Light travels in a straight line by virtue of what property?

• A. Wavelengths
• B. Frequency
• C. Amplitude
• D. Phase

Answer: (A)

Light travels in a straight line through a uniform medium because the wavefronts advance in step with a constant wavelength. In a homogeneous material the speed of light and its frequency stay the same, so the distance between successive crests (the wavelength) remains fixed as the wave propagates. When every part of the wavefront moves together in the same direction, the fronts are flat planes and the rays point straight ahead, giving a linear path. The other properties—frequency, amplitude, and phase—describe how the light oscillates or its energy content, not the direction of travel. If the medium were not uniform, the varying optical properties could bend the path (refraction), but in a uniform medium the constant wavelength supports straight-line propagation.