A laser produces 20.0 mW of light. In 1.00 hr, the laser emits 1.51*1020 photons. What is the wavelength of the laser?

To determine the number of photons emitted per second, we need data on the total power of the system and the total energy of the photons. We use this concept to calculate the wavelength. We are given the power of the laser, the time and the number of photons that was emitted for that given time. We calculate as follows:

Number of photons = 1.51*10^20 per hour

Number of photons per second = 1.51 x 10^20 / hr (1 hr / 3600 s) = 4.19 x 10^16 per second

4.19 x 10^16 per second = Power / Energy

4.19 x 10^16 per second = 20x10^-3 J / s / Energy

Energy = 4.77x10^-19 J

Energy = hc / wavelength

where h is the planck's constant and c is the speed of light

4.77x10^-19 J = (6.63 x 10^-34) (3 x 10^8) / wavelength

Wavelength = 4.17x10^-7 m