the number of photons of light of wavelength 7000A equivalent to 1J

Hello Aspirant,

A photon can be described by either its energy, frequency, or wavelength. As frequency and wavelength are related by a constant (c) the energy can also be written in terms of wavelength,the energy equation in terms of wavelength is  E = h * c / λ,where h is the planck's constant= 6.626 × 10^-34 J s, c is the speed of light constant =3.00 × 10^8 m/s and λ is the wavelength of light.You question is to find the number of photons, so let is consider 'n' as the number of photons. So according to the formula Energy = n × hc/λ,

n (no. of photons) =Energy × λ / (hc)

= 1 J × 7000 × 10^-10 m / (6.626 × 10^-34 J s × 3.00 × 10^8 m/s)

= 3.5 × 10^18 photons

= 3.5 Quintillion photons or 3.5* 10^18 photons.

All the best

This is a simple question from Electrons and Photons. You need to practise the formulas well from this chapter as most questions are numerical and these types only where in you need to have all the formulas byheart on your finger tips. Let us now see how this one gets solved.

Energy, E= hc/lambda

where h is the Plank's constant, c is the speed of the light and lambda is the wavelength.

Energy of a single  photon of wavelength 7000 Angstrom is  hc / 7000×10^−10

put h=6.6×10^−34Joule−second and c=3×10^8m/s

we get energy of a single photon as E=2.829×10^−19Joule
Number of photon in 1 Joule=E1J=3.52×10^18

Best wishes. Thank you.