Questions: Refractive Index as a Material Property

Question 1

A glass prism separates white light into a spectrum, with blue light bending more than red light. Which statement correctly explains why?

• A. Blue light has higher energy, so it interacts more strongly with the glass surface and is deflected further
• B. Blue light has a higher refractive index in glass than red light, meaning it slows more and bends more at each glass-air boundary
• C. Blue light has a shorter wavelength, so it travels faster through the glass and experiences greater deflection
• D. The refractive index of glass is higher for red light, causing red to bend more toward the normal

Answer: (B)

Dispersion means n varies with wavelength — blue (shorter wavelength) has a higher n in glass than red (longer wavelength). Higher n means the light slows more (n = c/v, so larger n → smaller v). Slower light bends more toward the normal when entering glass and more away when exiting. Option A confuses energy with refractive behavior. Option C reverses the speed relationship — shorter wavelength means slower in glass, not faster. Option D simply has the direction of dispersion backwards.

Question 2

Light passes from water (n ≈ 1.33) into diamond (n ≈ 2.42). What happens to the light's speed and bending direction?

• A. Speed increases and the light bends away from the normal
• B. Speed decreases and the light bends toward the normal
• C. Speed decreases and the light bends away from the normal
• D. Speed is unchanged (c is constant), but direction changes

Answer: (B)

Moving from lower n (water) to higher n (diamond) means moving to a slower medium — v = c/n, so larger n → smaller v. When light enters a denser (higher-n) medium, Snell's law requires it to bend toward the normal. Option D confuses the vacuum speed of light c (which is constant) with the speed in a medium, which definitely changes. Option C would describe light going from higher n to lower n.

Question 3

The refractive index of a material is a fixed constant that does not depend on the color (wavelength) of light passing through it.

• T. True
• F. False

Answer: False

This is false — refractive index is wavelength-dependent, a property called dispersion. Blue light (shorter wavelength) typically has a higher n in glass than red light (longer wavelength), because blue light is closer to the ultraviolet resonance frequencies of the material's electrons and couples more strongly with them. If n were constant across wavelengths, prisms would produce no spectrum and rainbows would not exist.

Question 4

The refractive index of any ordinary material must be greater than or equal to 1.

• T. True
• F. False

Answer: True

True. By definition n = c/v, where c is the speed of light in vacuum and v is the speed in the material. Since light can only slow down in a material (v ≤ c), the ratio n = c/v ≥ 1 always. Vacuum has n = 1 exactly. No ordinary material allows light to travel faster than c, so n < 1 is not possible for real materials in normal conditions.

Question 5

Why is the refractive index described as a 'material property' rather than simply a number that describes how light bends at a surface? What does it actually encode about the material?

Model answer: The refractive index encodes how strongly the electrons in a material interact with electromagnetic radiation — specifically, how much the passing light wave drives those electrons to oscillate, which slows the wave down. This coupling depends on the electronic structure of the material and its proximity to natural resonance frequencies (typically in the ultraviolet). Because n reflects intrinsic electronic properties of the substance, it is a material property like density or electrical conductivity — not a geometric description of a surface. This is also why n varies with wavelength: blue light is closer to the electronic resonance than red light, so it couples more strongly and slows more.

The key is that n is not just a bending parameter — it quantifies how fast light propagates inside the material, which in turn reflects deep physics about electron-photon interaction. Two materials with the same n at one wavelength may have different ns at other wavelengths because their electronic structures respond differently across the spectrum. This is why different glass formulations are combined in camera lenses to control chromatic aberration.