A blackbody is an idealized object that absorbs all radiation falling on it and re-emits radiation with a spectrum that depends only on its temperature. It is a theoretical concept, but the Sun, stars, and a closed metal cavity with a small hole are all excellent practical approximations.

What is Wien's displacement law?

Wien's displacement law states λ_max × T = 2.898 × 10⁻³ m·K, where λ_max is the wavelength of peak emission and T is the absolute temperature in kelvin. As temperature rises, the peak shifts toward shorter wavelengths — which is why hotter stars appear blue and cooler stars appear red.

What is the Stefan-Boltzmann law?

The Stefan-Boltzmann law says the total radiated power per unit area of a blackbody is P = σT⁴, where σ is the Stefan-Boltzmann constant (5.67 × 10⁻⁸ W m⁻² K⁻⁴). Doubling the temperature increases the radiated power by a factor of 16, since the relationship is to the fourth power.

What is the ultraviolet catastrophe?

Classical physics (Rayleigh-Jeans law) predicted that a blackbody should emit infinite energy at short wavelengths, including the ultraviolet region. Real blackbodies do nothing of the sort — they show a finite peak and then drop off. The mismatch between theory and experiment was called the ultraviolet catastrophe.

How did Planck solve the ultraviolet catastrophe?

In 1900, Max Planck proposed that energy is exchanged in discrete packets called quanta, where E = nhν. This restriction made the high-frequency modes statistically rare, so the predicted intensity dropped off at short wavelengths and matched experiment exactly. The idea launched quantum mechanics.

If the temperature of a blackbody doubles, what happens to λ_max?

Wien's law gives λ_max × T = constant, so if T doubles then λ_max is halved. The peak emission wavelength shifts to half its original value. This is a frequent NEET question and the answer is always 'halved' for a doubling of absolute temperature.

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Blackbody Radiation

PhysicalMediumNEET: 1-2 Q3D

Physics lab

Blackbody Radiation — From Catastrophe to Quantum

Atoms & Modern Physics

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temperature slider ghumao · object ka colour + Planck curve dono live shift hote hain · UV catastrophe toggle karo

How to use this lab

1Temperature slider drag karo — curve aur peak shift hote dekho (Wien's law).
2Famous-object presets try karo (Sun, bulb, human body).
3“UV catastrophe” toggle karke classical vs quantum compare karo.

About this lab

Drag a temperature slider from human body heat to a blue supergiant. Watch Planck's curve shift, see why classical physics predicted an ultraviolet catastrophe, and understand Wien's law and Stefan-Boltzmann in real time.

What you're seeing

Kisi bhi garam object ka colour uske temperature se decide hota hai — material se nahi. Slider ghumao: red → white → blue, aur saath mein emission curve bhi shift hoti hai. Yahin se quantum physics shuru hui.

Key NEET facts

Wien's displacement law: λ_max · T = 2.898×10⁻³ m·K
Stefan-Boltzmann law: P = σT⁴
T double karo → power 16× (2⁴). Power area ke neeche dikhta hai
Planck's quantum: E = nhν
Sun's surface: 5778 K
peak green (~500 nm) ke paas — isiliye aankhein yahin best dekhti hain

Memory trick

Wien peak hilaata hai (left/right), Stefan-Boltzmann area badhaata hai (T⁴), Planck ne catastrophe ko maara. Teen naam, teen kaam.

Practice

Test Yourself

If the absolute temperature of a blackbody is doubled, the wavelength of maximum emission becomes:

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