Verified Solution • 2025–26 Session moderate

QPages 395-396: Q2: HC Verma: Concepts of Physics - Volume 2 X-Rays Pages 395-396: Q2 Solution: Iron emits X-ray of energy 6.4 keV. Calculate the times taken…

⚠️ Common Exam Traps & Conceptual Pitfalls

Students often try to use the photon’s energy (6.4 keV) to compute its speed, mistakenly believing higher‑energy photons travel faster; however, all electromagnetic radiation in vacuum travels at $c$ regardless of energy, so the time is simply $t=\frac{\text{distance}}{c}=\frac{3\times10^3\text{ m}}{3\times10^8\text{ m/s}}=1.0\times10^{-5}\text{ s}$.

Iron emits KαK_\alpha X-ray of energy 6.4 keV. Calculate the times taken by an iron KαK_\alpha photon to cross through a distance of 3 km. Use Planck constant h=4.14×10−15 eVsh = 4.14 \times 10^{-15} \text{ eVs}, speed of light c=3×108 m/sc = 3 \times 10^8 \text{ m/s}.)

Step-by-Step Verified Solution

**Answer:** Time taken = 10 �s = 1.0 _ 10__ s. **Step-by-Step Explanation:** X-rays are electromagnetic radiation and travel at the speed of light regardless of their energy. Time = Distance / Speed = (3 _ 10_ m) / (3 _ 10_ m/s) = 1.0 _ 10__ s = 10 �s [citation:8][citation:16].

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