試題詳解

20. A laser beam reads information from a compact disc with an initial diameter of1.900 mm as it strikes the disc. The laser forms a cone with a half-angle θ₁=30.0° (as shown in the figure). The beam then passes through a 1.50 mm thick layer of plastic with a refractive index of 1.60 before reaching the reflective information layer near the disc's top surface. What's the beam diameter d at the information layer? 

(A) 726 µm
(B) 846 µm
(C) 914 µm
(D) 1056 µm
(E) 1168 µm