Question

A small laser emits light at a power of 5.00 mW. The laser beam is focussed until its diameter matches the 1266 nm diameter of a sphere placed in its path. The sphere is perfectly absorbing and has a density of 5.00 x10 3 kg/m 3.

a. Calculate the average energy per unit time per unit area (S). b. Find the maximum values of E and B. c. Calculate the radiation pressure on the sphere and the force exerted on the sphere. d. Find the acceleration of the sphere.

Answer 1

a). The half of the sphere facing the beam has surface area of

2 (pi) (R²)

= (2 pi) (633 x 10⁻⁹ m)²

= (2 pi) (4.01 x 10⁻¹³) m²

= 2.52 x 10⁻¹² m²

Average power = (5 x 10⁻³ W) / (2.52 x 10⁻¹² m²)

= 1.984 GW / m² .

b).
c).
d). I don't remember how to do these parts of the question.
However, I do sincerely believe that the solution of part-'a' was
a fair return for the penurious and parsimonious award of 5-points.