University of Central Florida (UCF) PHY2054 General Physics with Calculus II Practice Exam

When light enters a medium from vacuum, its frequency stays the same while its speed slows down to v = c/n. The wavelength in a medium is tied to speed and frequency by λ' = v/f. Since f is unchanged and v = c/n, you get λ' = (c/n)/f. But in vacuum, λ = c/f, so λ' = (c/f)/n = λ/n. So the wavelength becomes shorter by a factor of n. This aligns with the intuition that slowing the wave compresses its spatial spacing, not increasing it. The other options would imply the wavelength staying the same, growing, or would have incorrect dimensional form, which isn’t consistent with how speed and frequency relate to wavelength.