Question

1. What is photoelectric effect? State the result of photoelectric effect experiment that could
not be explained on the basis of laws of classical physics. Explain this effect on the basis on
quantum theory of electromagnetic radiations​

Answer 1

Answer:

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The photoelectric effect cannot be explained by classical theory. We can explain the photoelectric effect by assuming that radiation consists of photons (particles of light). Each photon carries a quantum of energy. The energy of a photon depends only on its frequency, which is the frequency of the radiation.

Photoelectric effect can only be explained by the quantum concept of radiation.

The observations of photoelectric effect experimentally provides with the following conclusions -

1) The photocurrent is proportional to the intensity of incident radiation.

2) The magnitude of stopping potential and hence the maximum kinetic energy of emitted photoelectrons is proportional to the frequency of emitted radiation.

3) There exists a minimum threshold frequency so that if radiation of frequency lesser than this threshold frequency is incident on the metal surface, there is no photoemission irrespective of intensity of radiation.

Einstein's explanation -

Einstein used Max Planck's concept of quantized radiation packets each of an energy

E

=

h

ν

where

h

is Planck's constant and

ν

is frequency of radiation.

According to Einstein, a single photon upon being incident on the metal surface interacts with one electron and transfers it's entire energy to the electron.

Then it is emitted with a maximum kinetic energy,

K

max=hν−ϕ0

Where,

ϕ0

is the work function of the metal.

This explains the fact that maximum kinetic energy increases linearly with frequency of incident radiation.

Now for a frequency,

ν0

so that

hν0=ϕ0

implies that for radiation of frequency less than

ν

0

the maximum kinetic energy of the photoelectron turns out to be negative and hence, no photoemission is possible.

This explains the concept of threshold frequency

ν

0

below which photoemission is not possible even with an extremely intense beam of light.

Since

ϕ

0

depends on the metal and the nature of the surface, the work function and hence the threshold frequency varies from substance to substance.

Also intensity of radiation from the quantum concept is associated with number of photons incident on a unit area in unit time.

That is, a greater intensity implies that a greater number of photons are incident on the metal surface per unit area in unit time.

Now since a single photon interacts with a single electron causing the emission of the electron, a higher number of incident photons implies that a greater number of photoelectrons are emitted in unit time. Consequently, the current increases with the number of photons incident and hence with the intensity of radiation used.

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Answer 2

Explanation:

Photoelectric effect: When certain metals are exposed to a beam of light, the electrons are ejected from the metal. This phenomenon is called photoelectric effect. The electrons ejected are called photoelectrons.

The results of this experiment were:

1. The electrons are ejected from the metal surface only as the beam of light strikes the surface.

2. The number of electrons ∝ intensity of radiation.

3. For each metal; there is a minimum frequency, v

0

below which photoelectric effect is not observed.

4. K.E. of electrons α frequency of light.

Explanation of photoelectric effect on the basis of quantum theory: When a photon of sufficient energy strikes an electron in the atom of the metal, it transfers its energy to electron and the electron is ejected without delay. Greater the energy of the photon, greater will be the kinetic energy of the ejected electron