Physics

Electrons and Photons

127 Questions

Electrons and photons are fundamental concepts in modern physics, focusing on quantum interactions and the photoelectric effect. The topic involves calculating work functions, kinetic energy, and photon wavelengths. These physics questions are highly relevant for competitive test preparation.

Photoelectric effectWork function calculationsPhoton energy and wavelengthElectron emissionAtomic transitions

Electrons and Photons Questions

Multiple choice physics wave optics interference

Electrons accelerated from rest by an electrostatic potential are collimated and sent through a Young's double slit setup. The figure width is w. If the accelerating potential is doubled then the width is now close to.

  1. $0.5$ w
  2. $0.7$ w
  3. $1.0$ w
  4. $2.0$ w
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation
$\beta=\dfrac{\lambda D}{d}$

$\lambda=\dfrac{h}{mV}=\dfrac{h}{\sqrt{2mq\Delta V}}$

$\beta \propto \lambda$

Therefore,

$\beta \propto \dfrac{1}{\sqrt{\Delta V}}$

$As $\Delta V$ is double,

$\beta$ is $\dfrac{1}{\sqrt 2}$ times of $\beta_{old}$

Therefore,

$\beta_{new}=0.7\beta=0.7\,w$
Multiple choice laws of heat transfer heat and thermodynamics physics

If the operating voltage of X-ray tube is $50$kV then velocity of X-ray?

  1. $7.5\times10^{25}$ m/sec
  2. $3\times 10^8$ m/sec
  3. $10^8$ m/sec
  4. $3$ m/sec
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

We know, wavelength of X-ray $\lambda =\dfrac { hv }{ eV } $

Where h is the Plank's constant and $v$ is the required velocity.
$v=\frac { \lambda \times eV }{ h } $
  the minimum wavelength of X-ray is 0.01 nm.
So velocity, $\ \quad \quad v=\frac { 0.01\times { 10 }^{ -9 }\times 50\times { 10 }^{ 3 } }{ 6.626\times { 10 }^{ -34 } } $
     v= $0.0754\times { 10 }^{ 28 }\quad m/s\ =7.5\times { 10 }^{ 25 }\quad m/s$


Multiple choice laws of heat transfer heat and thermodynamics physics

The radiation corresponding to 3 $\rightarrow $ s 2 transition of hydrogen atom falls on a metal surface to produce photoelectrons. These electrons are made to enter a magnetic field of $3 \times 10 ^ { - 4 } T$ . If the radius of the largest circular path followed by these electrons is 100 mm, the work function of the metal is close to 

  1. 3.8 eV

  2. 1.1 eV

  3. 1.8 eV

  4. 1.6 eV
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

$mv=qBR$

$K{E _{\max }} = \dfrac{{\left( {m{v^2}} \right)}}{{2m}} = 0.8\,eV$
$hv = 13.6\left[ {\dfrac{1}{4} - \dfrac{1}{6}} \right]$
$\therefore W = hv - K{E _{\max }}$
$ = 13.6 \times \dfrac{5}{{36}} - 0.8 = 1.1eV$
Hence,
option $(B)$ is correct answer.

Multiple choice electromagnetic spectrum electromagnetic waves physics

Radiations of intensity 0.5 W/m2 are striking a metal plate. The pressure on the plate is

  1. $0.166 \times 10^{-8} N/m2$
  2. $0.332 \times 10-8 N/m^2$
  3. $0.111 \times 10-8 N/m^2$
  4. $0.083 \times 10-8 N/m^2$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Intensity or power per unit area of the radiations,
$P = pv$
$\Rightarrow p=\dfrac{p}{v} = \dfrac{0.5}{3\times 10^8}=0.166\times 10^{-8}N/m^2$

Multiple choice physics electric circuits introduction to diodes circuit components junction diode and its voltage current characteristics

The relation between dynamic plate resistance $\left( r _ { p } \right)$ of a vacuum diode and plate current in the space charge limited region, is 

  1. $r _ { p } \propto I _ { p }$
  2. $r _ { p } \propto I _ { p } ^ { 3 / 2 }$
  3. $r _ { p } \propto \frac { 1 } { I _ { p } }$
  4. $r _ { p } \propto \frac { 1 } { \left( I _ { p } \right) ^ { 1 / 3 } }$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

In a vacuum diode operating in the space charge limited region, Langmuir-Child law states that plate current Ip is proportional to V^(3/2). Dynamic plate resistance rp = dV/dIp. Differentiating the relation gives rp proportional to 1/(Ip^(1/3)).

Multiple choice physics electric circuits introduction to diodes circuit components junction diode and its voltage current characteristics

What is the work function of tungsten at $1500\ K$ temperature, when a diode valve with a tungsten filament works at $1500\ K$? Assume the work function of tungsten at $0\ K$ is $4.52\ eV$:

  1. $4.71\ eV$
  2. $0.39\ eV$
  3. $8.86\ eV$
  4. $1.25\ eV$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The work function at temperature T is given by phi(T) = phi(0) + 2kT. Given phi(0) = 4.52 eV, k = 8.617 * 10^-5 eV/K, and T = 1500 K, the increase is 2 * 8.617 * 10^-5 * 1500 = 0.2585 eV. 4.52 + 0.26 = 4.78 eV. Option A is the closest.

Multiple choice deflection of electron beam by magnetic field observing the force and electron beam tubes charged particles electromagnetic forces physics

Metallic sphere of radius is removed from other bodies,  and is irradiated with the radiation of wavelength Lambda, so that it is capable of causing Photoelectric effect.

  1. The maximum potential gained by the sphere will be independent of its radius

  2. Net positive charge appearing on the sphere after a long time will depend on the radius of the sphere

  3. The kinetic energy of the most energetic electron emanating from the Sphere will keep on declining with time

  4. The kinetic energy of the most energetic electrons emitted from this sphere initially will be independent of the radius of the sphere
Reveal answer Fill a bubble to check yourself
A Correct answer
Multiple choice deflection of electron beam by magnetic field observing the force and electron beam tubes charged particles electromagnetic forces physics

Statement -1 :When ultraviolet light is incident on a photo cell, its potential is $ V _0 $ and the maximum kinetic energy of the photoelectrons is $ K _{max} $. When the ultravoilet light is replaced by X-rays, both $ V _0 $ and $ K _{max} $ increase. 

Statement-2: Photoelctrons are emitted with speeds ranging from zero to a maximum value because of the range of frequencies present in the incident light.

  1. Statement -1 is true,statement-2 is true ; statement -2 is correct explanation of statement-1.

  2. Statement -1 is true,statement-2 is true ; statement -2 is not correct explanation of statement-1.

  3. Statement -1 is false,statement-2 is true

  4. Statement -1 is true,statement-2 is false
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Statement 1 is true because X-rays have higher frequency than UV light, increasing the photon energy and thus Kmax and stopping potential. Statement 2 is false because the range of speeds is due to electrons being emitted from different depths within the material, not the frequency range of the light.

Multiple choice deflection of electron beam by magnetic field observing the force and electron beam tubes charged particles electromagnetic forces physics

Velocity ratio of the two cathode rays is 1:2. They are applied to the same electric field. What is the deflection ratio of the two cathode rays

  1. 1 : 2

  2. 1 : 4

  3. 4 : 1

  4. 8: 1
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The deflection of a charged particle in a transverse electric field is given by y = (1/2) * (q E / m) * (t^2), where t = l / v. Substituting this yields deflection proportional to 1 / v^2. Since the velocity ratio is 1:2, the ratio of deflections is (2/1)^2, which equals 4:1.

Multiple choice deflection of electron beam by magnetic field observing the force and electron beam tubes charged particles electromagnetic forces physics

In a photocell bichromatic light of wavelengths  $2475 \mathring { A } $  and  $6000 \mathring { A } $  are incident on cathode whose work function is  $4.8 e V.$  If a uniform magnetic field of  $3 \times 10 ^ { - 5 }$  Tesla exists parallel to the plate, the radius of the path describe by the photoelectron will be (mass of electron  $=9\times 10^{ { -31 } }{ kg }$ )

  1. $1 cm$
  2. $5 cm$
  3. $10 cm$
  4. $25 cm$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The radius of the path of a charged particle in a magnetic field is given by r = mv/qB. Using the photoelectric equation, the kinetic energy is E = hc/lambda - work function. Calculating for the shorter wavelength (2475 A) gives a kinetic energy of 0.2 eV, which leads to a radius of 5 cm.

Multiple choice deflection of electron beam by magnetic field observing the force and electron beam tubes charged particles electromagnetic forces physics

Cathode rays enter an electric field normal to the field. Then their path in the electric field is :

  1. A parabola

  2. A circle

  3. A straight line

  4. An ellipse
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Electric field applies a constant force  $eE$ on the electrons (i.e. cathode rays) perpendicular to the beam of electrons due to which the electrons will trace a parabolic path.

Multiple choice deflection of electron beam by magnetic field observing the force and electron beam tubes charged particles electromagnetic forces physics

When the energy of the incident radiation is increased by 20%, The kinetic energy of the photelectrons emitted from a metal surfce in creased from 0.5V to 0.8eV The work function of the metal is 

  1. 0.65 eV

  2. 1.0 eV

  3. 1.2 eV

  4. 1.5 eV
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Using Einstein's photoelectric equation, K = E - W. Let initial energy be E and final energy be 1.2E. We have 0.5 = E - W and 0.8 = 1.2E - W. Solving these simultaneous linear equations gives W = 1.0 eV.

Multiple choice deflection of electron beam by magnetic field observing the force and electron beam tubes charged particles electromagnetic forces physics

When electron is incident on molybdenum they by changing energy of electron:-

  1. $\lambda _{\text{min}}$ changes
  2. $\lambda _{\text{min}}$ remains constant
  3. $\lambda _{K _{\alpha}}, \lambda _{K _{\beta}}$ changes
  4. $\lambda _{\text{min}}, \lambda _{K _{\alpha}}$ and $\lambda _{K _{\beta}}$ all changes
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The minimum wavelength (lambda_min) of X-rays produced is determined by the accelerating voltage (or energy) of the incident electrons via the Duane-Hunt law, lambda_min = hc/eV. Characteristic X-ray wavelengths (K_alpha, K_beta) depend on the target material (molybdenum), not the incident electron energy.

Multiple choice physics dual nature of matter and radiation davisson and germer experiment and its conclusion matter waves wave nature of matter

In Davisson-Germer experiment an electron beam of energy $60\ eV$ falls normally on the surface of a crystal. If the maximum intensity is obtained at an angle of $60^{\circ}$ to the direction of incident beam. then inter-atomic distance in the lattice plane of the crystal will be-

  1. $18\ A^{\circ}$
  2. $3.6\ A^{\circ}$
  3. $1.8\ A^{\circ}$
  4. $0.18\ A^{\circ}$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Using Bragg's Law 2d sin(theta) = n * lambda. The wavelength lambda = sqrt(150/V) Angstroms = sqrt(150/60) = 1.58 Angstroms. Given the angle 60 degrees is to the incident beam, the glancing angle theta is 90 - 60/2 = 60 degrees (or 30 degrees depending on convention). Using standard diffraction geometry for this experiment, d = 1.8 Angstroms is the accepted value.