Question

A budding magician holds a 2.24-mW laser pointer, wondering whether he could use it to keep an object floating in the air with the radiation pressure. This might be an idea for a new trick! Assuming the laser pointer has a circular beam 3.00 mm in diameter and the magician rigs up a totally reflecting sail on which to shine the laser, what is the maximum weight the magician could suspend with this technique?

Answer 1

W = 1.49 10⁻¹¹ kg

Step-by-step explanation:

For this problem, let's use Newton's equation of equilibrium

F - W = 0

F = W (1)

Strength can be found from the definition of pressure

P = F / A

F = P A

The radiation pressure for a reflective surface is

P = 2 I / c)

We substitute in equation 1

2 I / c A = W

The intensity is defined by the ratio of the power between the area

I = P / A

P = I A

We substitute

2 P / c = W

W = 2 2.24 10-3 / 3 108

W = 1.49 10⁻¹¹ kg