Question

The ratio of velocities of two photons of wavelengths \(2000\,\text{\AA}\) and \(4000\,\text{\AA}\) is

• A. \(2:1\)
• B. \(1:2\)
• C. \(1:4\)
• D. \(1:1\)

Hint:

For photons in vacuum: \[ v = c \quad (\text{constant}) \] Wavelength or frequency changes energy, not speed.

Answer 1

The Correct Option is D

Step 1: Photons are electromagnetic waves and always travel with the speed of light in vacuum. \[ v = c = 3 \times 10^8\,\text{m s}^{-1} \]
Step 2: The speed of light does not depend on wavelength or frequency. Hence, photons of wavelengths \(2000\,\text{\AA}\) and \(4000\,\text{\AA}\) travel with the same velocity.
Step 3: Therefore, the ratio of their velocities is: \[ v_1 : v_2 = c : c = 1 : 1 \]