Final answer:
The power needed to levitate a foil with a laser beam can be calculated using the expression (mg) / t, where m is the mass of the foil and t is the time it takes for the laser beam to change the momentum of the foil.
Step-by-step explanation:
When an object is levitated by a laser beam, the power needed can be calculated using the following expression:
P = (mg) / t
Where P is the power, m is the mass of the foil, g is the acceleration due to gravity, and t is the time it takes for the laser beam to change the momentum of the foil.
To evaluate P for a foil with a mass of 25 μg, we need to know the specifics of the laser beam, including the time it takes to change the momentum of the foil. Without that information, we cannot provide a specific value for P.