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Physics Question 97 – AP-EAMCET 2026

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When an iron rod is heated, the variation of color from dull red to white can be explained by

All objects above absolute zero emit electromagnetic radiation. The characteristics of this radiation, including its color, depend on the object's temperature.

Step 1: Understand the Phenomenon✦ Active

When an iron rod is heated, it starts to glow, first dull red, then brighter red, orange, yellow, and eventually white or even blue-white at very high temperatures. This change in color is due to the emission of electromagnetic radiation, specifically thermal radiation.

💡 Teacher's Secret Hint

This phenomenon is a classic example of blackbody radiation, where an ideal emitter absorbs all incident electromagnetic radiation and emits radiation based solely on its temperature.

Step 2: Relate Color to Wavelength○ Expand

The color of the emitted light is related to its wavelength. Red light has a relatively long wavelength in the visible spectrum, while white light is a combination of all visible wavelengths, indicating a shift towards shorter wavelengths (including blue and violet) as the temperature increases.

Step 3: Identify the Governing Law○ Expand

The law that describes the relationship between the temperature of a black body and the wavelength at which it emits the most radiation is Wien's Displacement Law. This law states that the peak wavelength (λmax) of the emitted radiation is inversely proportional to the absolute temperature (T) of the object:

λmaxT=b

where b is Wien's displacement constant (2.898×10−3 m⋅K). Stefan's law (or Stefan-Boltzmann law) describes the total power radiated, and Newton's law of cooling describes the rate of heat loss, neither of which directly explains the color shift.

💡 Teacher's Secret Hint

Remember that Stefan-Boltzmann Law relates total radiated power to temperature (P∝T4), and Newton's Law of Cooling describes the rate of heat loss to the surroundings, neither of which directly explains the color shift.

Step 4: Conclude the Explanation○ Expand

As the iron rod's temperature increases, λmax decreases according to Wien's Displacement Law. This shift to shorter wavelengths means the emitted light progresses from predominantly red (longer wavelength) to white (a broader spectrum including shorter wavelengths), explaining the observed color variation.

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