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.

To calculate the number of photons emitted by the laser pointer, we need to use the formula:

Number of photons = Power / Energy per photon

First, we need to calculate the energy per photon. The energy of a photon can be calculated using the equation:

Energy per photon = Planck's constant × speed of light / wavelength

Given that the wavelength is 6300 Angstrom, we need to convert it to meters:

Wavelength = 6300 Angstrom × (1 × 10^-10 m / 1 Angstrom) = 6.3 × 10^-7 m

Now, we can calculate the energy per photon:

Energy per photon = (Planck's constant × speed of light) / wavelength
= (6.626 × 10^-34 J·s × 3 × 10^8 m/s) / (6.3 × 10^-7 m)
= 9.93 × 10^-19 J

Next, we calculate the power of the laser in watts:

Power = 5.0 mW × (1 W / 1000 mW)
= 5.0 × 10^-3 W

Now, we can determine the number of photons emitted by the laser pointer in 120 seconds:

Number of photons = Power / Energy per photon
= (5.0 × 10^-3 W) / (9.93 × 10^-19 J)
= 5.03 × 10^15 photons

Therefore, if the laser pointer is used for 120 seconds, approximately 5.03 × 10^15 photons of laser light will be emitted.