CBSE Class 12-science Answered

(A) there is a minimum frequency of incident radiation below which no photo electrons are emitted .

Above staement is true.

According to Einstein's photo electric equation , incident photon's energy equals sum of work function

of photo-electric material and maximum kinetic energy of photo electron.

If kinetic energy of photo electron is near to zero, then energy of incident radiation equals work function of

photo-electric material . If the energy of incident radiation is less than the work function of photo electric material ,

photo electrons are not ejected out of photo-electric material . Since energy of incident radiation is directly

proportional to frequency of radiation , if frequency of radiation less than the equivalent frequecy corresponding to work

function of material , then no photo electrons are emitted from phot electric material.

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(B) Maximum kinetic energy of photo electron depends only on frequency of incident radiation .

Above statement is true.

As explained before , maximum kinetic energy of photo electron is difference between energy of incident radiation

and work function of material . Since energy of incident  photons depend only on frequency , maximum kinetic energy of

photo electron depends only on frequency of incident radiation.

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(C) when the metal surface is illuminated , electrons are ejected from surface of metal after some time

Above statement is False .

Electrons are ejected from surface of metal spontaneously when the surface is irradiated with electromagnetic radiation.

Spontaneous nature of emission of photo electron is due to very high kinetic energy for the electron,

Requirement energy of incident radiation is few eV . Hence incident energy is order of 10^-19 J .

Mass of electron is 9.1 × 10^-31 kg . hence velocity of photo-electron is order of 10⁶ m/s 

Due to this high speed , emission of photon is spontaneous .

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(D) photoelectric current is independent of intensity of incident radiation .

Above statement is false .

Magnitude of photoelectric current depneds on number of photo electrons .

Number of photo electrons emiited is proportioal to number of photons incident on photo electric surface .

Number of photons depends on intensity of electromagentic radiation.

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