Physics · Ch 8 — Heat and Thermodynamics
Wien's Displacement Law
Wien's Displacement Law
Every hot object radiates across a whole spread of wavelengths at once, but always with one particular wavelength, , at which the radiated intensity is greatest. Wien's displacement law states this peak wavelength is inversely proportional to the body's absolute temperature: (Wien's constant) -- so as temperature rises, the peak shifts toward shorter wavelengths (Figure 8.13 shows this leftward shift across five black-body temperature curves). Applied to the Sun (surface temperature about 5700 K), this gives nm, which falls right in the middle of the visible spectrum (400-700 nm, Figure 8.14) -- exactly why the human eye evolved to be sensitive precisely to visible light: a species that evolved under a much hotter star, such as …
What this figure shows. A graph of radiation intensity (Jm⁻³Hz⁻¹, y-axis) against wavelength in nanometres (x-axis, from 0 to 2000 nm), showing five separate bell-shaped curves for five different black-body temperatures: 3500 K, 4000 K, 4500 K, 5000 K and 5500 K. Each curve rises from zero, peaks at some particular wavelength, then falls back toward zero at longer wavelengths; crucially, the peak of each successive curve (higher temperature) sits further to the LEFT (shorter wavelength) than the peak of the curve below it, visually demonstrating Wien's law that the wavelength of peak emission shifts to shorter wavelengths as temperature rises. The curves are collectively called the 'black b …
What this figure shows. A composite figure with two parts. The upper part is a radiation-intensity-vs-wavelength graph for the Sun at 5700 K, peaking at a labelled wavelength λmax around 508 nm. The lower part is a full electromagnetic spectrum bar running from very short wavelengths (0.0001 nm gamma rays, X-rays) through ultraviolet, then a narrow highlighted band labelled 'VISIBLE SPECTRUM' spanning 400 nm to 700 nm (with 400, 500, 600, 700 nm individually marked), then infrared, radar, TV/FM, AM and long radio waves out to 100 m. The two parts are aligned so that the Sun's radiation peak (508 nm) is shown falling squarely inside the visible band -- the figure's whole point is to show that human vision evolved …