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CBSE-XI-Physics

17: Light Waves

with Solutions - page 2

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  • Qstn #2
    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
    (d) neither on elasticity nor on inertia
    digAnsr:   d
    Ans : (d) neither on elasticity nor on inertia
    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.
    Page No 379:
  • Qstn #3
    The equation of a light wave is written as
    y=A sinkx-ωt. Here, y represents
    (a) displacement of ether particles
    (b) pressure in the medium
    (c) density of the medium
    (d) electric field
    digAnsr:   d
    Ans : (d) electric field
    Light consists of mutually perpendicular electric and magnetic fields. So, the equation of a light wave is represented by its field vector.
    Page No 379:
  • Qstn #4
    Which of the following properties shows that light is a transverse wave?
    (a) Reflection
    (b) Interference
    (c) Diffraction
    (d) Polarization
    digAnsr:   d
    Ans : (d) Polarization
    Reflection, interference and diffraction are the phenomena shown by both transverse waves and longitudinal waves. Polarization is the phenomenon shown only by transverse waves.
    Page No 379:
  • Qstn #5
    When light is refracted into a medium,
    (a) its wavelength and frequency increase
    (b) its wavelength increases but frequency remains unchanged
    (c) its wavelength decreases but frequency remains unchanged
    (d) its wavelength and frequency decrease
    digAnsr:   c
    Ans : (c) its wavelength decreases but frequency remains unchanged
    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 }_{{\rm M}}=\frac{\lambda }{\mu },``
    `` where{\lambda }_{{\rm M}}=wavelengthinmedium``
    `` \lambda =wavelengthinvacuum``
    `` \mu =refractiveindex``
    Page No 379:
  • Qstn #6
    When light is refracted, which of the following does not change?
    (a) Wavelength
    (b) Frequency
    (c) Velocity
    (d) Amplitude
    digAnsr:   b
    Ans : (b) Frequency
    Frequency of a light wave doesn't change on changing the medium of propagation of light.
    Page No 379:
  • Qstn #7
    An amplitude modulated (AM) radio wave bends appreciably round the corners of a 1 m × 1 m board but a frequency modulated (FM) wave only bends negligibly. If the average wavelengths of the AM and FM waves are
    λa and λf,
    (a)
    λa>λf
    (b)
    λa=λf
    (c)
    λa<λf
    (d) We don’t have sufficient information to evaluate the relation of
    λa and λf.
    digAnsr:   a
    Ans : (a) `` {\lambda }_{a}>{\lambda }_{f}``
    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 corners of a 1 m `` \times `` 1m board.
    λ
    Page No 379:
  • Qstn #8
    Which of the following sources provides the best monochromatic light?
    (a) A candle
    (b) A bulb
    (c) A mercury tube
    (d) A laser
    digAnsr:   d
    Ans : (d) A laser
    Among the given sources, laser is the best coherent source providing monochromatic light with constant phase difference.
    Page No 379:
  • Qstn #9
    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°
    (b) 45°
    (c) 90°
    (d)
    cos-1 1/3
    digAnsr:   d
    Ans : (d) `` {\,\mathrm{\,cos\,}}^{-1}\left(1/\sqrt{3}\right)``
    On writing the given equation in the plane equation form lx + my + nz = p,
    where l2 + m2 + n2 = 1 and p>0, we get:
    `` \frac{1}{\sqrt{3}}x+\frac{1}{\sqrt{3}}y+\frac{1}{\sqrt{3}}z=\frac{c}{\sqrt{3}}``
    If `` \theta `` is the angle between the normal and +X axis, then
    `` \,\mathrm{\,cos\,}\theta =\frac{1}{\sqrt{3}}``
    `` \Rightarrow \theta ={\,\mathrm{\,cos\,}}^{-1}\left(\frac{1}{\sqrt{3}}\right)``
    Page No 379:
  • Qstn #10
    The wavefronts of light coming from a distant source of unknown shape are nearly
    (a) plane
    (b) elliptical
    (c) cylindrical
    (d) spherical
    digAnsr:   a
    Ans : (a) plane
    Wave travelling from a distant source always has plane wavefront.
    Page No 379:
  • Qstn #11
    The inverse square law of intensity (i.e. the intensity
    ∞1r2) is valid for a
    (a) point source
    (b) line source
    (c) plane source
    (d) cylindrical source
    digAnsr:   a
    Ans : (a) point source
    Intensity of a point source obeys the inverse square law.
    Intensity of light at distance r from the point source is given by
    `` I=S/\left(4{\,\mathrm{\,\pi r\,}}^{2}\right)`` ,
    where S is the source strength.
    Page No 379:
  • Qstn #12
    Two sources are called coherent if they produce waves
    (a) of equal wavelength
    (b) of equal velocity
    (c) having same shape of wave front
    (d) having a constant phase difference
    digAnsr:   a
    Ans :
    (d) having a constant phase difference
    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.
    Page No 379:
  • Qstn #13
    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
    (d) interference of light
    digAnsr:   d
    Ans : (d) interference of light
    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))In 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.
  • Qstn #14
    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
    (c) 9 : 4
    (d) 625 : 1
    digAnsr:   a
    Ans :
    (c) 9 : 4
    Ratio of maximum intensity and minimum intensity is given by
    `` \frac{{I}_{max}}{{I}_{min}}=\frac{{\left(\sqrt{{I}_{1}}+\sqrt{{I}_{2}}\right)}^{2}}{{\left(\sqrt{{I}_{1}}-\sqrt{{I}_{2}}\right)}^{2}}=\frac{25}{1}``
    `` \Rightarrow \sqrt{{I}_{1}}=3and\sqrt{{I}_{2}}=2``
    `` \Rightarrow {I}_{1}=9and{I}_{2}=4``
    `` ``
    Then,
    `` \frac{{I}_{1}}{{I}_{2}}=\frac{9}{4}``
    Page No 380:
  • Qstn #15
    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 I0. If one of the slits is closed, the intensity at this point will be
    (a) I0
    (b) I0/4
    (c) I0/2
    (d) 4I0
    digAnsr:   b
    Ans : (b) I0/​4
    Total intensity coming from the source is I0 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 1/4th of intensity.
    Page No 380:
  • Qstn #16
    A thin transparent sheet is placed in front of a Young’s double slit. The fringe-width will
    (a) increase
    (b) decrease
    (c) remain same
    (d) become non-uniform
    digAnsr:   c
    Ans : (c) remain same
    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.
    Page No 380:
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