IB PHYSICS · THERMAL RADIATION

Hotter. Brighter. Shorter.

Change the temperature. Watch the spectrum shift.
Compare curves to see where the extra radiation goes.

Ideal blackbody · emissivity = 1
Wavelength distribution · absolute scale

Set the temperature

Temperature / K

Adjust temperature

300 K12,000 K

Try a temperature

Keep up to three reference curves, then change the temperature.

Wavelength rangeAuto-scale vertical axisShow visible bandMark peak wavelength

The emission spectrum

Spectral intensity
MW m⁻² μm⁻¹

Visible wavelengths are approximately 0.38–0.75 μm.

Peak wavelengthWien’s displacement law

Total emitted power / areaAcross all wavelengths, not just the window shown

Visible share of emitted powerIntegrated from 0.38 to 0.75 μm

Predict, then explore

Keep a curve at 3000 K, then double the temperature. Does the peak wavelength double or halve? How much does the total emitted power increase?

With auto-scale on, the tallest peak stays at the same screen height while the Y-axis values change. This is a changing axis scale, not constant physical intensity. Save a curve for comparison, or turn auto-scale off to hold the vertical axis fixed.

IB takeaways: higher temperature means a shorter peak wavelength, greater intensity at every fixed wavelength, and a larger total area under the full curve. Graph clipped? Widen the wavelength range or enable auto-scale. All curves always share one scale.

The physics behind the curve

λ_maxT = 2.898 × 10⁻³ m K
M = σT⁴

IB graph convention: the Y-axis shows emitted intensity per unit wavelength. More precisely, this is spectral radiant exitance M_λ: emitted power per surface area per wavelength interval, integrated over the outward hemisphere. It is not spectral radiance per steradian.

Presets are ideal-blackbody approximations. A 300 K object radiates mainly in the infrared; a visible-band peak does not mean monochromatic body colour.