Increasing water content in a hydrogel lens has what effect on its refractive index?

• A. Higher refractive index
• B. Lower refractive index
• C. No change in refractive index
• D. Cannot be determined from water content

Answer: (B)

Increasing water content changes the lens toward a higher fraction of water, and water has a lower refractive index than the hydrogel polymer. Because the refractive index of the composite is effectively a blend of the polymer and water, adding more water lowers the overall index. For many hydrogels, the polymer alone has a higher n (around 1.48–1.50) while water is about 1.333, so increasing hydration shifts the effective index downward toward that water value. The exact number depends on the specific polymer and how much water is present, but the trend is clear: more water content leads to a lower refractive index.