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All questions of Wave Optics for Class 12 Exam

If the Young’s apparatus is immersed in water, the effect on fringe width will​
  • a)
    remain same
  • b)
    decrease
  • c)
    increase
  • d)
    Cant say because the experiment cannot be carried in any other medium except air
Correct answer is option 'B'. Can you explain this answer?

Kalyan Joshi answered
Effect of Immersing Young's Apparatus in Water on Fringe Width

When the Young's apparatus is immersed in water, the effect on the fringe width can be explained as follows:

1. Refractive Index of Water: Water has a higher refractive index than air. This means that light passing through water will be bent more than in air.

2. Path Difference: Path difference is the difference in the distance traveled by two waves from the source to the point where they interfere with each other. When the apparatus is immersed in water, the path difference between the two waves will change due to the change in refractive index.

3. Decrease in Fringe Width: As the path difference changes, the interference pattern will also change. The fringe width will decrease because the change in the path difference will cause the interference pattern to become more spread out.

4. Explanation: The fringe width is inversely proportional to the square root of the refractive index. Since the refractive index of water is higher than air, the fringe width will decrease when the apparatus is immersed in water.

Conclusion

In conclusion, when the Young's apparatus is immersed in water, the fringe width will decrease due to the change in refractive index, which leads to a change in the path difference and the interference pattern.
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Light of wavelength 500 nm is incident on a slit of width 0.1 mm. The width of the central bright line on the screen is 2m. What is the distance of the screen?​
a)1 m
b)200 m
c)0.75 m
d)0.5 m
Correct answer is option 'B'. Can you explain this answer?

Swara Sharma answered
Beta(central Maxima)=2lambda D(distance of the screen)/d(distance between slits).

beta=2m.

wavelength= 500nm=5×10^-7m.

d=0.1mm=1×10^-4m.

2=2×5×10^-7 D/10^-4.

1=5×10^-3 D .

1/5×10^-3=D .

1000/5=D.

200m=D

Which of the following is not an example of coherent source produced by the division of wavefront?
  • a)
    Fresnel’s biprism
  • b)
    Lloyd’s mirror
  • c)
    Young’s double slit experiment
  • d)
    Interference by thin films
Correct answer is option 'D'. Can you explain this answer?

Suresh Iyer answered
The correct answer is option D
In A,B,C there is a single source (coherent source)while in D there is an interference of thin films whose sources are different ,therefore not coherent.

An unpolarised beam of intensity Io is incident on a polarizer and analyser placed in contact. The angle between the transmission axes of the polarizer and the analyser is θ. What is the intensity of light emerging out of the analyser?​
  • a)
    (I0 cos 2θ)/2
  • b)
    I0 cos2θ
  • c)
    I0 cosθ
  • d)
    I0
Correct answer is option 'A'. Can you explain this answer?

Nikita Singh answered
 Suppose the angle between the transmission axes of the analyser and the polarizer is θ. The completely plane polarized light form the polarizer is incident on the analyser. If E0 is the amplitude of the electric vector transmitted by the polarizer, then intensity I0 of the light incident on the analyser is
I ∞ E02
 
The electric field vector E0 can be resolved into two rectangular components i.e E0 cosθ and E0 sinθ. The analyzer will transmit only the component ( i.e E0 cosθ ) which is parallel to its transmission axis. However, the component E0sinθ will be absorbed by the analyser. Therefore, the intensity I of light transmitted by the analyzer is,
 
I ∞ ( E0 x cosθ )2
 
I / I0 = ( E0 x cosθ )2 / E02 = cos2θ
 
I = I0 x cos2θ
when light passes from polarizer it's intensity becomes half and when passed through analyser it becomes,
I = I0 x cos2θ/2
 

Diffraction of light gives the information of
  • a)
    Transverse nature
  • b)
    Longitudinal nature
  • c)
    Both transverse and longitudinal
  • d)
    Neither transverse nor longitudina
Correct answer is option 'D'. Can you explain this answer?

Vijay Bansal answered
Transverse and longitudinal waves. Light and other types of electromagnetic radiation are transverse waves. Water waves and S waves (a type of seismic wave) are also transverse waves. In transverse waves, the vibrations are at right angles to the direction of travel.

The phenomena diffraction can take place in sound waves.
  • a)
    Yes
  • b)
    No
  • c)
    Only Interference
  • d)
    Under certain conditions only
Correct answer is option 'A'. Can you explain this answer?

Rohan Singh answered
YES, Sound waves, on the other hand, are longitudinal, meaning that they oscillate parallel to the direction of their motion. Since there is no component of a sound wave's oscillation that is perpendicular to its motion, sound waves cannot be polarized

Two coherent sources produce a dark fringe when phase difference between the interfering waves is(n integer)​
  • a)
    2π
  • b)
    (2n – 1)π
  • c)
    zero
  • d)
    n
Correct answer is option 'B'. Can you explain this answer?

Shruti Sarkar answered
Dark fringes will be produced when there are destructive interference. The condition for that is the two waves should have a phase difference of an odd integral multiple of π.

A rotating calcite crystal is placed over an ink dot. On seeing through the crystal one finds:
  • a)
    Two stationary dots
  • b)
    Two dots moving along straight lines
  • c)
    One dot rotating about the other
  • d)
    Both dots rotating about a common axis
Correct answer is option 'C'. Can you explain this answer?

Anaya Patel answered
In calcite crystal there is a phenomenon of double refraction. So, we see one dot as a stationary object whereas the other being refracted at a little displaced position seems to be rotating about the first dot.

Diffraction pattern cannot be observed with:
a)one wide slit
b)two narrow slits
c)one narrow slit
d)large number of narrow slits
Correct answer is option 'A'. Can you explain this answer?

Pooja Mehta answered
If one illuminates two parallel slits, the light from the two slits again interferes. Here the interference is a more pronounced pattern with a series of alternating light and dark bands. ... He also proposed (as a thought experiment) that if detectors were placed before each slit, the interference pattern would disappear.

A beam of light has a wavelength of 650 nm in vacuum. What is the wavelength of these waves in the liquid whose index of refraction at this wavelength is 1.47?
  • a)
    472nm
  • b)
    442nm
  • c)
    462nm
  • d)
    452nm
Correct answer is option 'B'. Can you explain this answer?

Geetika Shah answered
We know that,
Wavelength of light in a material:
λ= λ0/n
where, λ=wavelength of material, λ0=wavelength of light in vacuum,
n=index of refraction of material.
So, 650x10-9m/1.47=442.177nm.

Which of the following undergoes largest diffraction?
  • a)
    Ultraviolet light
  • b)
    Infra red light
  • c)
    Radio waves
  • d)
    Y – rays
Correct answer is option 'C'. Can you explain this answer?

Anjana Sharma answered
Maximum diffraction occurs when size of obstacle is almost equal to wavelength of light wave. Hence maximum diffraction occurs for larger wavelength . As wavelength of radio wave is higher than others, maximum diffraction will occur for it.   

In a Fresnel’s biprism experiment, the two coherent sources are obtained by:
  • a)
    two slits
  • b)
    reflection
  • c)
    Refraction
  • d)
    Internal reflection
Correct answer is option 'C'. Can you explain this answer?

Om Desai answered
When a light travels from one medium to another then deviation in the path of light known as refraction. And in the prism surrounding medium is air and material medium is glass due to the change of medium refraction will occur in prism.

The Brewster’s angle for a transparent medium is 600.The angle of incidence is​
  • a)
    450
  • b)
    900
  • c)
    600
  • d)
    300
Correct answer is option 'C'. Can you explain this answer?

Hansa Sharma answered
Brewster's angle is an angle of incidence at which light with a particular polarization is perfectly transmitted through a transparent dielectric surface, with no reflection. When unpolarized light is incident at this angle, the light that is reflected from the surface is therefore perfectly polarized.
So the angle of incidence would be 60°

The change in diffraction pattern of a single slit, when the monochromatic source of light is replaced by a source of white light will be
  • a)
    Diffracted image is not clear
  • b)
    Diffracted image gets dispersed into constituent colours of white light
  • c)
    The image of the slit becomes infinitely wide
  • d)
    Clear colourful fringe pattern
Correct answer is option 'B'. Can you explain this answer?

Dr Manju Sen answered
  • When a source of white light is used instead of a monochromatic source, the diffracted image of the slit gets dispersed into constituent colours of white light.
  • The central maximum will be white and on either side of the central maximum, there will be coloured fringes.

Emission and absorption is best described by,
  • a)
    dual / schizophrenic model
  • b)
    a wave model
  • c)
    None of the above
  • d)
    particle model
Correct answer is option 'D'. Can you explain this answer?

Nandini Iyer answered
Distance between the slits, d=0.28×10−3 m
Distance between the slits and the screen, D=1.4m
Distance between the central fringe and the fourth (n=4) fringe, 
u=1.2×10−2 m
In case of a constructive interference, we have the relation for the distance between the two fringes as:
u=nλD/d
⇒λ=ud/nD=6×10−7m=600nm

If the light is completely polarized by reflection, then angle between the reflected and refracted light is
  • a)
    π
  • b)
    2π
  • c)
    π/2
  • d)
    π/4
Correct answer is option 'C'. Can you explain this answer?

Sant Singh answered
This is in accordance with Brewsters law. When light is incident. When light is incident at Brewsters angle then the refracted and reflected ray are perpendicular to reach other.

Phase difference between two coherent sources should be:​
  • a)
    zero
  • b)
    πn rad
  • c)
    π rad
  • d)
    π/2 rad
Correct answer is option 'A'. Can you explain this answer?

Kalyan Joshi answered
Phase difference between two coherent sources should be zero.

Coherent Sources:
Coherent sources are the sources of light that emit light waves of the same frequency and have a constant phase difference with respect to each other. When two coherent sources emit light waves, the waves superimpose to form an interference pattern.

Phase Difference:
The phase difference is the difference in the phase of two waves at a given point in space and time. It is measured in radians or degrees. When two coherent sources emit light waves, the phase difference between them determines the interference pattern.

Explanation:
When two coherent sources emit light waves of the same frequency and zero phase difference, the waves superimpose constructively and form a bright fringe in the interference pattern. If the phase difference between the two sources is not zero, the waves may superimpose destructively, resulting in a dark fringe.

Therefore, to obtain a clear interference pattern, the phase difference between the two sources should be zero. This can be achieved by ensuring that the two sources are of the same frequency and are in phase with each other.

Conclusion:
In conclusion, the phase difference between two coherent sources should be zero to obtain a clear interference pattern. This can be achieved by ensuring that the two sources are of the same frequency and are in phase with each other.

In an interference experiment monochromatic light is replaced by white light, we will see:​
  • a)
    uniform darkness on the screen
  • b)
    a few coloured bands and then uniform illumination
  • c)
    equally spaced white and dark bands
  • d)
    uniform illumination on the screen
Correct answer is option 'B'. Can you explain this answer?

Arun Khanna answered
Therefore if monochromatic light in Young's interference experiment is replaced by white light, then the waves of each wavelength form their separate interference patterns. The resultant effect of all these patterns is obtained on the screen. i.e., the waves of all colours reach at mid point M in same phase we will see​.a few coloured bands and then uniform illumination

The intensity of light transmitted by the analyzer is maximum when
  • a)
    The transmission axes of the analyzer and the polarizer are perpendicular
  • b)
    The reflected ray is fully polarized
  • c)
    The transmission axes of the analyzer and the polarizer are parallel
  • d)
    The reflected ray is partially polarized
Correct answer is option 'C'. Can you explain this answer?

Alfiya I answered
From malu's law, If I0 is intensity of plane polarised light incident on an analyser and (thete) is the angle between axes of polariser and analyser, then intensity of polarised light transmitted from analyser ,I = I0 cos(^2) thete, ie, when (thete)=0, cos(^2) 0=1, then I= I0 ie, the transmission axes of polariser and analyser are paralell.

In the Young’s double slit experiment, the distance of p th dark fringe from the central maximum is:
  • a)
  • b)
  • c)
  • d)
Correct answer is option 'B'. Can you explain this answer?

Neha Sharma answered
Position of the pth dark fringe is at a distance of 
y=(2p−1) (λD/2d)​ from the centre
where 
λ: Wavelength
D: Distance between slits 
d: slit width

When viewed in white light, a soap bubble shows colours because of
  • a)
    Interference
  • b)
    Scattering
  • c)
    Dispersion
  • d)
    Diffraction
Correct answer is option 'A'. Can you explain this answer?

Sibi Roopan answered
When the rays are entering the surface some part of light is reflected back and some part of light is refracted then it against reflects in the inner surface of the bubble and get outside of the bubble.this ray is parallel to the reflected.now the both rays of wavelength are "out of phase". this is called destructive interference result in reduce in Color of intensity.

Shape of the wave front of portion of the wave front of light from a distant star intercepted by the Earth
  • a)
    plane
  • b)
    spherical
  • c)
    hyperboloid
  • d)
    conical
Correct answer is option 'A'. Can you explain this answer?

Aryan Keshri answered
Option A ....Since the light coming from the distance star to the earth will be at Infinity , the rays coming from it will be parallel rays , thus the wavefront will be a plane .

Shape of the wave front of light emerging out of a convex lens when a point source is placed at its focus.
  • a)
    spherical
  • b)
    plane
  • c)
    conical
  • d)
    hyperboloid
Correct answer is option 'B'. Can you explain this answer?

Geetika Tiwari answered
Explanation:

When a point source is placed at the focus of a convex lens, the emerging wave front of light will be plane. This can be explained using the following points:

  • Definition: A wave front of light is a continuous and connected surface of points where the phase of all points is the same.

  • Convex Lens: A convex lens is thicker in the middle and thinner at the edges. It is also known as a converging lens because it converges the beam of light passing through it.

  • Focus: The focus of a convex lens is a point on the principal axis where all the parallel rays of light converge after passing through the lens.

  • Point Source: A point source is a source of light that emits light in all directions from a single point.



Working:

When a point source is placed at the focus of a convex lens, the light rays emerging from the lens will be parallel to each other. This is because the convex lens converges the light rays that pass through it. When the light rays converge at the focus, they become parallel to each other.

Since the light rays are parallel to each other, the wave front of light emerging from the lens will be a plane surface. This is because all the points on the wave front have the same phase and are equidistant from the source.

Therefore, the correct answer is option B, plane.

Coherent sources can be created by the division of:
  • a)
    only amplitude
  • b)
    only wave front
  • c)
    only wavelength
  • d)
    both amplitude and wave front
Correct answer is option 'D'. Can you explain this answer?

Rajeev Saxena answered
 Two sources are said to be coherent if they have exactly the same frequency and zero or constant phase difference between them. Methods to produce them: ✓ Division of wavefront where wavefront is divided into two parts by reflection,refraction or diffraction .

A parallel beam of light of wavelength 500 nm falls on a narrow slit and the resulting diffraction pattern is observed on a screen 1 m away. It is observed that the first minimum is at a distance of 2.5 mm from the centre of the screen. Find the width of the slit.
  • a)
    0.32 mm
  • b)
    0.2 mm
  • c)
    0.3 mm
  • d)
    0.25 mm
Correct answer is option 'B'. Can you explain this answer?

Poulomi Desai answered
Wavelength of light beam, λ
Distance of the screen from the slit, D=1m
For first minima, n=1
Distance between the slits is d
Distance of the first minimum from the centre of the screen can be obtained as, x = 2.5mm = 2.5×10−3
Now, nλ = xd/D
⇒ d= nλD/x = 0.2mm
Therefore, the width of the slits is 0.2 mm.

Which of the following phenomenon cannot be explained by diffraction?
  • a)
    lack of sharp boundary when a point source of light illuminates a straightedge
  • b)
    The rainbow fringes of an oil slick on water
  • c)
    fringes observed from a large number of parallel slits
  • d)
    A bulb filament viewed through two blades held so that the edges form a slit
Correct answer is option 'B'. Can you explain this answer?

Nisha Kulkarni answered
The bright colors seen in an oil slick floating on water or in a sunlit soap bubble are caused by interference. The brightest colors are those that interfere constructively. This interference is between light reflected from different surfaces of a thin film; thus, the effect is known as thin film interference.

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