BPSC 69th CCE Prelims General Studies · Question 16 of 144

Correct — A, converts light energy into electric energy. A photoelectric cell — a photocell — is a device that turns incident radiant energy straight into an electrical output, and the mechanism is the photoelectric effect that Heinrich Hertz stumbled on in 1887 and Albert Einstein explained in 1905. In the classical photoemissive cell the working parts sit inside an evacuated glass bulb: a semi-cylindrical cathode coated with a low work-function material — caesium, or caesium-antimonide, caesium's work function being about 2.14 eV, the lowest among the common metals — faces a thin wire anode held at a positive potential. Light falling on that cathode is absorbed one photon at a time, each photon of energy hν handing its whole energy to a single electron; if hν exceeds the work function φ the electron escapes, with kinetic energy at most hν − φ. The anode collects those electrons, so a current flows in the external circuit, and because one photon can liberate at most one electron the photocurrent rises in proportion to the light intensity — which is exactly what makes the cell useful as a light-measuring element. A photovoltaic cell, the silicon solar cell of band gap about 1.1 eV first made practical at Bell Laboratories in 1954, reaches the same end by a different route: absorbed light creates electron-hole pairs across a p-n junction and the junction's built-in field sweeps them out as a direct current, with no vacuum and no external supply. Every member of the family is light in, electricity out — the direction of conversion is precisely what this stem tests, and only option (a) states it.

• (b)converts electric energy into light energy — This is the exact reverse, and it describes a lamp, not a cell. An incandescent bulb heats a tungsten filament to roughly 2,500–3,000 K until it glows; a fluorescent tube excites mercury vapour whose ultraviolet output a phosphor coating re-emits as visible light; a light-emitting diode drives current across a forward-biased p-n junction so that recombining electrons and holes emit photons of energy about equal to the band gap. A candidate who registers only that the name contains both 'photo' and 'electric' can pick this without ever checking which way the arrow points.
• (c)stores light energy — No device stores light as light — radiation that is not absorbed simply travels on. Storage always means converting the radiant energy into another form first: chemically, as photosynthesis does when it fixes roughly 1–2 per cent of incident sunlight into carbohydrate, or electrically, as a solar installation does when its panels charge a battery. The nearest real phenomenon is phosphorescence, where a glow-in-the-dark material holds absorbed energy in a metastable excited state and releases it slowly, and even that is absorption followed by re-emission rather than storage of light. Nothing in that family is called a photoelectric cell.
• (d)None of the above — 'None of the above' can only stand if (a), (b) and (c) all fail. Option (a) is the standard textbook description of the device — the photoelectric effect converting light energy into electrical energy — so this option is unavailable the moment (a) is read properly. On a paper that deducts one-third of a mark for a wrong response, using 'None of the above' as a hedge when one option is textbook-exact is an expensive habit.

The photoelectric effect is the emission of electrons from a surface when electromagnetic radiation falls on it. Its puzzle — unsolvable in pure wave physics — is that emission depends on the frequency of the light rather than on how bright it is: below a threshold frequency nothing at all is emitted, however intensely or however long the surface is lit, and above that threshold electrons appear with no measurable delay. Einstein resolved this in 1905 by treating light as quanta of energy hν, one photon absorbed by one electron, giving the photoelectric equation K_max = hν − φ, where the work function φ is the minimum energy needed to pull an electron out of that particular material. Devices built on this effect and on its semiconductor cousins are together called photocells: photoemissive cells eject electrons into a vacuum, photovoltaic cells generate an emf across a p-n junction, and photoconductive cells such as the cadmium-sulphide LDR simply change their resistance under illumination. All three take light as input and deliver an electrical output.

Energy-conversion questions have exactly one discriminator — which form goes in and which comes out — and once that is fixed the options collapse. Break the word open: 'photo' is the input, light; 'electric' is the output; and a cell is a source, not a sink. Then cross-check against the devices you already know by their conversion. A dynamo turns mechanical energy into electrical, a motor electrical into mechanical, a microphone sound into electrical, a loudspeaker electrical into sound, a thermocouple heat into electrical, an LED electrical into light. The photocell's partner in that table is the LED, sitting at the opposite end of the same arrow — which is exactly why option (b) is placed second in the list. It offers the same two forms in the other order, and a candidate reading at speed recognises the familiar words rather than the direction. Option (c) probes a different confusion, the idea that a device which responds to light must be soaking it up and holding it; the one place light energy is genuinely stored is a chemical bond, and the process that does it is photosynthesis, not a photocell. Note finally that the answer does not depend on which kind of photocell the setter had in mind — photoemissive, photovoltaic and photoconductive devices all convert in the same direction.

• Heinrich Hertz observed the effect in 1887 while generating electromagnetic waves; Einstein explained it with light quanta in 1905, and it is for that work — not for relativity — that he was awarded the Nobel Prize in Physics for 1921, presented in 1922.
• Einstein's photoelectric equation is K_max = hν − φ, with Planck's constant h = 6.626 × 10⁻³⁴ J s. Below the threshold frequency ν₀ = φ/h no electron is emitted at any intensity; above it the maximum kinetic energy depends only on frequency, while the number of photoelectrons emitted per second is proportional to intensity.
• The work function decides which light will do the job: caesium about 2.14 eV, so ordinary visible light suffices and caesium-coated photocathodes became standard; zinc about 4.3 eV and platinum about 5.65 eV need ultraviolet.
• Three device families share the principle — photoemissive (the vacuum photocell), photovoltaic (the silicon solar cell, band gap about 1.1 eV, first made practical by Chapin, Fuller and Pearson at Bell Laboratories in 1954 at roughly 6 per cent efficiency), and photoconductive (the cadmium-sulphide light-dependent resistor).
• Applications run from camera exposure meters, automatic street lighting and burglar or fire alarms to reading the optical sound track on cinema film, photomultiplier tubes in radiation detection, and rooftop solar arrays — whose native output is direct current, converted to alternating current by an inverter before it reaches the grid.

• Reversing the arrow — reading 'photoelectric' as a light-producing device and picking the LED-type option; a cell is a source of electricity, not a source of light
• Believing brighter light gives faster electrons: intensity raises the number of photoelectrons per second, while their maximum kinetic energy is set by the frequency alone, and below the threshold frequency no intensity works at all
• Assuming a solar panel supplies alternating current directly — photovoltaic output is DC, and an inverter is what makes it usable on the AC grid

BPSC keeps this at definition level — a one-line stem naming a device and four energy-conversion phrases, answerable in seconds once you know which form goes in and which comes out, and the 69th paper runs several such single-line science items back to back. UPSC almost never asks the bare definition; it wraps the same physics either in a statement set on solar technology (photovoltaic versus solar thermal, DC versus AC output, domestic manufacturing base) or in a scientist-and-discovery matching item in which 'Photoelectric effect : Albert Einstein' is one of the pairs to be judged.

• practice — not a real PYQ

  Which one of the following devices converts electrical energy into light energy?

  • (a)Photoelectric cell
  • (b)Light-emitting diode
  • (c)Thermocouple
  • (d)Dynamo

  Answer(b) Light-emitting diode — a forward-biased p-n junction in which recombining electrons and holes emit photons; the photocell, thermocouple and dynamo all produce electricity, from light, heat and mechanical motion respectively.

• practice — not a real PYQ

  In the photoelectric effect, if the intensity of the incident light is increased while its frequency is kept unchanged, what increases?

  • (a)The maximum kinetic energy of the emitted electrons
  • (b)The number of photoelectrons emitted per second
  • (c)The threshold frequency of the metal
  • (d)The work function of the metal

  Answer(b) The number of photoelectrons emitted per second — intensity means more photons per second, hence more ejected electrons; maximum kinetic energy is hν − φ and depends only on frequency, while threshold frequency and work function are properties of the metal.