A black body is an idealised body that absorbs all electromagnetic radiation falling on it and emits thermal radiation depending on its temperature. (p.224)
As temperature rises, the wavelength of maximum radiation intensity becomes shorter. (p.225)
Classical theory failed to explain the black-body radiation graph, which sparked quantum theory. (p.226)
Max Planck introduced the quantum of energy in 1900; Einstein introduced the photon in 1905. (p.227)
A line spectrum has unique lines for each element; the visible spectrum is continuous (400–750 nm). (p.228)
A quantum is a discrete energy packet; photon energy E = hf = hc/λ, with h = 6.63 × 10−34 J s. (p.229)
de Broglie hypothesis: particles have wave properties, λ = h/p = h/mv. (p.230)
Electron diffraction confirmed the wave properties of electrons in 1927; electron microscopes exploit short de Broglie wavelengths. (p.231)
Light and electrons have wave-particle duality. (p.232)
Power of a light source: P = nhc/λ, where n = photons emitted per second. (p.232)
7.2 Photoelectric Effect
Photoelectric effect: electrons (photoelectrons) are emitted from a metal surface when light of suitable frequency shines on it. (p.234)
Planck's constant can be found from the gradient of the graph of activation voltage Va against 1/λ using LEDs: h = me/c, where m = gradient. (p.236)
Four characteristics: higher frequency gives higher kinetic energy; threshold frequency f0 exists; kinetic energy is independent of intensity; emission is instantaneous. (p.237)
Classical wave theory cannot explain these characteristics. (p.237)
7.3 Einstein's Photoelectric Theory
Einstein's photoelectric theory (1905) uses photons of energy E = hf; each photon is fully absorbed by one electron. (p.238)
Einstein's equation: hf = W + ½mv2max (= W + Kmax). (p.239)
Work function W is the minimum energy needed to emit a photoelectron; W = hf0. (p.239)
The graph of Kmax against f is a straight line with gradient h and frequency intercept f0. (p.239)
Threshold wavelength: λ0 = hc/W. (p.242)
In a photocell, higher light intensity gives a larger photoelectric current. (p.243)
Applications: solar cells, automatic doors, image sensors, solar panels on the ISS. (p.244)