Black Body Radiation, Quantum Theory of Light, Important Topics For JEE 2025

Black Body Radiation : In the world of chemistry, understanding the behaviour of light and its interaction with matter is crucial. Two important concepts that help us delve into this realm are Black Body Radiation and the Quantum Theory of Light. These concepts not only have profound implications in chemistry but also lay the foundation for various scientific advancements. Let's embark on a journey to unravel the mysteries of Black Body Radiation and the Quantum Theory of Light.

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Black Body Radiation

Imagine a perfect absorber and emitter of light, which we call a black body. When a black body absorbs light, it heats up and then emits radiation. This emitted radiation covers a wide range of wavelengths and intensities, depending on the temperature of the black body. This phenomenon is known as Black Body Radiation.

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Think of a stove burner. As it heats up, it changes colour from red to orange and then to blue. This change in color represents a shift in the emitted radiation's wavelength due to the increase in temperature. Similarly, any object, when heated, emits radiation according to its temperature, as described by the black body radiation curve.

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An ideal body, which emits and absorbs radiations of all frequencies, is called black body and radiation emitted by a black body is called black body radiation.

The variation of intensity with wavelength at different temperatures for a black body is shown below:

So it is observed that with increasing temperature, the dominant wavelength in the emitted radiations decreases and the frequency increases. That is at higher temperatures, though the intensity rises as predicted by Maxwell’s theory but the wavelength decreases.

If T ₁ > T ₂ > T ₃ then λ ₁ < λ ₂ < λ ₃

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The Quantum Theory of Light

The Quantum Theory of Light : Light, which was once thought to behave purely as a wave, exhibits characteristics of both waves and particles. This duality is at the heart of the Quantum Theory of Light, proposed by Albert Einstein and Max Planck.

According to this theory, light consists of discrete packets of energy called photons. These photons have properties of both waves and particles. They carry energy proportional to their frequency, as described by Planck's equation

E = hf

Where E is energy, h is Planck's constant, and f is frequency.

In simpler terms, imagine light as a stream of tiny balls (photons) travelling through space. Each ball carries a specific amount of energy, and the colour or wavelength of light depends on how many of these balls are present and their individual energies.

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Solved Example Of Black Body Radiation

Q. Calculate number of photon coming out per sec. from the bulb of 100 watt.  If it is 50% efficient and wavelength coming out is 600 nm.

Ans. Energy = 100J

Energy of one photon

No. of photon

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