How many photons are produced in a laser pulse of 0.258 j at 683 nm?

We can calculate the energy of a single photon through Plank-Einstein relation. We have



where is the Plank's constant and is the frequency. Also, recall that to solve for the frequency, we have



where is the speed of light and is the wavelength of the  laser, in meters. So for this problem we can compute for with



Going back to the Planck-Einstein relation, we have



Hence, we have .

Given that the laser emits an energy of 0.258 J, then there are

.

7.5 ⋅ 10^18 photons 

Answer(a)   the matererials that are in contact, the two materials and the nature of their surfaces/// (b)/// the force pushing the two surfaces together, pushing the surfaces together causes the more of the asperities to come together and increases the surface area in contact with each other