How much light is reflected off of a plastic lens that does not have an AR coating on it?

• A. 4%
• B. 12%
• C. 2%
• D. 8%

Answer: (D)

Light reflects from any surface where air meets lens material because their refractive indices differ. At normal incidence, the amount reflected at one air–plastic interface can be estimated by a simple formula that depends on the indices. With typical plastic used in lenses (around 1.50) and air at 1.00, the reflection per surface works out to about 4%. A lens has two surfaces, so the total reflectance without any coating is roughly doubled, giving about 8% of the light reflected overall. AR coatings are designed to bring that per-surface reflectance down, which is why uncoated lenses commonly show around eight percent total reflection. The other values don’t fit the usual per-surface or total reflectance for standard uncoated plastics: 4% would be per surface, 2% is too low for typical plastics, and 12% would imply a much larger index contrast or different incidence conditions.