Video Lecture
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Practice Questions (Basic Level)
1.
If a surface has a work function 4.0 eV, what is the maximum velocity of electrons liberated from the surface when it is irradiated with ultra violet radiation of wavelength 0.2 µm?
(a)4.4 × 105 m/s
(b)8.8 × 107 m/s
(c)8.8 × 105 m/s
(d)4.4 × 107 m/s.
Ans (c)
2.
The work function for Aluminium surface is 4.2 eV. The cut off wavelength for the photoelectric effect for the surface is
(a)1500 Å
(b)2955 Å
(c)3100 Å
(d)4200 Å.
Ans (b)
3.
If the frequency of light in a photoelectric experiment is doubled the stopping potential will be
(a) halved
(b) doubled
(c) more than double
(d) less than double
Ans (c)
4.
If the distance of 100 watt lamp is increased from the photo cell, the saturation current i in the photocell varies with the distance d as
(a) \displaystyle i\propto {{d}^{2}}
(b) \displaystyle i\propto d
(c) i\propto \frac{1}{d}
(d) i\propto \frac{1}{{{{d}^{2}}}}.
Ans (d)
5.
Light of wavelength 3500 Å is incident on two metals A and B whose work functions are 4.2 eV and 1.9 eV respectively. Photoelectrons will be emitted by
(a) metal A only
(b) metal B only
(c) both A and B
(d) none
Ans (b)
6.
If the speed of photoelectrons is 104 ms–1, what should be the frequency of incident radiation on the potassium metal? Work function of potassium = 2.3 eV.
(a) 8.5 × 1018 Hz
(b) 6.9 × 1018 Hz
(c) 5.5 × 1018 Hz
(d) 2.0 × 1018 Hz
Ans (b)
6.
The work function for a metal surface is 2.0 eV. Determine the longest wavelength that will eject photoelectrons from the metal surface.
(a) 6000 Å
(b) 1000 Å
(c) 4000 Å
(d) 8000 Å
Ans (a)
7.
A photosensitive metallic surface has work function hn0. If photons of energy 2hn0 falls on this surface, the electrons come out with a maximum velocity of 4 × 106 m/s. When the photon energy is increased to 5hn0, then maximum velocity of photoelectrons will be:
(a) 2 × 106 m/s
(b) 2 × 107 m/s
(c) 8 × 107 m/s
(d) 8 × 106 m/s
Ans (d)
8.
If the maximum kinetic energy of emitted photo electrons from a metal surface of work function 2.5 eV, is 1.7 eV. If wavelength of incident radiation is halved, then stopping potential will be
(a) 2.5 V
(b) 6.7 V
(c) 5 V
(d) 1.1 V
Ans (b)
9.
In a photoelectric experiment, the wavelength of incident radiation is reduced from 6000Å to 4000Å then
(a) Stopping potential will decrease
(b) Stopping potential will increase
(c) Kinetic energy of emitted electrons will decrease
(d) The value of work function will decrease
Ans (b)
10.
Figure represents the graph of kinetic energy (K) of photoelectrons (in eV) and frequency (n) for a metal used as cathode in photoelectric experiment. The work function of metal is
(a) 1 eV
(b) 1.5 eV
(c) 2 eV
(d) 3 eV
Ans (c)
11.
In a photoelectric effect experiment
(a) on increasing intensity and keeping frequency fixed the saturation current decreases.
(b) on increasing intensity and keeping frequency fixed the saturation current remains constant.
(c) on increasing intensity, saturation current may increase.
(d) on increasing frequency saturation current may increase.
Ans (c)
12.
When a certain metallic surface is illuminated with monochromatic light of wavelength \displaystyle \lambda , the stopping potential for photoelectric current is 3V0. When the same surface is illuminated with the light of wavelength 2 \displaystyle \lambda , the stopping potential is V0. The threshold wavelength for the surface for photoelectric effect is
(a) \frac{{4\lambda }}{3}
(b) 4 \displaystyle \lambda
(c) 6 \displaystyle \lambda
(d) 8 \displaystyle \lambda
Ans (b)
13.
The work function of aluminium is 4.2 eV. If two photons, each of energy 3.5 eV strike an electron of aluminium, then emission of electrons will be
(a) possible
(b) not possible
(c) data is incomplete
(d) depend upon the density of the surface
Ans (b)
14.
The work function for potassium is 2.25 eV. A beam with a wavelength of 400 nm has an intensity of 10-9 W/m2. Find
(a)
the maximum kinetic energy of the photoelectrons,
(a) 0.245 eV
(b) 1.045 eV
(c) 0.855 eV
(d) 10.674 eV
Ans (c)
(b)
the number of electrons emitted per meter square per second from the surface assuming 3% of the incident photons are effective in ejecting electrons.
(a) 2 x 107 m-2s-1
(b) 9 x 107 m-2s-1
(c) 13 x 107 m-2s-1
(d) 6 x 107 m-2s-1
Ans (d)
15.
The cutoff wavelength for photoemission from a material is 360 nm. What is the maximum speed of ejected electrons when it is illuminated by photons of wavelength 280 nm?
(a) 0.39 x 105 m/s
(b) 4.26 x 105 m/s
(c) 2.33 x 105 m/s
(d) 5.89 x 105 m/s
Ans (d)
16.
The threshold wavelength for cesium is 686 nm. If light of wavelength 470 nm illuminates the surface, what is the maximum speed of the photoelectrons?
(a) 5.4 x 105 m/s
(b) 10.7 x 105 m/s
(c) 4.6 x 105 m/s
(d) 3.2 x 105 m/s
Ans (a)
17.
The work function for lithium is 2.3 eV.
(a)
What is the maximum kinetic energy of photoelectrons when the surface is illuminated with light of wavelength 400 nm?
(a) 0.2 eV
(b) 0.4 eV
(c) 0.8 eV
(d) 1.5 eV
Ans (c)
(b)
If the stopping potential is 0.6 V, what is the wavelength?
(a) 428 nm
(b) 450 nm
(c) 476 nm
(d) 328 nm
Ans (a)
18.
When radiation of wavelength 350 nm is incident on a surface, the maximum kinetic energy of the photoelectrons is 1.2 eV. What is the stopping potential for a wavelength of 230 nm?
(a) 2.56 V
(b) 3.00 V
(c) 3.05 V
(d) 3.65 V
Ans (c)
Practice Questions (JEE Main Level)
1.
Radiation of two photon energies twice and five times the work function of metal are incident successively on the metal surface. The ratio of the maximum velocity of photo-electrons emitted in the two cases will be
(a)1 : 1
(b)1 : 2
(c)1 : 3
(d)1 : 4.
Ans (b)
2.
A cesium photocell with a steady potential difference of 60V across it, is illuminated by a small bright light placed 1 m away. When the same light is placed 2 m away, the electrons crossing the photocell
(a)each carry one quarter of their previous momentum
(b)each carry one quarter of their previous energy
(c)are one quarter as numerous
(d)are half as numerous.
Ans (c)
3.
The stopping potential Vs as a function of frequency of incident radiation is plotted for two different photoelectric surfaces A and B. The graphs show that the work function of A is
(a)greater than that of B
(b)smaller than that of B
(c)same as that of B
(d)no comparison can be done from the given graph.
Ans (b)
4.
If the wavelength of light incident on a photoelectric cell be reduced from 4000 Å to
3600 Å, then the change in the cut off potential will be
(a) 3.4 V
(b) 0.34 V
(c) 1.34 V
(d) 2.34 V
Ans (b)
5.
When a certain metallic surface is illuminated with monochromatic light of wavelength \displaystyle \lambda , the stopping potential for photoelectric current is 3V0 and when the same surface is illuminated with light of wavelength 2 \displaystyle \lambda the stopping potential is V0. The threshold wavelength of this surface for photoelectric effect is
(a)6 \displaystyle \lambda
(b) \frac{{4\lambda }}{3}
(c)4 \displaystyle \lambda
(d)8 \displaystyle \lambda .
Ans (c)
6.
Light of wavelength 0.6 mm from a sodium lamp falls on a photo cell and causes the emission of photoelectron for which the stopping potential is 0.5 volt. With light of wavelength 0.40 mm from a mercury vapour lamp the stopping potential is 1.5 volt, then value of \frac{h}{e} is
(a)4 × 10–59Vs
(b)0.25 × 1015 Vs
(c)4 × 10–15 Vs
(d)4 × 10–8 Vs.
Ans (c)
7.
The photoelectric threshold wavelength for a tungsten is 2400 Å. Determines the energy of the electrons ejected from its surface when exposed to ultraviolet radiations of wavelength 1200 Å.
(a) 1.22 eV
(b) 1.73 eV
(c) 1.30 eV
(d) 1.48 eV
Ans (d)
8.
The cathode of a photocell is irradiated with light of wavelength 3000Å. The current through the cell is found to stop when the potential of the plate is 2.1V just below the cathode. Calculate the work function of the cathode.
(a) 2.58 eV
(b) 2.00 eV
(c) 2.04 eV
(d) 1.39 eV
Ans (c)
9.
When photons of energy 4.25 eV strike the surface of a metal A, the ejected photoelectrons have maximum kinetic energy TAeV and de Broglie wavelength \displaystyle \lambda A. The maximum kinetic energy of photoelectrons liberated from another metal B by photons of energy 4.70 eV is eV. If the de Broglie wavelength of these photoelectrons is \displaystyle \lambda B = 2 \displaystyle \lambda A , then choose the incorrect statement.
(a) the work function of A is 1.25 eV
(b) the work function of B is 4.20 eV
(c) TA= 2.00 eV
(d) TB = 0.5 eV
Ans (a)
10.
In the following diagram, the relation between \displaystyle \lambda 1 and \displaystyle \lambda 2 will be
(a) {{\lambda }_{1}}=\sqrt{{{{\lambda }_{2}}}}
(b) {{\lambda }_{1}}<{{\lambda }_{2}}
(c) {{\lambda }_{1}}={{\lambda }_{2}}
(d) {{\lambda }_{1}}>{{\lambda }_{2}}
Ans (d)
11.
Two electrons of kinetic energy 2.5 eV fall on a metal plate, which has work function of
4.0 eV. Number of electrons ejected from the metal surface is
(a) one
(b) two
(c) zero
(d) more than two
Ans (c)
12.
A plate is kept in front of a beam of photons. The plate reflects 40% of the incident photons and absorbs the remainder. If the plate absorbs energy at the rate of 2000 W, find the net force acting on it.
(a) 9.33 x 10-6 N
(b) 2.83 x 10-6 N
(c) 0.33 x 10-6 N
(d) 5.22 x 10-6 N
Ans (a)
13.
A source emits monochromatic light of frequency 5.5 ´ 1014 Hz at a rate of 0.1 W. Of the photons given out, 0.15% fall on the cathode of a photocell which gives a current of 6mA in an external circuit.
(i)
Find the energy of a photon
(a) 3.8 x 10-19 J
(b) 1.9 x 10-19 J
(c) 0.3 x 10-19 J
(d) 3.6 x 10-19 J
Ans (d)
(ii)
Find the number of photons leaving the source per second.
(a) 2.8 x 1017 s-1
(b) – 2.5 x 1017 s-1
(c) 1.5 x 1017 s-1
(d) – 1.3 x 1017 s-1
Ans (a)
(iii)
Find the percentage of the photons falling on the cathode which produce photoelectrons.
(a) 1%
(b) 5%
(c) 9%
(d) 12%
Ans (c)
Practice Questions (JEE Advance Level)
1.
When a beam of 10.6 eV photons of intensity 2.0 W/m2 falls on a platinum surface of area 1.0 x 10-4 m2 and work function 5.6 eV, 0.53% of the incident photons eject photoelectrons. Find the number of photoelectrons emitted per second and their minimum and maximum energies (in eV). Take 1 eV = 1.6 x 10-19 J.
(a) 0, 5.0 eV, 3.15 x 1011
(b) 0, 5.0 eV, 8.45 x 1011
(c) 0, 5.0 eV, 6.25 x 1011
(d) 0, 5.0 eV, 0.56 x 1011
Ans (c)