How does the refractive index n vary with wavelength in normal dispersion?

• A. n increases with wavelength
• B. n decreases as wavelength increases
• C. n remains constant with wavelength
• D. n varies non-monotonically with wavelength

Answer: (B)

In normal dispersion, the refractive index n decreases as the wavelength of light increases. Shorter wavelengths experience a larger n and thus slow down more in the medium, bending more when entering or exiting the material. Conversely, longer wavelengths have a smaller n, slow down less, and bend less. This is why prisms spread white light into colors: blue light (shorter wavelength) is refracted more than red light (longer wavelength). Near absorption bands, dispersion can behave differently (anomalous dispersion), but in the typical visible range the trend is toward decreasing n with increasing wavelength.