Which one of the following correctly describes the principle of the working of an atomic clock?
- (a)Vibration of a small quartz crystal
- (b)Simple harmonic motion of atoms inside a crystal
- (c)Resonant frequency in cesium (or rubidium) atom
- (d)Excitation and de-excitation of hydrogen atoms
Correct — C, resonant frequency in cesium (or rubidium) atom. An atomic clock keeps time by locking an oscillator onto the one microwave frequency at which atoms of a chosen element flip between the two hyperfine levels of their ground state. That resonant frequency is a property of the atom itself, identical for every atom of that species anywhere, which is why it makes a far better standard than any manufactured object. For caesium-133 the figure is exact by definition: the BIPM fixes the SI second by declaring the unperturbed ground-state hyperfine transition frequency of caesium-133 to be 9 192 631 770 hertz. Rubidium clocks work on the same principle at a different frequency and are the compact, lower-cost version, widely flown on navigation satellites.
- (a)Vibration of a small quartz crystal — That is the principle of an ordinary quartz watch, whose crystal is cut to vibrate at 32,768 hertz. A quartz oscillator is in fact used inside an atomic clock, but only as the local oscillator that the atomic resonance corrects; on its own it drifts with temperature and ageing, which is exactly what the atoms are there to prevent.
- (b)Simple harmonic motion of atoms inside a crystal — Atoms in a crystal lattice do vibrate about their positions, but that vibration is thermal, its frequency depends on temperature and on the particular crystal, and no timekeeping standard is built on it.
- (d)Excitation and de-excitation of hydrogen atoms — This is the most tempting wrong option, because a hydrogen maser really is a kind of atomic clock. But it works on a hyperfine resonance of hydrogen, not on generic excitation and de-excitation, and it is used for short-term stability rather than to define the unit. The SI second is realised on caesium, which is why the key names caesium and rubidium.
Every clock is an oscillator plus a counter: something that repeats at a steady rate, and something that counts the repetitions. A pendulum clock counts swings, a quartz watch counts crystal vibrations, and an atomic clock counts cycles of the microwave radiation that drives a specific transition inside an atom. The atomic version wins because the transition frequency is set by the structure of the atom and cannot be manufactured wrongly, worn out, or knocked out of tune.
The stem asks for the principle, and the word doing the work in the correct option is 'resonant'. Options (a) and (b) are both crystal-based, which is the giveaway that they belong to quartz timekeeping rather than to atomic timekeeping. Option (d) deserves an honest note: hydrogen masers exist and are genuine atomic frequency standards, so a well-read candidate can feel the pull of it. The distinction the key relies on is that the SI second is defined and realised on caesium, and rubidium is the common practical variant, while (d) also describes the mechanism loosely rather than as a hyperfine resonance. As of the 2024 exam that definition stood on caesium; since then the international metrology bodies have continued to work towards a future redefinition of the second on an optical transition, using strontium or ytterbium clocks that are already more stable than caesium.
- The BIPM fixes the SI second by taking the caesium-133 unperturbed ground-state hyperfine transition frequency to be exactly 9,192,631,770 hertz.
- The second is therefore the duration of 9,192,631,770 periods of that radiation.
- Rubidium atomic clocks use the same hyperfine-resonance principle in a smaller, cheaper package and are widely carried on navigation satellites.
- A quartz watch crystal is typically cut to vibrate at 32,768 hertz — precise, but it drifts, unlike an atomic resonance.
- Choosing the quartz option because quartz appears in everyday watches; quartz is the oscillator an atomic clock disciplines, not the source of its accuracy.
- Rejecting the caesium option because rubidium is mentioned alongside it — both work on the same hyperfine-resonance principle.
- Thinking a hydrogen maser is not an atomic clock at all; it is, but the unit of time is defined on caesium.
Asked as a 'which describes the principle' item — identify the mechanism, not the device; the SI definition of the second is the fact that settles it.
No directly related past PYQ was found.
- practice — not a real PYQ
The SI unit of time, the second, is defined in terms of a transition in which atom?
- (a)Hydrogen
- (b)Caesium-133
- (c)Carbon-12
- (d)Argon-40
Answer(b) Caesium-133 — the second is fixed by taking its ground-state hyperfine transition frequency as exactly 9,192,631,770 hertz.
- practice — not a real PYQ
An ordinary quartz wristwatch keeps time by counting which of the following?
- (a)Swings of a pendulum
- (b)Vibrations of a quartz crystal driven by an applied voltage
- (c)Microwave transitions in rubidium atoms
- (d)Rotations of a balance wheel
Answer(b) Vibrations of a quartz crystal driven by an applied voltage — the piezoelectric crystal vibrates at a fixed frequency and the circuit counts those vibrations.