In thick-lens calculations, what does D1 denote?

• A. Front surface power
• B. Back surface power
• C. Index of refraction
• D. Central thickness

Answer: (A)

In thick-lens calculations, the lens is treated as two refracting surfaces separated by thickness, and the overall power comes from the contributions of the front surface, the back surface, and a correction term due to thickness. The power attributed to the front surface, known as D1, is the dioptric power introduced as light first enters the lens. It is defined by D1 = (n2 − n1)/R1, where n1 is the index of the incident medium (usually air), n2 is the index inside the lens, and R1 is the radius of curvature of the front surface with the appropriate sign convention. The back surface has its own power, D2, defined similarly with the indices on that surface. The central thickness affects the overall power through the thickness correction term, but it is not D1 itself. This is why D1 corresponds to the front surface power.