Question

Suppose that all the energy from a 75-watt light bulb came in the form of photons with wavelength 600 nm (This is not quite realistic.)

A. Calculate the energy of a single photon with wavelength 600 nm.

B. How many 600-nm photons must be emitted each second to account for all the light from this 75-watt light bulb?

Answer 1

Power P = 75w

wavelength   = 600nm = 600 x 10^-9 m

A. Energy of each Photon E = h = hc /

=( 6.626 x 10^-34 x 3 x 10⁸ ) / 600 x 10^-9

= 3.31 x 10^-19 J

B) 75 watt means 75joules of energy must be emitted in one second

No. of photons are emitted in one second = Total energy / Energy of each photon

= 75 / 3.31 x 10^-19

⁼ 22.66 x10¹⁹

= 2.3 x 10²⁰ photons