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Physics NEET MCQ
Particle Nature Of Light Mcq
Quiz 9
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Q.1
Energy of photon is E=hν and its momentum is p=h/λ, where λ is the wavelength of photon, with this assumption, speed of light wave is
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a)3×108 m/s
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b) E/p
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c)Ep
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d)(E/p)2
Explanation
E=fν=hc/λ But h/λ=p ∴ E=pc C=E/pAnswer: (b)
Q.2
Monochromatic light is incident on a phtocell from a source lying at the distance of 0.2 m. The stopping potential and the saturated current are 0.6V and 18.0 mA respectively. Now on displacing the source at the distance 0.6m from the photocell...
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a) stopping potential will be 0.2V
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b) stopping potential will be 0.9V
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c)saturated current will be 6mA
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d)Saturated current will be 2mA
Explanation
Intensity of light I ∝ (1/d2) here d is distance of source)∴ current=2mA. But stopping potential does not depend upon intensity , it remains sameAnswer: (d)
Q.3
A proton and alpha particle are passed through same potential difference. If their initial velocity is zero, the ratio of their de-broglie wavelength after getting accelerated is .. [ GUJCET 2007]
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a) 1:1
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b) 1:2
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c)2:1
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d)2√2 :1
Explanation
USe formula for wave length for proton and α particle Answer:(d)
Q.4
Mass of photon in motion is ..[GUJCET 2008]
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a) c/hν
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b) h/λ
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c) hν
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d) hν/c2
Explanation
use equations E=mc2 and E=hν Answer: (d)
Q.5
Wavelength of an electron having energy 10keV is ..Å
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a)0.12
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b) 1.2
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c)12
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d)120
Explanation
Wavelength in Å of electron=12.36/√V=12.36 / √(104 )=0.12ÅAnswer: (a)
Q.6
If the momentum of an electron is required to be same as that of wave of 5200Å wave length, its velocity should be [GUJCET 2008]
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a) 103 m/s
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b) 1.2 ×103
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c)1.4 × 103
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d)2.8 × 103
Explanation
we know that p=h/λ and p=mv mv=h/λ v=h/mλ substitute the values to get answerAnswer: (c)
Q.7
If the uncertainty in position of a particle is same as it's de-Broglie wavelength, then the uncertainty in its momentum is
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a) h/(2πλ)
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b) h/(3πλ)
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c) 2πh/(λ)
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d)3λ/π
Explanation
Answer:(a)
Q.8
A proton and electron are lying in a box having un-penetrable walls, the ratio of uncertainty in their velocities are ...., me is mass of electron, mp is mass of proton
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a) me/mp
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b) me × mp
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c) √(me × mp)
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d) √(me/me × mp)
Explanation
since both the particle are in same box their uncertainty in position is same ∴ Δp is also same mp Δ vp=me Δ ve ∴ vp/ve=me / mpAnswer: (a)
Q.9
When α-particles are accelerated under the p.d of V volt, their de-Broglie's wavelength is ...Å [ Mass of alpha particle=6.4 ×10-27 kg and its charge is 3.2×10-19 C.
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a)0.287/√V
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b) 12.27/√V
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c)0.103/√V
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d)1.22/√V
Explanation
use formula λ=h/ √(2mqV) Answer: (c)
Q.10
To reduce de_broglie wavelength of an electron from 10-10m to 0.5×10-10m, its energy should be ...
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a) increased to four times
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b) doubled
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c)halved
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d)decreased
Explanation
Use formula λ=h/ √(2mK) λ ∝ 1/√K Answer: (a)
Q.11
Which of the following energies are not possible for a microscopic oscillator having frequency f?
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a) 1.6 hf
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b) 3.5 hf
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c)4.8 hf
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d)all of them
Explanation
As E=nhf here n=1, 2, 3, ...... Answer:(d)
Q.12
If photoelectric effect is not seen with X-ray in a given metal, photoelectron may be emitted by
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a) infra-red rays
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b) γ rays
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c) visible
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d) ultra violet rays
Explanation
Answer: (b)
Q.13
The dimensional formula of Plank's constant is
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a)M1L1T-2
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b)M1L2T-3
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c)M1L2T-2
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d)M1L2T-1
Explanation
Answer: (d)
Q.14
An image of sun is formed by a lens of focal length 30 cm on the metal surface of a photoelectric cell and it produces current I. The lens forming the image is replaced by another lens of the same diameter but focal length 15 cm. the photoeletric current in this case will be ..
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a)I/2
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b) I
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c)2I
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d)4I
Explanation
Since diameter of lens and distance of sun remain uncharged. Intensity of light will remain same hence same currentAnswer: (b)
Q.15
When U.V. light is incident on a photocell its work function is φ and maximum kinetic energy of the photoelectrons is Kmax. When X-ray are incident on the same cell...
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a) Both φ and Kmax increase
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b) both decrease
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c)Kmax increases , φ remains same
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d)Kmax remains same, φ increases
Explanation
Frequency of X-ray > frequency of U.VAnswer: (c)
Q.16
The threshold frequency of a photo-sensitive metal is fo. When it is illuminated by light of frequency 2fo the stopping potential is Vo and maximum velocity of electron is vo. What will be the stopping potential when the same metal is illuminated by light of frequency 3fo
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a) Vo
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b) 2Vo
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c)3Vo
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d)4Vo
Explanation
Use formula eVo=h(f - fo) Answer:(b)
Q.17
In situation of Q255, what will be the maximum velocity of the photoelectrons?
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a) √2 Vo
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b) 2 Vo
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c) 2√2 Vo
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d) 4Vo
Explanation
From formula eVo=h( f- fo)=½ mv2Case I eVo=h(2fo - fo)=½ mvo2 ½ mvo2=hfo --eq(1) Case II ½ mv'2=h(3fo - fo) ½ mv'2=h(2fo)=2hfo=2eVo ½ mv'2=2eVo --eq(2) from equation 1 and 2 we get v'=√2 vo Answer: (c)
Q.18
Violet light can cause photoelectric emission from a metal but blue cannot. If sodium light is incident on the metal, then ....
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a)photoelectric current decreases
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b) photoelectric current increases
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c)photoelectric current is zero
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d)photoelectric current remains same
Explanation
Soudium light is yellow and frequency of yellow light is less than frequency of blue lightAnswer: (c)
Q.19
The maximum kinetic energy of photoelectrons emitted from a metal surface is doubled when the wavelength of the incident radiation is reduced from λ1 to λ2, then the work function of the metal is .....
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a)
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b)
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c)
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d)
Explanation
use following equation and solve them for φAnswer: (a)
Q.20
1020 photons of wavelength 660nm are emitted from a lamp per second. What is the wattage of the lamp
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a) 30 W
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b) 60 W
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c)100 W
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d)120 W
Explanation
use formula Power of lamp=nhc / λ here n is number of photons emitted per second Answer:(a)
Q.21
When a certain metallic surface is illuminated with light of wavelength λ, the stopping potential of the photoelectrons is 3Vo. when the same surface is illuminated by light of wavelength 2λ, the stopping potential is Vo. Then threshold wavelength of the metal is ..
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a) 2λ
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b) 4λ
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c) 6λ
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d) 8λ
Explanation
Use following equation for both cases and solve equations for λo Answer: (b)
Q.22
A proton is moving with velocity v in a region containing uniform electric field E and magnetic field B. All are mutually perpendicular. the proton will move un-deflected if
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a)v ≥ E/B
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b) V < E/B
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c)v=E/B
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d)v=B/E
Explanation
Proton will move un-deflected if Electric force=magnetic force qE=qVB or V=E/BAnswer: (c)
Q.23
When green light is incident on a certain metal surface, electrons are emitted, but no electrons are emitted by yellow light. If now red light is made incident on the same metal, then
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a) more energic electrons are emitted
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b) less enerrgic electrons are emitted
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c)no electrons are emitted
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d)emission of electrons will depend on the intensity of light
Explanation
since frequency of red light is less than yellow light. given that yellow light could not emit electronAnswer: (c)
Q.24
The work function of a substance is 4eV. the longest wavelength of light that can cause photoelectric emission from the substance is approximately [ GUJCET 2006]
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a) 400 nm
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b) 310 nm
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c)220 nm
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d)540 nm
Explanation
Use formula φ=hc/λ solve equation for λ Answer:(b)
Q.25
λo is the de-Broglie wavelength for proton accelerated through a potential difference of 110V, the de-Broglie wavelength for α-particles accelerated through the same potential difference is ...[ GUJCET 2006]
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a) λo /2
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b) λo / 2√2
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c) λo / √2
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d) (2√2 )λo
Explanation
use equation λ=h/ √(2mqV) for proton and alpha particle note mα=4mp and qα=2qp Answer: (b)
Q.26
Monochromatic light of frequency f1 is incident on a photocell and the stopping potential is found to be VWhat is the new stopping potential of the cell it is irradiated by monochromatic light of frequency f2 [ GUJCET 2006]
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a)
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b)
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c)
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d)
Explanation
eV1=hf1 - φ and eV2=hf2 - φ∴ e( V2 - V1)=h ( f2 - f1)∴ V2=(h/e) (f2 - f1) + V1Answer: (d)
Q.27
The threshold wavelength for a metal having workfunction Wo is λo. What is the threshold wavelength for a metal whose workfunction is Wo /2? [ GUJCET 2007]
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a) 4λo
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b) 2λo
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c)λo/2
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d)λo /4
Explanation
Wo=hc/λo∴ Wo/2=hc/2λo∴ b is correct optionAnswer: (b)
Q.28
Which of the following has the longest de-Broglie wavelength if they are moving with same velocity [ GUJCET 2007]
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a) Neutron
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b) Proton
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c)α-particle
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d)β-particle
Explanation
velocity is same thus λ ∝ (1/m) , β particle has smallest mast so wavelength longest Answer:(d)
Q.29
When a point source of light is at a distance of 50cm from a photoelectric cell, the stopping voltage is found to be Vo. If the same source is placed at a distance of 1m from the cell, the stopping voltage will be ...[ GUJCET 2007]
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a) 2Vo
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b) Vo
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c) Vo/2
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d) Vo/4
Explanation
Stopping potential is independent of intensity Answer: (b)
Q.30
The wavelength of matter waves is independent of ... [GUJCET 2007]
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a)charge
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b) momentum
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c)velocity
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d)mass
Explanation
Answer: (a)
0 h : 0 m : 1 s
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