posted by qwerty123 .
A typical laser pointer is rated at 5.0 mW. It outputs red light with a wavelength of about 6300 Angstrom. How many photons of laser light will be emitted from the pointer if it is used 120 seconds.
E = hc/wavelength, solve for E in joules.
1 watt = 1 J/s, make the conversion to 5 mW, then see how many joules is needed to run 120 sec. I would then use a proportion to determine how many of the photons are needed.