Iron emits `K_alpha` X-ray of energy 6.4 keV and Calcium emits `K_alpha` X-ray of energy 3.69 keV. Calculate the times taken by an iron `K_a` photon and a calcium `K_a` photon to cross through a distance of 3 km.

Explanation

To solve the problem of calculating the time taken by an iron Kα photon and a calcium Kα photon to cross a distance of 3 km, we can follow these steps:

### Step 1: Understand the speed of photons
Photons, regardless of their energy or type (in this case, Kα X-rays from iron and calcium), travel at the speed of light, denoted as \( c \). The speed of light is approximately:
\[ c = 3 \times 10^8 \text{ m/s} \]

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