A photoelectric cell is a device which

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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.

Why the other options are wrong — (b) 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) 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' 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.

Concept: 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.

State PCS 2023 Q16 — Science & Technology PYQ with Answer

State PCS 2023 Q16 — Science & Technology PYQ with Answer