Q#1
Light is(a) wave phenomenon
(b) particle phenomenon
(c) both particle and wave phenomenon.
Answer: (c)
The photoelectric effect shows it as a particle phenomenon while the interference and diffraction of light shows it as a wave phenomenon. Hence duel nature.
Q#2
The speed of light depends on
(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
Answer: (d)
The light does not require even a medium. The speed of light in a medium is the intrinsic property of that medium which is related to the refractive index of the medium.
Q#3
The equation of a light wave is written as
y = A sin(kx - ⍵t).
Here y represents
(a) displacement of ether particles
(b) pressure in the medium
(c) density of the medium
(d) electric field.
Answer:(d)
The light wave does not require a medium, hence the displacement of particles, the variation of pressure and density in the medium has no relation to the light wave. Light is an electromagnetic wave in which the electric field transverse to the motion of wave changes with time and place. Here y represents the electric field in the wave equation.
Q#4
Which of the following properties show that light is a transverse wave?
(a) Reflection.
(b) Interference.
(c) Diffraction.
(d) Polarization.
Answer:(d)
The reflection, interference and diffraction effects are shown by both transverse and longitudinal waves. But polarization is only possible with a transverse wave because the transverse displacement of the particles of a medium or the transverse change of the electric field (as in lightwave) are in a plane and a slit perpendicular to this plane will not allow it to pass.
Q#5
When light is refracted into a medium,
(a) its frequency and wavelength both increase
(b) its wavelength increases but frequency remains unchanged
(c) its wavelength decreases but frequency remains unchanged
(d) its wavelength and frequency both decrease.
Answer: (c)
When light is refracted into a medium its speed changes but the frequency remains the same because the energy associated with a photon of this light is proportional to the frequency of the light and it remains unchanged. Since the wavelength ๐ = c/๐, i.e. ๐ ∝ c. So the wavelength changes. Option (c) is true.
Q#6
When light is refracted, which of the following does not change?
(a) wavelength.
(b) frequency.
(c) velocity.
(d) amplitude.
Answer:(b)
Same explanation as in question above.
Q#7
The amplitude modulated (AM) radio wave bends appreciably round the corners of 1 m x 1 m board but frequency modulated (FM) wave only negligibly bends. If the average wavelengths of AM and FM waves are ๐โ and ๐แตฉ,
(a) ๐โ > ๐แตฉ
(b) ๐โ = ๐แตฉ
(c) ๐โ < ๐แตฉ
(d) we don't have sufficient information to decide about the relation of ๐โ and ๐แตฉ.
Answer: (a)
The wave having greater wavelength will bend more.
In fact, the frequency of the FM waves is nearly 100 times more than the AM waves hence the wavelength of the AM waves is 100 times more than FM waves.
Q#8
Which of the following sources gives the best monochromatic light?
(a) A candle
(b) A bulb
(c) A mercury tube
(d) A laser.
Answer: (d)
The candle, bulb and mercury tube give lights of a mixture of wavelengths. In a laser, the same wavelength of light is processed and amplified. Hence the option (d).
Q#9
The wavefronts of a light wave traveling in the vacuum are given by x + y + z = c. The angle made by the direction of propagation of light with X-axis is
(a) 0°
(b) 45°
(c) 90°
(d) cos⁻¹(1/√3).
Answer: (d)
The given equation of the wavefronts represent a plane that intersects each of the axes at a distance c from the origin. Let these points be named A, B and C. ABC is an equilateral triangle of side c.
The direction of propagation of light will be normal to the plane. In this case, the normal makes equal angles with each of the axes say ฮฑ. Since the sum of the squares of the direction cosines equals one, here for the normal
cos²ฮฑ + cos²ฮฑ + cos²ฮฑ = 1
3 cos²ฮฑ = 1
cos²ฮฑ = 1/3
cos ฮฑ = 1/√3
ฮฑ = cos⁻¹ (1/√3)
Hence the option (d).
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