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QPage 393: Q5: HC Verma: Concepts of Physics - Volume 2 X-Rays Page 393: Q5 Solution: X-ray and visible light travel at the same speed in vacuum. Do…

⚠️ Common Exam Traps & Conceptual Pitfalls

Students frequently assume that the refractive index of a material is the same for all electromagnetic wavelengths, so they claim X‑rays travel at the same speed as visible light in glass. In reality $n$ depends on wavelength (dispersion); for X‑rays $n$ is slightly below 1, giving a different speed $v=c/n$ than that of visible light.

X-ray and visible light travel at the same speed in vacuum. Do they travel at the same speed in glass?

Step-by-Step Verified Solution

**Answer:** No, they do not travel at the same speed in glass. **Step-by-Step Explanation:** The speed of light in a medium is given by v = c/n, where n is the refractive index. The refractive index of glass depends on the wavelength of the radiation. Since X-rays have a much shorter wavelength than visible light, their refractive index in glass is slightly different (typically slightly less than 1), so their speed is very close to c but not exactly the same as visible light [citation:2].

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