In the thick-lens shape factor equation Ms = 1/(1 - (t/n)D1), what does t represent?

• A. Central thickness of lens
• B. Total thickness
• C. Edge thickness
• D. Optical thickness

Answer: (A)

In this thick-lens expression, t is the central thickness of the lens—the physical distance along the optical axis between the two refracting surfaces, measured at the center of the lens. This axial separation is the quantity that influences how much the first surface’s power D1 is “felt” by the ray as it travels through the lens, which is why it appears as (t/n)·D1 in the formula. The product is dimensionless and captures the effect of the lens’s thickness on the overall shape factor. Using central thickness here makes the model consistent, because edge thickness or total thickness do not consistently represent the actual separation the light traverses between the two surfaces in the thick-lens approximation.