Question

1. Thirty-five years ago, a green laser might cost $100,000 and occupy a table. Today, you can buy one the size of a ball point pen for around $15. A green laser pointer has a beam power of 5.00 mW at a wavelength of 532 nm. Imagine that a lens is then used to focus the beam of such a laser to a circular spot 1 mm in diameter. Calculate (a) the intensity of the focused beam and (b) the radiation force exerted on a tiny sphere whose diameter is that of the focal spot. Assume that the sphere reflects 85% of the light that falls on it, while absorbing the rest. (Hint: the cross-sectional area of a sphere is a circle!)

2. Why do astronomers, when looking at distant objects such as stars or galaxies, say they are looking at the past? Strictly speaking, is it also true when looking at nearby objects? What about when looking at reflections, such as when you look at yourself in the mirror? Explain.

3. An object is 24.0 cm from the center of a silvered spherical glass Christmas tree ornament 6.00 cm in diameter. What are the position and magnification of its image?

Answer 1