A laser produces 17.0 mW of light. In 4.00 hr, the laser emits 6.04*1020 photons. What is the wavelength of the laser?

The unit 'mW' means milliwatts. It is a unit of work. There are 1,000 milliwatts in a 1 Watt of work. In 4 hours, there are 14,400 seconds.

Work = Energy/time

17 mW * 1 W/1000 mW = Energy / (14,400 seconds)

Solving for energy,

Energy = 244.8 J

Energy/photon = 244.8 J / (6.04*10²⁰) = 4.053*10⁻¹⁹ J/photon

Using the Planck's equation:

E = hc/λ

where h = 6.626*10⁻³⁴ m²·kg/s, c = 3,00,000,000 m/s and λ is the wavelength

4.053*10⁻¹⁹ J/photon = (6.626*10⁻³⁴ m²·kg/s) (3,00,000,000 m/s) / λ

λ = 4.9*10⁻⁷ m or 49 micrometers