Question

Energy conversion in a photoelectric cell takes place from
A. chemical to electrical
B. magnetic to electrical
C. optical to electrical
D. mechanical to electrical

Answer 1

Hint:The equation for photoelectric effect is\[h\nu = {\phi _o} + {E_k}\]. Comparing the L.H.S and R.H.S. of this photoelectric equation, the energy of the photons in the incident radiation is used for emission of electrons, that is the energy of photons is converted into electrical energy. In this equation, the minimum amount of energy required for electron emission from the metal surface is the work function of that metal and the frequency of light corresponding to this minimum energy is called threshold frequency and the corresponding wavelength is called threshold wavelength.

Formula(e) used:
Energy, \[E = h\nu = \dfrac{{hc}}{\lambda }\]
Equation for photoelectric effect, \[h\nu = {\phi _o} + {E_k}\]
Work function, \[{\phi _o} = h{\nu _o} = \dfrac{{hc}}{{{\lambda _o}}}\]
Where,
E = energy of incident radiation
\[{\phi _o}\]= Work function of the metal
\[{E_k}\]= energy of the emitted photoelectron
\[c{\rm{ }} = \text{speed of light}= 3 \times {10^8}m/s\]
\[\nu \]= frequency of the light,
\[{\nu _o}\]= threshold frequency,
\[\lambda \]= wavelength of the light,
\[{\lambda _o}\]= threshold wavelength

Complete step by step solution:
A Light beam can be considered as a train of energetic but massless particles called photons. When the beam of light strikes the surface of a metal, these photons transfer their energy in quanta (i.e. energy of each photon equal to \[h\nu \]) to atoms inside the metal. The electrons present in the ground state in the atom gain this energy as kinetic energy and are basically ejected out of the metal atom.

This movement of electrons leads to generation of electric current. This phenomenon is called the photoelectric effect. It was discovered by Einstein in 1905 for which he also won the Nobel prize in physics. Photoelectric effect takes place only if this energy is higher than the threshold energy of the metal. These emitted electrons are called photoelectrons whose energy is less than the energy of the incident light as some of the energy is utilised in overcoming the barrier energy or the work function.

Work function thus can be defined as the minimum energy required for photoelectron emission from the surface of a particular metal. Equation for photoelectric effect is,
\[h\nu = {\phi _o} + {E_k}\]---- (1)
Work function,
\[{\phi _o} = h{\nu _o} = \dfrac{{hc}}{{{\lambda _o}}}\]---(2)
Here the energy of photons present in the beam of incident light is used for emission of electrons from the metal surface. So, the form of energy conversion is from optical to electrical. The current produced due to these emitted photoelectrons is called photocurrent and thus we get photoelectricity. Hence the Energy conversion in a photoelectric cell takes place from optical to electrical.

Hence option C is the correct answer.

Note: The conversion of optical to electrical energy occurs in three different ways: photoconductive, photo emissive and photovoltaic. An example of photoconductive effect is an LDR(light dependent resistor) in which incident light affects the mobility of electrons and thus the resistance of this device. Solar cells are the main example of photovoltaic effect where light incident on the device drives its circuit into producing an electric current. Photoemissive effect is used in a photomultiplier tube which has a light sensitive anode that collects photons to generate electric impulse but amplifies.