Tag: photons

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Questions Related to photons

Multiple choice physics quantum physics photons concept of photon photons and photoelectric effect

If the work function of the metal is W and the frequency of the incident light is v , then there is no emission of photo-electrons if

  1. $v< W/h$

  2. $v > W/h$

  3. $v \geq W/h$

  4. $v \leq W/h$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Maximum Kinetic Energy $=h\nu -\phi $
$=h\nu -W $
Since there is no emission
So, $h\nu -W < 0$
or $\nu < \dfrac{W}{h}$

Multiple choice physics quantum physics photons concept of photon photons and photoelectric effect

A proton, an electron and an $\alpha$ particle is accelerated through the same potential difference enter a region of uniform magnetic field, moving at right angles to the magnetic field $\vec{B}$. The ratio of their kinetic energies is

  1. 2:1:1

  2. 2:2;1

  3. 1:2:1

  4. 1:1:2
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Kinetic energy = qV where q is charge and V is potential
But potential is same
So, $K. E \propto q$
$q _p = q _e = 2q$
Putting these values,
we get the ratio of 1:1 :2

Multiple choice physics quantum physics photons concept of photon photons and photoelectric effect

Identify which of the following statement is true about the momentum of a photon?

  1. It is proportional to the wavelength of the photon

  2. It is inversely proportional to the wavelength of the photon.

  3. It is inversely proportional to the square of the wavelength of the photon.

  4. It is proportional to the mass of the photon.

  5. It is equal to the energy of the photon.
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Momentum of the photon $p = \dfrac{h}{\lambda}$ where $\lambda$ is the wavelength of the photon

$\implies$$p  \propto \dfrac{1}{\lambda}$
Hence momentum of the photon is inversely proportional to the wavelength of the photon.

Multiple choice physics quantum physics photons concept of photon photons and photoelectric effect

What should be the velocity of an electron so that its momentum becomes equal to that of a photon of wavelength $5200\overset {\circ}{A}$?

  1. $700\ m/s$

  2. $1000\ m/s$

  3. $1400\ m/s$

  4. $2800\ m/s$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Momentum, $p = mv = \dfrac {h}{\lambda}$


or $v = \dfrac {h}{m\lambda}$

$\therefore v = \dfrac {6.62\times 10^{-34}}{9.1\times 10^{-31}\times 5.2\times 10^{-7}}$

$\Rightarrow v = \dfrac {6.2\times 10^{4}}{9.1\times 5.1}$

$\Rightarrow v = 1400\ m/s$.

Multiple choice physics quantum physics photons concept of photon photons and photoelectric effect

Ultraviolet radiation of 6.2 eV falls on an aluminium surface (work function 4.2 eV). The kinetic energy (in joule) of the fastest electron emitted is :

  1. $3.2 \times 10^{-21}$

  2. $1.6 \times 10^{-17}$

  3. $3.2 \times 10^{-19}$

  4. $3.2 \times 10^{-15}$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

$Energy\quad of\quad radiation,\quad h\nu =6.2eV\ =6.2\times 1.6\times { 10 }^{ -19 }J\ Work\quad function\quad W=4.2eV\ =4.2\times 1.6\times { 10 }^{ -19 }J\ KE=h\nu -W\ =6.2\times 1.6\times { 10 }^{ -19 }-4.2\times 1.6\times { 10 }^{ -19 }\ =2\times 1.6\times { 10 }^{ -19 }\ =3.2{ \times 10 }^{ -19 }J$

Multiple choice physics quantum physics photons concept of photon photons and photoelectric effect

Momentum of a photon having frequency $1.5\times 10^{13}Hz$?

  1. $3.13\times 10^{-29}kg m/s$

  2. $3.3\times 10^{-34}kg m/s$

  3. $6.6\times 10^{-34}kg m/s$

  4. $6.6\times 10^{-30}kg m/s$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

$\quad Momentum\quad of\quad a\quad photon\quad is\quad P=\quad h/\lambda \ \quad And\quad \quad \quad \lambda \nu =c\quad ,\quad where\quad c=\quad speed\quad of\quad light\quad h=\quad Planck's\quad constant\ \qquad \qquad \qquad \qquad \qquad \qquad \lambda =\quad wavelength\quad of\quad photon\ \qquad \qquad \qquad \qquad \qquad \qquad \nu =\quad frequency\quad of\quad photon\ so\quad \lambda =\quad \dfrac { 3\times { 10 }^{ 8 } }{ 1.5\times { 10 }^{ 13 } } =2\times { 10 }^{ -5 }{ m }^{ }\quad P=\dfrac { 6.26\times { 10 }^{ -34 } }{ 2\times { 10 }^{ -5 } } =3.13\times { 10 }^{ -29 }kg m/s\ Therefore\quad option\quad A.\quad$

Multiple choice physics quantum physics photons concept of photon photons and photoelectric effect

A beam of light has two wavelengths $4971\mathring{A}$ and $6216\mathring{A}$ with a total intensity of $3.6\times { 10 }^{ -3 }W{ m }^{ -2 }$ equally distributed among the two wavelengths. The beam falls normally on an area of $1{cm}^{2}$ of a clean metallic surface of work function $2.3eV$. Assume that there is no loss of light by reflection and that each capable photon ejects one electron. The number of photo electrons liberated in $2s$ approximately :

  1. $6\times { 10 }^{ 11 }$

  2. $9\times { 10 }^{ 11 }$

  3. $11\times { 10 }^{ 11 }$

  4. $15\times { 10 }^{ 11 }$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation
${ E } _{ 1 }=\cfrac { 1242 }{ 497.2 } =2.50eV;{ E } _{ 2 }=\cfrac { 1242 }{ 6621. } =2.0eV$
so, photoelectron emission takes place only due to first wavelength
$\therefore$ No. of photoelectrons emitted $=\cfrac { 1.8\times { 10 }^{ -3 }\times { 10 }^{ -4 }\times 2 }{ 2.5\times 1.6\times 10^{-19} } hv=9\times { 10 }^{ 11 }$
Multiple choice physics quantum physics photons concept of photon photons and photoelectric effect

The threshold frequency for a metallic surface corresponds to an energy of $6.2eV$, and the stopping potential for a radiation incident on this surface $5V$. The incident radiation lies in.

  1. X-ray

  2. ultra-violet region

  3. infra-red region

  4. visible region
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

$hv=5eV+6.2eV=11.2eV$
$\lambda =\cfrac { 1242 }{ 11.2 } nm=1109\mathring { A } $
it lies in ultraviolet region

Multiple choice physics quantum physics photons concept of photon photons and photoelectric effect

An electron of mass $m$ and charge $e$ is accelerated from rest through a potential difference of $V$ volt in vacuum. Its final speed will be

  1. $\cfrac { eV }{ 2m } $

  2. $\cfrac { eV }{ m } $

  3. $\sqrt { \cfrac { 2eV }{ m } } $

  4. $\sqrt { \cfrac { eV }{ 2m } } $
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation
  • Energy gained by electron when accelerated through a potential $V $ is $ charge\times voltage$ i.e., $eV$ 
  • As Kinetic energy 
  •  of any mass $m$ is given as $\dfrac{mv^2}{2}$ 
  • so velocity $v$= $\sqrt[2]{ \dfrac{2K.E.}{m}}$ 
  • for final speed put $K.E.= eV$ we get final speed $v$= $ \sqrt[2]{ \dfrac {2eV}{m}}$. Option C is correct