Classical wave theory could not explain black‑body spectra or the photoelectric effect, prompting a shift to a quantised, dual model of light.

Planck solved the black‑body problem by assuming energy exchanges occur in discrete quanta. He derived the spectrum by letting oscillator energies be E=nhv introducing the constant ℎ and replacing the prediction that led to the ultraviolet catastrophe.

Einstein proposed that light itself is composed of quanta (photons) with energy E=hv. He explained the photoelectric effect with the relation Kmax=, predicting a threshold frequency and that electron kinetic energy depends on frequency, not intensity.

Photoelectric experiments Millikan, 1916:

Precise measurements showed 𝐾max varies linearly with frequency and a clear threshold frequency exists; intensity changed current but not 𝐾max. Millikan’s data gave an accurate value for ℎ.

Compton scattering (1923):

X‑ray scattering from electrons produced a wavelength shift. The result matched a particle collision model conserving energy and momentum, showing photons carry momentum.

Planck’s quantisation was initially a mathematical fix; Einstein gave it physical meaning by treating light as quantised particles. High‑precision experiments forced acceptance of particle aspects while wave phenomena remained valid.

The result was a reconceptualisation: light exhibits wave–particle duality, described by quantum theory where electromagnetic fields are quantised and interactions exchange discrete energy and momentum.

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Physics Trials

Reconceptualisation of light

Question	Answer
Intro	Classical wave theory could not explain black‑body spectra or the photoelectric effect, prompting a shift to a quantised, dual model of light.
Planck 1900	Planck solved the black‑body problem by assuming energy exchanges occur in discrete quanta. He derived the spectrum by letting oscillator energies be E=nhv introducing the constant ℎ and replacing the prediction that led to the ultraviolet catastrophe.
Einstein 1905	Einstein proposed that light itself is composed of quanta (photons) with energy E=hv. He explained the photoelectric effect with the relation Kmax=, predicting a threshold frequency and that electron kinetic energy depends on frequency, not intensity.
Photoelectric experiments Millikan, 1916:	Precise measurements showed 𝐾max varies linearly with frequency and a clear threshold frequency exists; intensity changed current but not 𝐾max. Millikan’s data gave an accurate value for ℎ.
Compton scattering (1923):	X‑ray scattering from electrons produced a wavelength shift. The result matched a particle collision model conserving energy and momentum, showing photons carry momentum.
Critical analysis	Planck’s quantisation was initially a mathematical fix; Einstein gave it physical meaning by treating light as quantised particles. High‑precision experiments forced acceptance of particle aspects while wave phenomena remained valid.
Conclusion	The result was a reconceptualisation: light exhibits wave–particle duality, described by quantum theory where electromagnetic fields are quantised and interactions exchange discrete energy and momentum.

Created by: zac taylor

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