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 answer
Explanation
The correct answer is C, Resonant frequency in cesium (or rubidium) atom. An atomic clock keeps time by locking onto the fixed frequency of radiation that makes atoms such as cesium-133 or rubidium jump between two energy levels.
In a cesium clock, microwaves are tuned until they exactly match the transition between the two hyperfine levels of the cesium-133 ground state, where the atoms respond most strongly. That frequency never changes, and since 1967 the SI second has been defined as 9,192,631,770 cycles of this radiation. India's national time, Indian Standard Time, is kept by cesium atomic clocks at the CSIR-National Physical Laboratory in New Delhi.
A is wrong: a vibrating quartz crystal is the principle of ordinary quartz watches, not atomic clocks.
B is wrong: the vibration of atoms in a crystal lattice is not used to keep time; the clock uses an energy-level transition.
D is wrong: hydrogen masers exist, but the standard principle the question describes is the resonance of cesium or rubidium atoms.
Exam tip: one second equals 9,192,631,770 oscillations of the cesium-133 atom; quartz watches use a crystal vibrating at 32,768 Hz.
Practice Questions
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- C.They act as immunosuppressants in the body.
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- A.12.5 cm
- B.5 cm
- C.2 cm
- D.10 cm
Show answer
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