Light color determines frequency

Steps:

• Red and green are different colors of visible light.
• Color corresponds to specific wavelengths in the electromagnetic spectrum.
• Wavelength and frequency are inversely related by the wave equation $c = f \lambda$, where $c$ is constant in vacuum.
• Thus, different colors imply different frequencies.

Why B is correct:

• Frequency defines the color of light; red light has a lower frequency (around 4.3 × 10¹⁴ Hz) than green (around 5.5 × 10¹⁴ Hz), per the visible spectrum.

Why the others are wrong:

• A: Amplitude relates to wave height and can be identical for both lasers.
• C: Intensity measures power per area and can be adjusted to match between lasers.
• D: Speed of light in vacuum is constant ($3 \times 10^8$ m/s) for all electromagnetic waves, per special relativity.

Final answer: B