PMDC Unit 50 · 1 outcome

Dawn of Modern Physics

Unit 14 is one outcome: light as particles (photons) with energy E = h f. Intensity is how many packets. Colour is how big each packet is.

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14.1

Photons and the particle model of light

PMDC: Explain the particle model of light in terms of photons with energy E = h f.

Classical light was a continuous wave. Planck and Einstein: energy comes in packets. One packet is a photon. Its energy is set by the frequency of that light: $E = h$ f. Using $c = f λ$ you can also write $E = h$ c / $λ$ .

A brighter lamp of the same colour sends more photons per second. Each photon is still h f. Dim violet can eject electrons from a metal; bright red of too-low f cannot — the packet is too small, no matter how many arrive.

A photon has zero rest mass, zero charge, and speed c in vacuum. Its momentum is $p = h / λ$ = E / c. Atoms emit or absorb whole packets, never half an h f.

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p h o t o n = o n e

quantum

f = 4

× 10¹⁴ Hz

$E = h f h = 6.63 \times 1 0^{- 34} J s$
$E = h c / λ$
$E (e V) \approx 1240/ λ (nm)$
$p = h / λ = E / c$
$P = n$ h f (n photons per second)
$1 eV = 1.6 \times 1 0^{- 19} J$

Hold these

Colour (f or $λ$ ) sets the energy of one photon. Brightness sets n.
Double f, double E. Halve $λ$ , double E.
Wave face: interference. Particle face: photoelectric effect, packets of h f.

Unit 50 formula sheet

Write these until they are cheaper than thinking, then open the quiz.

$E = h f h = 6.63 \times 1 0^{- 34} J s$
$E = h c / λ$
$E (e V) \approx 1240/ λ (nm)$
$p = h / λ = E / c$
$P = n$ h f (n photons per second)
$1 eV = 1.6 \times 1 0^{- 19} J$

Open the Unit 50 quiz