What optical properties of polymers matter: refractive index, transmittance, photoelectric property and colour?

In one line

Optical properties describe how a polymer interacts with light: how much it bends light (refractive index), how much light passes through (transmittance), whether light changes its electrical behaviour (photoelectric property) and what colour it shows.

Explained simply

Why is a spectacle lens clear while a bucket is opaque? Why do some plastics turn yellow in sunlight? These are optical properties. They decide whether a polymer can be used for lenses, screens, optical fibres, solar cells or decorative products.

How it works

1. Refractive index (n): ratio of the speed of light in vacuum to its speed in the polymer. It depends on the polarisability of the groups in the chain; aromatic rings and heavy atoms increase n. PMMA ≈ 1.49, polycarbonate ≈ 1.58, polystyrene ≈ 1.59. Used in lenses and optical fibres (core with higher n than cladding).
2. Transmittance: percentage of incident light that passes through the material. Amorphous polymers (PMMA, PC, PS) are transparent because there are no crystallite boundaries to scatter light; PMMA transmits about 92% of visible light. Crystalline or filled polymers scatter light and are translucent or opaque.
3. Photoelectric (photoconductive) property: some polymers, especially those with conjugated π-electron systems or carbazole groups, generate charge carriers when light falls on them, so their conductivity increases. Example: poly(N-vinylcarbazole) (PVK), used historically in photocopier photoreceptors; conjugated polymers are used in organic solar cells and organic LEDs.
4. Colour: most pure polymers are colourless or white because they do not absorb visible light. Colour comes from added dyes (soluble) or pigments (insoluble, e.g., TiO₂ white, carbon black) or from conjugated structures in the chain. UV exposure can cause yellowing due to degradation.

Key formula

\( n = \dfrac{c}{v} \qquad \%T = \dfrac{I_t}{I_0} \times 100 \)

• c = speed of light in vacuum (3 × 10⁸ m/s); v = speed of light in the polymer; n has no unit
• I₀ = intensity of incident light; I_t = intensity of transmitted light

Quick example

Light travels in PMMA at about 2.01 × 10⁸ m/s: n = 3 × 10⁸ / 2.01 × 10⁸ ≈ 1.49. Applications: PMMA for aquarium walls, display covers and lenses; polycarbonate for headlamp covers and eyewear; carbon black in tyres for UV protection and black colour.

Common mistakes

• Thinking crystalline polymers are more transparent; crystallites scatter light.
• Confusing transmittance with refractive index.
• Saying all polymers are naturally coloured; colour usually comes from additives.

How MU asks it

Introductory level: "Explain optical properties of polymers" or "Write a note on refractive index and transmittance of polymers" (3–5 marks). For full marks: define each of the four properties, give the factor that controls it (structure/crystallinity/conjugation/additives), and an application for each.