CBSE Class 12 Physics Wave Optics MCQs Set E

Question: A parallel beam of fast moving electrons is incident normally on a narrow slit. A fluorescent screen is placed at a large distance from the slit. If the speed of the electrons is increased, which of the following statements is correct?
(a) The angular width of the central maximum will decrease.
(b) The angular width of the central maximum will be unaffected.
(c) Diffraction pattern is not observed on the screen in the case of electrons.
(d) The angular width of the central maximum of the diffraction pattern will increase.
Answer: a

Question: Diameter of human eye lens is 2 mm. What will be the minimum distance between two points to resolve them, which are situated at a distance of 50 meter from eye? (The wavelength of light is 5000 Å.)
(a) 2.32 m
(b) 4.28 mm
(c) 1.25 cm
(d) 12.48 cm
Answer: c

Question: Interference was observed in interference chamber where air was present, now the chamber is evacuated, and if the same light is used, a careful observer will see
(a) no interference
(b) interference with brighter bands
(c) interference with dark bands
(d) interference with larger width. 
Answer: d

Question:  Ratio of intensities of two waves are given by 4 : 1. Then ratio of the amplitudes of the two waves is
(a) 2 : 1
(b) 1 : 2
(c) 4 : 1
(d) 1 : 4
Answer: a

Question: A star, which is emitting radiation at a wavelength of 5000 Å, is approaching the earth with a velocity of 1.5 × 10⁴ m/s. The change in wavelength of the radiation as received on the earth is
(a) 25 Å
(b) 100 Å
(c) zero
(d) 2.5 Å
Answer: a

Question: Green light of wavelength 5460 Å is incident on an air-glass interface. If the refractive index of glass is 1.5, the wavelength of light in glass would be (c = 3 × 10⁸ m s^–1)
(a) 3640 Å
(b) 5460 Å
(c) 4861 Å
(d) none of these.
Answer: a

Question: Assume that light of wavelength 600 nm is comingfrom a star. The limit of resolution of telescopewhose objective has a diameter of 2 m is
(a) 3.66 × 10^–7 rad
(b) 1.83 × 10^–7 rad
(c) 7.32 × 10^–7 rad
(d) 6.00 × 10^–7 rad
Answer: a

Question: A parallel beam of monochromatic light of wavelength 5000 Å is incident normally on a single narrow slit of width 0.001 mm. The light is focussed by a convex lens on a screen placed in focal plane. The first minimum will be formed for the angle of diffraction equal to
(a) 0°
(b) 15°
(c) 30°
(d) 50°
Answer: c

Question: Two polaroids P₁ and P₂ are placed with their axis perpendicular to each other. Unpolarised light I0 is incident on P₁. A third polaroid P₃ is kept in between P₁ and P₂ such that its axis makes an angle 45° with that of P₁. The intensity of transmitted light through P₂ is
(a) I₀/4
(b) I₀/8
(c) I₀/16
(d) I₀/2
Answer: b

Question: Interference is possible in
(a) light waves only
(b) sound waves only
(c) both light and sound waves
(d) neither light nor sound waves. 
Answer: c

Question: In Young’s double slit experiment, if the separation between coherent sources is halved and the distance of the screen from the coherent sources is doubled, then the fringe width becomes
(a) double
(b) half
(c) four times
(d) one-fourth
Answer: c

Question: The refractive index of water is 1.33. What will be the speed of light in water?
(a) 4 × 10⁸ m/s
(b) 1.33 × 10⁸ m/s
(c) 3 × 10⁸ m/s
(d) 2.25 × 10⁸ m/s
Answer: d

Question: A linear aperture whose width is 0.02 cm is placed immediately in front of a lens of focal length 60 cm. The aperture is illuminated normally by a parallel beam of wavelength 5 × 10^–5 cm. The distance of the first dark band of the diffraction pattern from the centre of the screen is
(a) 0.10 cm
(b) 0.25 cm
(c) 0.20 cm
(d) 0.15 cm
Answer: d

Question: In a double slit experiment, when light of wavelength 400 nm was used, the angular width of the first minima formed on a screen placed 1 m away, was found to be 0.2°. What will be the angular width of the first minima, if the entire experimental apparatus is immersed in water? (μ_water = 4/3)
(a) 0.1°
(b) 0.266°
(c) 0.15°
(d) 0.05°
Answer: c

 Question: The angular resolution of a 10 cm diameter telescope at a wavelength of 5000 Å is of the order of
(a) 10⁶ rad
(b) 10^–2 rad
(c) 10^–4 rad
(d) 10^–6 rad
Answer: c

Question: In the Young’s double slit experiment, the intensity of light at a point on the screen where the path difference λ is K, (λ being the wavelength of light used). The intensity at a point where the path difference is λ/4 will be
(a) K
(b) K/4
(c) K/2
(d) zero
Answer: c

Question: In Young’s double slit experiment carried out with light of wavelength (λ) = 5000 Å, the distance between the slits is 0.2 mm and the screen is at 200 cm from the slits. The central maximum is at x = 0. The third maximum (taking the central maximum as zeroth maximum) will be at x equal to
(a) 1.67 cm
(b) 1.5 cm
(c) 0.5 cm
(d) 5.0 cm
Answer: b

Question: Ray optics is valid, when characteristic dimensions are
(a) much smaller than the wavelength of light
(b) of the same order as the wavelength of light
(c) of the order of one millimetre
(d) much larger than the wavelength of light.
Answer: d

Question: In Young’s double slit experiment, the slits are 2 mm apart and are illuminated by photons of two wavelengths λ₁ = 12000 Å and λ₂ = 10000 Å. At what minimum distance from the common central bright fringe on the screen 2 m from the slit will a bright fringe from one interference pattern coincide with a bright fringe from the other?
(a) 4 mm
(b) 3 mm
(c) 8 mm
(d) 6 mm
Answer: d

 Question: Which of the phenomenon is not common to sound and light waves ?
(a) Interference
(b) Diffraction
(c) Coherence
(d) Polarisation
Answer: d

Question: In Young’s double slit experiment the distance between the slits and the screen is doubled. The separation between the slits is reduced to half. As a result the fringe width
(a) is halved
(b) becomes four times
(c) remains unchanged
(d) is doubled.
Answer: b

Question: In a Young’ double slit experiment if there is no initial phase difference between the light from the two slits, a point on the screen corresponding to the fifth minimum has path difference.
(a) 5(λ/2)
(b) 10(λ/2)
(c) 9(λ/2)
(d) 11(λ/2)
Answer: c

Question: The Young’s double slit experiment is performed with blue and with green light of wavelengths 4360 Å and 5460 Å respectively. If x is the distance of 4th maxima from the central one, then
(a) x(blue) = x(green)
(b) x(blue) > x(green)
(c) x(blue) < x(green)
(d) x(blue)/x(green) = 5460/4360
Answer: c

Question: In Young’s double slit experiment the separation d between the slits is 2 mm, the wavelength λ of the light used is 5896 Å and distance D between the screen and slits is 100 cm. It is found that the angular width of the fringes is 0.20°. To increase the fringe angular width to 0.21° (with same λ and D) the separation between the slits needs to be changed to
(a) 1.8 mm
(b) 1.9 mm
(c) 2.1 mm
(d) 1.7 mm
Answer: b

Question: Colours appear on a thin soap film and on soap bubbles due to the phenomenon of
(a) interference
(b) dispersion
(c) refraction
(d) diffraction.
Answer: a

Question: In a Fresnel biprism experiment, the two positions of lens give separation between the slits as 16 cm and 9 cm respectively. What is the actual distance of separation?
(a) 13 cm
(b) 14 cm
(c) 12.5 cm
(d) 12 cm
Answer: d

Question: A telescope has an objective lens of 10 cm diameter and is situated at a distance of one kilometre from two objects. The minimum distance between these two objects, which can be resolved by the telescope, when the mean wavelength of light is 5000 Å, is of the order of
(a) 0.5 m
(b) 5 m
(c) 5 mm
(d) 5 cm
Answer: c 

Question: In a double slit experiment, the two slits are 1 mm apart and the screen is placed 1 m away. A monochromatic light of wavelength 500 nm is used. What will be the width of each slit for obtaining ten maxima of double slit within the central maxima of single slit pattern?
(a) 0.5 mm
(b) 0.02 mm
(c) 0.2 mm
(d) 0.1 mm
Answer: c

Question: At the first minimum adjacent to the central maximum of a single-slit diffraction pattern, the phase difference between the Huygen’s wavelet from the edge of the slit and the wavelet from the midpoint of the slit is
(a) π radian
(b) (π/8)radian
(c) (π/4)radian
(d) (π/2)radian
Answer: a

Question: Which one of the following phenomena is not explained by Huygen’s construction of wavefront ?
(a) Refraction
(b) Reflection
(c) Diffraction
(d) Origin of spectra
Answer: d

Question: A beam of monochromatic light is refracted from vacuum into a medium of refractive index 1.5. The wavelength of refracted light will be
(a) depend on intensity of refracted light
(b) same
(c) smaller
(d) larger.
Answer: c

Question: An astronomical refracting telescope will have large angular magnification and high angular resolution, when it has an objective lens of
(a) small focal length and large diameter
(b) large focal length and small diameter
(c) large focal length and large diameter
(d) small focal length and small diameter.
Answer: c

Question: If yellow light emitted by sodium lamp in Young’s double slit experiment is replaced by monochromatic blue light of the same intensity
(a) fringe width will decrease
(b) fringe width will increase
(c) fringe width will remain unchanged
(d) fringes will becomes less intense
Answer: a

Question: In Young’s double slit experiment, the fringes width is found to be 0.4 mm. If the whole apparatus is immersed in water of refractive index 4/3, without disturbing the geometrical arrangement, the new fringe width will be
(a) 0.30 mm
(b) 0.40 mm
(c) 0.53 mm
(d) 450 micron.
Answer: a

Question: Two slits in Young’s experiment have widths in the ratio 1 : 25. The ratio of intensity at the maxima and minima in the interference pattern, I_max I_min is
(a) 49/121
(b) 4/9
(c) 9/4
(d) 121/49
Answer: c

Question: A beam of light of λ = 600 nm from a distant source falls on a single slit 1 mm wide and the resulting diffraction pattern is observed on a screen 2 m away. The distance between first dark fringes on either side of the central bright fringe is
(a) 1.2 cm
(b) 1.2 mm
(c) 2.4 cm
(d) 2.4 mm
Answer: d

Question: The frequency of a light wave in a material is 2 × 10¹⁴ Hz and wavelength is 5000 Å. The refractive index of material will be
(a) 1.50
(b) 3.00
(c) 1.33
(d) 1.40
Answer: b

Question: The Brewsters angle i_b for an interface should be
(a) 0° < i_b < 30°
(b) 30° < i_b < 45°
(c) 45° < i_b < 90°
(d) i_b = 90° 
Answer: c

Question: Time taken by sunlight to pass through a window of thickness 4 mm whose refractive index is 3/2 is
(a) 2 × 10^–4 s
(b) 2 × 10⁸ s
(c) 2 × 10^–11 s
(d) 2 × 10¹¹ s
Answer: c

Question: The intensity at the maximum in a Young’s double slit experiment is I₀. Distance between two slits is d = 5λ, where l is the wavelength of light used in the experiment. What will be the intensity in front of one of the slits on the screen placed at a distance D = 10d ?
(a) (3/4) I₀
(b) I₀/2
(c) I₀
(d) I₀/4
Answer: b

Question: Unpolarised light is incident from air on a plane surface of a material of refractive index m. At a particular angle of incidence i, it is found that the reflected and refracted rays are perpendicular to each other. Which of the following options is correct for this situation?
(a) Reflected light is polarised with its electric vector parallel to the plane of incidence.
(b) Reflected light is polarised with its electric vector perpendicular to the plane of incidence.
(c) i = sin⁻¹ (1/μ)
(d) i = tan⁻¹ (1/μ)
Answer: b

Question: Light travels through a glass plate of thickness t and having a refractive index m. If c is the velocity of light in vacuum, the time taken by light to travel this thickness of glass is
(a) t/μc
(b) μt/c
(c) tmc
(d) tc/μ
Answer: b

Question: A parallel beam of light of wavelength l is incident normally on a narrow slit. A diffraction pattern formed on a screen placed perpendicular to the direction of the incident beam. At the second minimum of the diffraction pattern, the phase difference between the rays coming from the two edges of slit is
(a) 2π
(b) 3π
(c) 4π
(d) πλ
Answer: c 

Question: In Young’s experiment, two coherent sources are placed 0.90 mm apart and fringes are observed one metre away. If it produces second dark fringe at a distance of 1 mm from central fringe, the wavelength of monochromatic light is used would be
(a) 60 × 10^–4 cm
(b) 10 × 10^–4 cm
(c) 10 × 10^–5 cm
(d) 6 × 10^–5 cm
Answer: d

Question: The ratio of resolving powers of an optical microscope for two wavelengths λ₁ = 4000 Å and λ₂ = 6000 Å is
(a) 9 : 4
(b) 3 : 2
(c) 16 : 81
(d) 8 : 27
Answer: b

Question: Young’s double slit experiment is first performed in air and then in a medium other than air. It is found that 8th bright fringe in the medium lies where 5^th dark fringe lies in air. The refractive index of the medium is nearly
(a) 1.59
(b) 1.69
(c) 1.78
(d) 1.25
Answer: c