Question

what is the number of photons of light with a wavelength of 4000 pm that provide 1J of energy?

Answer 1

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c = velocity of light in vacuum = 3 × 108 m/s.
h = Planck's constant = 6.626 × 10–34 Js.
thefore the energy of photon (E)
= 6.626 × 10–34 Js X 3 × 108 m/s / 4x 10-9 m = 4.965 x 10-16 J.

Answer 2

Energy (E) of a photon = hc/λ

Where,λ = wavelength of light = 4000 pm = 4000 ×10–12 m = 4x 10-9 m

c = velocity of light in vacuum = 3 × 108 m/s

h = Planck’s constant = 6.626 × 10–34 Js

thefore the energy of photon (E)

= 6.626 × 10–34 Js X 3 × 108 m/s / 4x 10-9 m = 4.965 x 10-16 J

Now 4.965 x 10-16 J is the energy of = 1 photon

Therefore 1 J will be the energy of = 1/4.965 x 10-16 J = 2.014 x 1015 photons