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Physics M.C.Q (1 Marks)

Light Waves · Rajasthan Board (RBSE) STD 12 Science (Rajasthan - English Medium). 20 questions.

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M.C.Q (1 Marks)

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20 questions · auto-graded multiple-choice test.

MCQ 11 Mark
Two coherent sources of different intensities send waves which interfere. The ratio of maximum intensity to the minimum intensity is 25. The intensities of the sources are in the ratio:
  • A
    25 : 1
  • B
    5 : 1
  • ✓
    9 : 4
  • D
    625 : 1
Answer
Correct option: C.
9 : 4
Explanation:
Ratio of maximum intensity and minimum intensity is given by
$\frac{\text{I}_\text{max}}{\text{I}_\text{min}}=\frac{(\sqrt{\text{I}_1}+\sqrt{\text{I}_2})^2}{(\sqrt{\text{I}_1}-\sqrt{\text{I}_2})^2}=\frac{25}{1}$
$\Rightarrow\sqrt{\text{I}_1}=3 \ \text{and}\ \sqrt{\text{I}_2}=2$
$\Rightarrow\text{I}_1=9\ \text{and}\ \text{I}_2=4$
Then,
$\frac{\text{I}_1}{\text{I}_2}=\frac{9}{4}$
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MCQ 21 Mark
The inverse square law of intensity $\Big(\text{i.e., the intensity}\propto\frac{1}{\text{r}^2}\Big)$ is valid for a:
  • ✓
    Point source.
  • B
    Line source.
  • C
    Plane source.
  • D
    Cylindrical source.
Answer
Correct option: A.
Point source.
Explanation:
Intensity of a point source obeys the inverse square law.
Intensity of light at distance r from the point source is given by
$\text{I}=\frac{\text{S}}{(4\pi\text{r}^2)}$
Where S is the source strength.
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MCQ 31 Mark
The slits in a Young's double slit experiment have equal width and the source is placed symmetrically with respect to the slits. The intensity at the central fringe is $I_0$. If one of the slits is closed, the intensity at this point will be:
  • A
    $\text{I}_0$
  • ✓
    $\frac{\text{I}_0}{4}$
  • C
    $\frac{\text{I}_0}{2}$
  • D
    $4\text{I}_0$
Answer
Correct option: B.
$\frac{\text{I}_0}{4}$
Total intensity coming from the source is $I_0$ which is present at the central maxima. In case of two slits, the intensity is getting distributed between the two slits and for a single slit, the amplitude of light coming from the slit is reduced to half which leads to $\frac{1}{4}\text{th}$ of intensity.
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MCQ 41 Mark
The equation of a light wave is written as $\text{y}=\text{A}\ \sin(\kappa\text{x}-\omega\text{t}).$ Here, y represents:
  • A
    Displacement of either particles.
  • B
    Pressure in the medium.
  • C
    Density of the medium.
  • ✓
    Electric field.
Answer
Correct option: D.
Electric field.
Explanation:
Light consists of mutually perpendicular electric and magnetic fields. So, the equation of a light wave is represented by its field vector.
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MCQ 51 Mark
Which of the following properties show that light is a transverse wave?
  • A
    Reflection.
  • B
    Interference.
  • C
    Diffraction.
  • ✓
    Polarization.
Answer
Correct option: D.
Polarization.
Explanation:
Reflection, interference and diffraction are the phenomena shown by both transverse waves and longitudinal waves. Polarization is the phenomenon shown only by transverse waves.
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MCQ 61 Mark
If Young's double slit experiment is performed in water:
  • ✓
    The fringe width will decrease.
  • B
    The fringe width will increase.
  • C
    The fringe width will remain unchanged.
  • D
    There will be no fringe.
Answer
Correct option: A.
The fringe width will decrease.
Explanation:
As fringe width is proportional to the wavelength and wavelength of light is inversely proportional to the refractive index of the medium,
Here,
$\lambda_\text{M}=\frac{\lambda}{\eta}$
$\lambda_\text{M}$ = wavelength in medium
$\lambda$ = wavelength in vacuum
$\eta$ = refractive index of medium
Hence, fringe width decreases when Young's double slit experiment is performed under water.
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MCQ 71 Mark
If the source of light used in a Young's double slit experiment is changed from red to violet:
  • A
    The fringes will become brighter.
  • ✓
    Consecutive fringes will come closer.
  • C
    The intensity of minima will increase.
  • D
    The central bright fringe will become a dark fringe.
Answer
Correct option: B.
Consecutive fringes will come closer.
Explanation:
Fringe width, $\beta=\frac{\lambda\text{D}}{\text{d}}$
Wavelength of red light is greater than wavelength of violet light; so, the fringe width will reduce.
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MCQ 81 Mark
The wavefronts of a light wave travelling in vacuum are given by $x + y + z = c$. The angle made by the direction of propagation of light with the $X-$axis is:
  • A
    $0^\circ$
  • B
    $45^\circ$
  • C
    $90^\circ$
  • ✓
    $\cos^{-1}\Big(\frac{1}{\sqrt{3}}\Big)$
Answer
Correct option: D.
$\cos^{-1}\Big(\frac{1}{\sqrt{3}}\Big)$
On writing the given equation in the plane equation form $ lx + my + nz = p,$
Where $\mathrm{l}^2+\mathrm{m}^2+\mathrm{n}^2$ and $\mathrm{p}>0$, we get:
$\frac{1}{\sqrt{3}}\text{x}+\frac{1}{\sqrt{3}}\text{y}+\frac{1}{\sqrt{3}}\text{z}=\frac{\text{c}}{\sqrt{3}}$
If $\theta$ is the angle between the normal and $+x$ axis, then
$\cos\theta=\frac{1}{\sqrt{3}}$
$\Rightarrow\theta=\cos^{-1}\Big(\frac{1}{\sqrt{3}}\Big) $
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MCQ 91 Mark
When light is refracted, which of the following does not change?
  • A
    Wavelepgth.
  • ✓
    Frequency.
  • C
    Velocity.
  • D
    Amplitude.
Answer
Correct option: B.
Frequency.
Explanation:
Frequency of a light wave doesnt change on changing the medium of propagation of light.
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MCQ 101 Mark
When a drop of oil is spread on a water surface, it displays beautiful colours in daylight because of:
  • A
    Disperson of light.
  • B
    Reflection of light.
  • C
    Polarization of light.
  • ✓
    Interference of light.
Answer
Correct option: D.
Interference of light.
Explanation:
Interference effect is produced by a thin film (coating of a thin layer of a translucent material on a medium of different refractive index which allows light to pass through it). ln the present case, oil floating on water forms a thin film on the surface of water, leading to the display of beautiful colours in daylight because of the interference of sunlight.
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MCQ 111 Mark
Which of the following sources gives best monochromatic light?
  • A
    A candle.
  • B
    A bulb.
  • C
    A mercury tube.
  • ✓
    A laser.
Answer
Correct option: D.
A laser.
Explanation:
Among the given sources, laser is the best coherent source providing monochromatic light with constant phase difference.
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MCQ 121 Mark
When light is refracted into a medium:
  • A
    Its wavelength and frequency both increase
  • B
    Its wavelength increases but frequency remains unchanged
  • ✓
    Its wavelength decreases but frequency remains unchanged.
  • D
    Its wavelength and frequency both decrease.
Answer
Correct option: C.
Its wavelength decreases but frequency remains unchanged.
Explanation:
Frequency of a light wave, as it travels from one medium to another, always remains unchanged, while wavelength decreases.
Decrease in the wavelength of light entering a medium of refractive index $\mu$, is given by,
$\lambda_\text{M}=\frac{\lambda}{\mu},$
Where $\lambda_\text{M}$ = wavelength in medium
$\lambda$ = wavelength in vacuum
$\mu$ = refractive index
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MCQ 131 Mark
A thin transparent sheet is placed in front of a Young's double slit. The fringe-width will:
  • A
    Increase.
  • B
    Decrease.
  • ✓
    Remain same.
  • D
    Become nonuniform.
Answer
Correct option: C.
Remain same.
Explanation:
On the introduction of a transparent sheet in front of one of the slits, the fringe pattern will shift slightly but the width will remain the same.
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MCQ 141 Mark
Two sources are called coherent if they produce waves:
  • A
    Of equal wavelength.
  • B
    Of equal velocity.
  • C
    Having same shape of wavefront.
  • ✓
    Having a constant phase difference.
Answer
Correct option: D.
Having a constant phase difference.
Explanation:
For light waves emitted by two sources of light to remain coherent, the initial phase difference between waves should remain constant in time. If the phase difference changes continuously or randomly with time, then the sources are incoherent.
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MCQ 151 Mark
The amplitude modulated (AM) radio wave bends appreciably round the corners of a 1m × 1m board but the frequency modulated (FM) wave only negligibly bends. If the average wavelengths of AM and FM waves are $\lambda_\text{a}$ and $\lambda_\text{f}:$
  • A
    $\lambda_\text{a}>\lambda_\text{f}$
  • ✓
    $\lambda_\text{a}=\lambda_\text{f}$
  • C
    $\lambda_\text{a}<\lambda_\text{f}$
  • D
    We don't have sufficient information to decide about the relation of $\lambda_\text{a}$ and $\lambda_\text{f}$
Answer
Correct option: B.
$\lambda_\text{a}=\lambda_\text{f}$
Explanation:
An electromagnetic wave bends round the corners of an obstacle if the size of the obstacle is comparable to the wavelength of the wave. An AM wave has less frequency than an FM wave, So, an AM wave has a higher wavelength than an FM wave and it bends round the comers of a 1m × 1m board.
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MCQ 161 Mark
Light waves travel in vacuum along the X-axis. Which of the following may represent the wavefronts?
  • ✓
    x = c.
  • B
    y = c.
  • C
    z = c.
  • D
    x + y + z = c.
Answer
Correct option: A.
x = c.
Explanation:
​The wave is travelling along the X-axis. So, it'll have planar wavefront perpendicular to the X-axis.
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MCQ 171 Mark
The wavefronts of light coming from a distant source of unknown shape are nearly:
  • ✓
    Plane.
  • B
    Elliptical.
  • C
    Cylindrical.
  • D
    Spherical.
Answer
Correct option: A.
Plane.
Explanation:
Wave travelling from a distant source always has plane wavefront.
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MCQ 181 Mark
Huygens' principle of secondary wavelets may be used to:
  1. Find the velocity of light in vacuum.
  2. Explain the particle behaviour of light.
  3. Find the new position of a wavefront.
  4. Explain Snell's law.
  • A
    $a$ and $b$ 
  • B
    $b$ and $c$
  • ✓
    $c$ and $d$
  • D
    $b$ and $d$
Answer
Correct option: C.
$c$ and $d$
Huygen's wave theory explains the origin of points for the new wavefront proceeding successively.
It also explains the variation in speed of light on moving from one medium to another,
i.e. it proves Snell's Law.
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MCQ 191 Mark
The speed of light depends:
  • A
    On elasticity of the medium only.
  • B
    On inertia of the medium only.
  • C
    On elasticity as well as inertia.
  • ✓
    Neither on elasticity nor on inertia.
Answer
Correct option: D.
Neither on elasticity nor on inertia.
Explanation:
The speed of light in any medium depends on the refractive index of that medium, which is an intensive property. Hence, speed of light is not affected by the elasticity and inertia of the medium.
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MCQ 201 Mark
Three observers $A, B$ and $C$ measure the speed of light coming from a source to be $vA, 0B$ and $vc.$ The observer $A$ moves towards the source and $C$ moves away from the source at the same speed. The observer $B$ stays stationary. The surrounding space is vacuum everywhere.
  • A
    $\text{v}_\text{A}>\text{v}_\text{B}>\text{v}_\text{C}.$
  • B
    $\text{v}_\text{A}<\text{v}_\text{B}<\text{v}_\text{C}$
  • ✓
    $\text{v}_\text{A}=\text{v}_\text{B}=\text{v}_\text{C}$
  • D
    $\text{v}_\text{B}=\frac{1}{2}(\text{v}_\text{A}+\text{v}_\text{C})$
Answer
Correct option: C.
$\text{v}_\text{A}=\text{v}_\text{B}=\text{v}_\text{C}$
Since the speed of light is a universal constant, $\text{v}_\text{A}=\text{v}_\text{B}=\text{v}_\text{C}=3\times10^8\text{m/s.}$
$\text{v}_\text{B}=\frac{1}{2}(\text{u}_\text{A}+\text{u}_\text{C})$ This expression also implies that $\mathrm{V}_{\mathrm{A}}=\mathrm{V}_{\mathrm{B}}=\mathrm{V}_{\mathrm{C}}$.
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