Light has different speeds
Page 16This page includes a diagram or table.
OBSERVATION / THE MEDIUM MATTERS
Light has different speeds
Light does not travel at one speed in every medium.
Passage actions
VACUUM REFERENCE: c = 299,792,458 m/s
Passage actions
Vacuum 1.00000 c
Passage actions
Air 0.99970 c
Passage actions
Water 0.75019 c
Passage actions
Crown glass 0.65920 c
Passage actions
FIGURE 6 / Calculated phase speeds using representative visible-light indices: 1, 1.0003, 1.333 and 1.517. Actual values depend on wavelength and conditions; glass composition matters. [35,47-49]
Passage actions
OBSERVATION
Light propagates at different speeds through vacuum, air, water and glass. The medium-dependent variation is established physics. [35,47-49]
Passage actions
MODEL
Phase speed is described by v = c/n. The refractive index, n, depends on wavelength, material and physical conditions.
Passage actions
COMPARISON
Air differs slightly from vacuum; water and glass produce much larger reductions. Phase, pulse and signal speeds must be distinguished. [35,47]
Passage actions
SCOPE
The symbol c identifies the vacuum reference. Material-dependent speed and proposed variation of vacuum physics are different claims, so they are recorded separately.
Passage actions
The speed of light comes with a sentence around it.