
In Young’s double slit blue, red and green light are used separately. In which case will be the
fringe width maximum?
Fringe width  = D/d, thus the fringe width is directly proportional to  as D and d are
constant. Thus, it will be maximum for red as its wavelength is maximum.
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Q.10
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Q.11
A.11
Out of speed, frequency and wavelength which physical quanitity remains unchanged with the
refraction of wave?
The frequency of the wave remains unchanged with the change in medium due to refraction,
whereas velocity and wavelength changes.
What will be the effect on phase in refraction and reflection?
In refraction of wave no change of phase occurs, whereas in reflection from denser medium
there is change of phase , thus incident and reflected waves have phase difference of .
Q.12
A.12
How is wavefront related to the direction of propagation of the waves?
Wavefront is always perpendicular to the direction of propagation of the wave. If the beam of
rays is parallel, the Wavefront is plane wavefront perpendicular to the rays.
Q.13
A.13
Explain the statement’ light falling on light can produce darkness’
This is possible if the two sources of light are coherent sources and the waves from the two
sources at that point interfere destructively. If amplitude of the two waves is equal it results
in total darkness on the screen.
Q.14
A.14
What happens to the interference pattern if one of the slits is closed?
NO superposition or interference of waves takes place if one of the slits is closed. Light from
one source falls on the screen and there is general illumination on the screen.
Q.15
In YDS experiment the intensity of central maximum is I, what will be the intensity if one of
the slits is closed?
If light from both the slits reaches point at the center of the screen, the intensity is I. If one
slit is closed no interference of light takes place, thus amplitude of resultant wave is halved
and intensity becomes one fourth.
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Q.16
Q.16
IN YDS experiment, performed with a white source of light, only black and white fringes are
observed. Is this true?
No, if white light is used the central fringe will e white as all the colors interfere constructively
at that point but central white fringe is surrounded by few colored fringes.
Q.17
A.17
Can interference be observed using sound waves?
Yes, interferences can be observed using sound waves also. It is a wave phenomenon, any
two waves which are coherent can produce interference and the waves should be same type.
Q.18
A.18
Which color travels slowest in glass prism?
The velocity of light in any medium is v = c/. . Also  is inversely proportional to the
wavelength. Thus,  is minimum for red color as its wavelength is maximum. Thus velocity of
light will be maximum for red light and minimum for violet light.
Q.19
A.19
What happens to the energy at the points of destructive interference?
The energy which disappears at the points of destructive interference will reappear at the
points of constructive interference. Thus, interference is the phenomenon of redistribution of
energy.
Q.20
A.20
What will be effect on the fringe width if YDS set is immersed in water?
As  = D/d. in water D and ‘d’ remains unchanged whereas the density of