9702 · 22.1
Energy and momentum of a photon flashcards
Revision flashcards for Cambridge 9702 Energy and momentum of a photon (syllabus 22.1). Flip, recall, then mark a real past-paper question.
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What is the fundamental nature of a photon?
A discrete packet or quantum of electromagnetic energy.
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State the formula relating a photon's energy to its frequency.
E = hf, where E is energy, h is Planck's constant, and f is frequency.
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How is the momentum of a photon calculated using its energy?
p = E/c, where p is momentum, E is energy, and c is the speed of light.
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What is the approximate value of Planck's constant?
6.63 × 10⁻³⁴ Js.
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Define one electronvolt (eV) and provide its equivalent in joules.
1 eV is the kinetic energy gained by one electron accelerated through a potential difference of 1 volt, equivalent to 1.60 × 10⁻¹⁹ J.
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How can photon energy be expressed using its wavelength?
E = hc/λ, where h is Planck's constant, c is speed of light, and λ is wavelength.
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Do photons have rest mass?
No, photons have zero rest mass, but they still possess momentum.
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What is the relationship between photon momentum and wavelength?
p = h/λ, where p is momentum, h is Planck's constant, and λ is wavelength.
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What constant links frequency, wavelength, and speed of light?
The speed of light (c), where c = fλ.
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Why is the electronvolt (eV) a useful unit in quantum physics?
It's a convenient unit for expressing the very small energy values typically associated with individual photons and electrons.
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How do you calculate the number of photons emitted per second from a light source of known power P and wavelength λ?
First, find the energy of one photon (E = hc/λ). Then, divide the total power (energy per second) by the energy of one photon: Number per second = P / E.
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What is the de Broglie relation, and how does it relate to photons?
The de Broglie relation λ = h/p links a particle's wavelength to its momentum. For photons, this is usually written as p = h/λ, confirming that photons exhibit this wave-particle duality.
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A blue photon has a higher frequency than a red photon. Which has more momentum?
The blue photon. Since momentum p = hf/c, momentum is directly proportional to frequency. Higher frequency means higher energy and higher momentum.
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What key feature of the photoelectric effect supports the photon model of light?
The existence of a 'threshold frequency'. Below this frequency, no electrons are emitted, no matter how intense the light is. This shows energy is delivered in discrete packets (photons), not continuously like a wave.