Which one of the following quantities is the same for all photons in vacuum?

Options

A. Speed
B. Kinetic energy
C. Frequency
D. Wavelength

Answer

To determine which quantity is the same for all photons in a vacuum, we can analyze the properties of photons and how they relate to each other.

### Step-by-Step Solution:

1. **Understanding Photons**: Photons are particles of light and are a form of electromagnetic radiation. They travel at the speed of light in a vacuum.

2. **Properties of Photons**: Photons have several key properties, including:
- Speed (c)
- Wavelength (λ)
- Frequency (ν)
- Energy (E)

3. **Speed of Photons**: In a vacuum, all photons travel at the same speed, which is the speed of light (approximately \(3 \times 10^8\) m/s). This is a fundamental property of light.

4. **Wavelength and Frequency**: The wavelength and frequency of photons can vary. The relationship between wavelength (λ), frequency (ν), and speed (c) is given by the equation:
\[ c = ν \cdot λ \]
This means that if the frequency changes, the wavelength must change accordingly to keep the speed constant.

5. **Energy of Photons**: The energy of a photon is given by the equation:
\[ E = hν \]
where \(h\) is Planck's constant. Since frequency can vary, the energy of photons also varies.

6. **Conclusion**: The only quantity that remains the same for all photons traveling in a vacuum is their speed. Therefore, the answer to the question is that the speed of all photons in vacuum is the same.

### Final Answer:
The quantity that is the same for all photons in vacuum is their speed.

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