Q: Light of frequency 1.5 times the threshold frequency is incident on photosensitive material. If the frequency is halved and intensity is doubled, the photo current becomes

When the frequency of incident light is halved of its original value i.e. \$1.5v_0\$ then it becomes less than the threshold value. In that case no photoelectric effect takes place. No photoelectrons would be emitted. The photoelectric current becomes zero.

Q: In photoelectric effect, the slope of the straight line graph between stopping potential and frequency of the incident light gives the ratio of Planck's constant to

\$hv=hv_{0}+K.E\$at stopping potential K.E\$=\$eV\$\Rightarrow hv=hv_{0}+eV\$\$V=\dfrac{h}{e}(v-v_{0})\$The slope of graph b/w stopping potential and frequency is \$\dfrac{h}{e}\$, where e is charge of electron.

Question 1

Light of frequency 1.5 times the threshold frequency is incident on photosensitive material. If the frequency is halved and intensity is doubled, the photo current becomes

Accepted Answer

When the frequency of incident light is halved&#10;of its original value i.e. $1.5v_0$ then it becomes less than the threshold value. In&#10;that case no photoelectric effect takes place. No photoelectrons would be emitted.&#10;The photoelectric current becomes zero.

Question 2

Draw graphs showing variation of photoelectric current with applied voltage for two incident radiations of equal frequency and different intensities. Mark the graph for the radiation of higher intensity.

Accepted Answer

The above graph is showing the variation of photoelectric current with the applied voltage. They have different intensities but same frequency.Here the upper one is for high intensity and lower one is for low intensity.

Question 3

Write the experimental observations of Photoelectric effect.

Accepted Answer

(1) For a given metal, there exists a certain minimum frequency above which the photoelectric effect takes place which is known as threshold frequency.(2) Maximum kinetic energy of the emitted photoelectrons increases as the frequency of the incident light is increased keeping the number of incident photons to be fixed.(3) Above threshold frequency, maximum kinetic energy of the photoelectrons depends only on the frequency of incident light but is independent of the intensity of incident light.(4) For a given  metal and frequency of incident light, the rate at which the photoelectrons ejected is directly proportional to the intensity of incident light. An increase in the magnitude of the incident light increases the magnitude of the photoelectric current.(5) The time lag between the incidence of photons and radiation of photoelectric effect is very small, nearly $10^{-9}$ second.

Question 4

The threshold frequency for certain metal is $3.3 \times 10^{14} H z$ . If light of $8.2 \times 10^{14} H z$ frequency is incident on the metal, then the cut-off voltage of the photoelectric current will be :

Accepted Answer

In photoelectric effect, energy is conserved.In photoelectric effect, Einstein's equation is given byPhoton energy $=$ kinetic energy of electron $+$ work functioni.e. $hv=e{ V }_{ s }+h{ v }_{ 0 }$where, ${ V }_{ s }$ is the stopping potential and ${ v }_{ 0 }$ is the threshold frequency.$\therefore { V }_{ s }=\dfrac { h }{ e } \left( v-{ v }_{ 0 } \right)$              .....(i) Given, ${ v }_{ 0 }=3.3\times { 10 }^{ 14 }Hz$, $v=8.2\times { 10 }^{ 14 }Hz$            $h=6.6\times { 10 }^{ -34 }J-s$, $e=1.6\times { 10 }^{ -19 }C$Substituting these values in equation (i), we get${&#10; V }_{ s }=\dfrac { 6.6\times { 10 }^{ -34 }\left( 8.2\times { 10 }^{ 14&#10; }-3.3\times { 10 }^{ 14 } \right)  }{ 1.6\times { 10 }^{ -19 } } $${ V }_{ s }=\dfrac { 6.6\times 4.9 }{ 1.6 } \times { 10 }^{ -1 }=2V$$\Rightarrow { V }_{ s }=2V$

Question 5

Write four laws of photoelectric effect.

Accepted Answer

Photo electric effect: When some photosensitive substances are exposed to ultraviolet light, electrons are ejected from their surface which are called photoelectrons. This process is called photoelectric emission.Laws of photoelectric emission:(i) There is a definite cut off value of frequency below which electrons cannot be ejected by any substance.(ii) Number of emitted electrons are directly proportional to the intensity of light incident.(iii) Kinetic energy of emitted electrons depends on the frequency of incident light on substance.(iv) There is no time logging between the incident of light and emission of electrons.

Question 6

(a) Why photoelectric effect cannot be explained on the basis of wave nature of light? Give reasons.
(b) Write the basis features of photon picture of electron magnetic radiation on which Einstein's photoelectric equation is based.

Accepted Answer

(a)Wave nature of radiation cannot explain the photo electric effects because of :(i) The immediate ejection of photo electrons (ii) The presence of threshold frequency for a metal surface.(iii) The fact that kinetic energy of the emitted electrons is independent of the intensity of light and depends upon its frequency. Thus, the photoelectric effect cannot be explained on the basis of wave nature of light (b) Photon picture of electron magnetic radiation on which Einstein's photoelectric equation is based on particle nature of light . Its basic features are:(i) In interaction with matter radiation behaves as if it is made up of particles called photons.(ii) Each photon has energy $(E=hv)$, momentum $p=\dfrac{h\nu}{c}$(p=and speed c, the speed of light(iii) All photon of light of a particular frequency v, or wavelength &#955;&#955; , have the same energy $E=\dfrac{hc}{\lambda}$and momentum $p=\dfrac{h\nu }{c}$(p=hvc). (iv)By increasing the intensity of light of given wavelength there is only an increase in the number of photons per second crossing a given area, with each photon having the same energy Thus, photon energy is independent of intensity of radiation (v) Photons are electrically neutral and are not deflected by electric and magnetic fields.(vi) In a photon-particle collision (such as photon-electron collision ),the total energy and total momentum are conserved. However number of Photon may not be observed.

Question 7

What is photoelectric effect? Explain the effect of increase (i) frequency (ii) intensity of incident radiation on photoelectric current with suitable graphs.

Accepted Answer

Photoelectric effect is a phenomenon which is based on the idea that electromagnetic radiation is made of a series of particles called photons.When a photon hits an electron  on a metal surface,the electron can be emitted.The emitted electrons are called photoelectrons.(i)On increasing the frequency the saturation currents remains the same as seen in the first figure.(ii)On increasing the Intensity the saturation current also increases.

Question 8

The given graph shows the variations of photo-electric current (I) versus applied voltage (V) for two different photosensitive materials and for two different intensities of the incident radiations. Identify the pairs of curves that correspond to different materials but same intensity of incident radiations

Accepted Answer

Curves 1 and 2 correspond to similar materials while curves and 4 represent different materials, since the value of stopping potential for 1,2 and 3, 4 are the same,For the given frequency of the incident radiation, the stopping potential is independent of its intensitySo, the pairs of curves (1 and 3) and (2 and 4) correspond to different materials but same intensity of incident radiations.

Question 9

Explain how does (i) photoelectric current and (ii) kinetic energy of the photoelectrons emitted in a photocell vary if the frequency of incident radiation is doubled, but keeping the intensity same? Show the graphical variation in the above two cases.

Accepted Answer

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Question 10

In photoelectric effect, the slope of the straight line graph between stopping potential and frequency of the incident light gives the ratio of Planck's constant to

Accepted Answer

$hv=hv_{0}+K.E$at stopping potential K.E$=$eV$\Rightarrow hv=hv_{0}+eV$$V=\dfrac{h}{e}(v-v_{0})$The slope of graph b/w stopping potential and frequency is $\dfrac{h}{e}$, where e is charge of  electron.

Photoelectric Effect - II

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Observations on Photo Electric Effect

Light of frequency 1.5 times the threshold frequency is incident on photosensitive material. If the frequency is halved and intensity is doubled, the photo current becomes

Draw graphs showing variation of photoelectric current with applied voltage for two incident radiations of equal frequency and different intensities. Mark the graph for the radiation of higher intensity.

Write the experimental observations of Photoelectric effect.

The threshold frequency for certain metal is $3.3 \times 10^{14} H z$ . If light of $8.2 \times 10^{14} H z$ frequency is incident on the metal, then the cut-off voltage of the photoelectric current will be :

Write four laws of photoelectric effect.

(a) Why photoelectric effect cannot be explained on the basis of wave nature of light? Give reasons.
(b) Write the basis features of photon picture of electron magnetic radiation on which Einstein's photoelectric equation is based.

What is photoelectric effect? Explain the effect of increase (i) frequency (ii) intensity of incident radiation on photoelectric current with suitable graphs.

Explain how does (i) photoelectric current and (ii) kinetic energy of the photoelectrons emitted in a photocell vary if the frequency of incident radiation is doubled, but keeping the intensity same? Show the graphical variation in the above two cases.

In photoelectric effect, the slope of the straight line graph between stopping potential and frequency of the incident light gives the ratio of Planck's constant to

Photoelectric Effect - II

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Rate

REVISE WITH CONCEPTS

Observations on Photo Electric Effect

Light of frequency 1.5 times the threshold frequency is incident on photosensitive material. If the frequency is halved and intensity is doubled, the photo current becomes

Draw graphs showing variation of photoelectric current with applied voltage for two incident radiations of equal frequency and different intensities. Mark the graph for the radiation of higher intensity.

Write the experimental observations of Photoelectric effect.

The threshold frequency for certain metal is 3.3×1014Hz. If light of 8.2×1014Hz frequency is incident on the metal, then the cut-off voltage of the photoelectric current will be :

Write four laws of photoelectric effect.

(a) Why photoelectric effect cannot be explained on the basis of wave nature of light? Give reasons. (b) Write the basis features of photon picture of electron magnetic radiation on which Einstein's photoelectric equation is based.

What is photoelectric effect? Explain the effect of increase (i) frequency (ii) intensity of incident radiation on photoelectric current with suitable graphs.

Explain how does (i) photoelectric current and (ii) kinetic energy of the photoelectrons emitted in a photocell vary if the frequency of incident radiation is doubled, but keeping the intensity same? Show the graphical variation in the above two cases.

In photoelectric effect, the slope of the straight line graph between stopping potential and frequency of the incident light gives the ratio of Planck's constant to

The threshold frequency for certain metal is $3.3 \times 10^{14} H z$ . If light of $8.2 \times 10^{14} H z$ frequency is incident on the metal, then the cut-off voltage of the photoelectric current will be :

(a) Why photoelectric effect cannot be explained on the basis of wave nature of light? Give reasons.
(b) Write the basis features of photon picture of electron magnetic radiation on which Einstein's photoelectric equation is based.