22.1 A2 Level

Energy and momentum of a photon

7 flashcards to master this topic

Definition Flip

What is a photon?

Answer Flip

A photon is a quantum of electromagnetic radiation. It's a discrete packet of energy exhibiting wave-particle duality; it has zero mass and travels at the speed of light in a vacuum.

Definition Flip

State the equation that relates the energy of a photon to its frequency.

Answer Flip

E = hf, where E is the energy of the photon, h is Planck's constant (6.63 x 10⁻³⁴ Js), and f is the frequency of the electromagnetic radiation.

Definition Flip

What is the electronvolt (eV) and how is it related to the joule (J)?

Answer Flip

An electronvolt (eV) is the amount of energy gained (or lost) by a single electron when it passes through a potential difference of 1 volt. 1 eV = 1.60 x 10⁻¹⁹ J.

Definition Flip

How is the momentum of a photon related to its energy and the speed of light?

Answer Flip

The momentum (p) of a photon is given by p = E / c, where E is the energy of the photon and c is the speed of light in a vacuum (approximately 3.0 x 10⁸ m/s).

Key Concept Flip

Explain how electromagnetic radiation exhibits particulate nature.

Answer Flip

Electromagnetic radiation, while exhibiting wave-like behavior, also comes in discrete packets of energy (photons). This is demonstrated in phenomena like the photoelectric effect, where electrons are emitted when light of sufficient frequency shines on a metal surface, behaving as discrete particles delivering energy.

Calculation Flip

A photon has an energy of 4.14 eV. Calculate its momentum.

Answer Flip

First convert eV to Joules: 4.14 eV * 1.60 x 10⁻¹⁹ J/eV = 6.624 x 10⁻¹⁹ J. Then, using p = E/c, p = (6.624 x 10⁻¹⁹ J) / (3.0 x 10⁸ m/s) = 2.21 x 10⁻²⁷ kg m/s.

Calculation Flip

A photon has a wavelength of 500 nm. Calculate its energy in electronvolts.

Answer Flip

First, find the frequency using c = fλ: f = c/λ = (3.0 x 10⁸ m/s) / (500 x 10⁻⁹ m) = 6 x 10¹⁴ Hz. Then, E = hf = (6.63 x 10⁻³⁴ Js) * (6 x 10¹⁴ Hz) = 3.978 x 10⁻¹⁹ J. Finally, convert to eV: (3.978 x 10⁻¹⁹ J) / (1.60 x 10⁻¹⁹ J/eV) = 2.49 eV.

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21.2 Rectification and smoothing 22.2 Photoelectric effect