A spherical convex surface separates an object space and an image space of refractive index 1.0 and 1.5 respectively. If the radius of curvature of the surface is 25 cm, its power is D.

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(3) 2. Given: n₁=1.0 in the object space, n₂=1.5 in the image space, and R=25 cm=0.25 m, positive because the surface is convex towards the object. Idea: refraction at a single spherical surface obeys (n₂)/v-(n₁)/u=(n₂-n₁)/R. The right-hand side does not depend on where the object is, and that fixed quantity is the power of the surface. P=(n₂-n₁)/R=(1.5-1.0)/(0.25) P=2 D Checking through the focal distances: parallel light focuses at f₂=(n₂R)/(n₂-n₁)=0.75 m, and P=(n₂)/(f₂)=(1.5)/(0.75)=2 D; from the object side f₁=(n₁R)/(n₂-n₁)=0.5 m and P=(n₁)/(f₁)=2 D again. Two surfaces like this, back to back in a thin lens, are what add up to the lens maker's formula.

A spherical convex surface separates an object space and an image space of refractive index 1.0 and 1.5 respectively. If the radius of curvature of the surface is 25 cm, its power is ________ D.

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