Question

Identify the mismatch of the following.
A) Photodiode- optical signal
B) LED - spontaneous emission
C) Diode laser- stimulated emission
D) Solar cell- electrical energy into light
E) Photo conducting cell - photo detector

Answer 1

Hint: Energy conservation gives a great freedom for the physicists to exploit different sources of energy and convert them as per their choices. Thus different transducing devices have been invented. The source energy and the process of the conversion to a certain form of energy is very crucial to understand and detect the fallacy in the list of the problem.

Complete step by step answer:
Step 1:
In a photodiode the light is incident and then the observed photons provide the energy to the semiconductor device and produce electricity.
So, as option (A) states it is true that the photodiodes take light and produce electricity that is it acts as optical signalling.
Step 2:
LEDs act in a bit different way. It acts with the mechanism of spontaneous emission. In this semiconductor device, electrons jump from a high energy level to the low energy level passing the band gap and thus a radiation in terms of photons happens. Thus the electricity ultimately results in the spontaneous radiation of light.
Hence, option (B) is a perfect match as LED acts following spontaneous emission.
Step 3:
For diode lasers a process called stimulated emission happens. Here a particle acting like a small electric dipole oscillates with the external field. Due to this oscillation the electrons decay to a lower energy state. This results in some photon radiation. These photons in the presence of other photons act coherently inheriting the same phase and the same direction. Thus we get the output of laser.
Hence, option (C) is also a correct match, where the stimulated emission is exploited in the diode laser.
Step 4:
Solar cells take photons from sunlight and use the energy to convert it completely to electric energy.
But, in the option (D) it is written that the solar cell converts electrical energy into light, which is false.
Step 5:
Photo conducting cells consist of a thin single-crystal or polycrystalline film of compound semiconductor substances. These substances have resistances which are dependent on light intensity. This is thus able to detect different intensities of light. Hence, this can be used as photo-detectors.
So, the option (E) is also a perfect match.

Final Answer:
The mismatch among the given list is (D) Solar cell- electrical energy into light.

Note: The solar cell takes light and produces electricity not the opposite. In LED the emission is spontaneous one. However, in LASER the radius is stimulated one. The sensitivity to light intensity in the photo conducting cell gives it the capability to detect light, and hence can be considered as photo-detector.