The wavelength of light in the visible region is about 390 nm for violet colour, about 550 nm (average wavelength) for yellowgreen colour and about 760 nm for red colour. What are the energies of photons in (eV) at the (i) violet end, (ii) average wavelength, yellow-green colour, and (iii) red end of the visible spectrum? `("Take h "= 6.63xx10^(-34)" J s and 1 eV" = 1.6xx10^(-19)J.)`

Explanation

Energy of the incident photon,
`E=hv=(hc)/(lambda)`
`E=(6.63xx10^(-34)Js)(3xx10^(8)m//s)//lambda`
`=(1.989xx10^(-25)Jm)/(lambda)`
(i) For voilet light,
`lambda_(1)=390` nm (lower wavelength end)
Incident photon energy, `E_(1)=(1.989xx10^(-25)Jm)/(390xx10^(-9)m)`
`=5.10xx10^(-19)J`
`=(5.10xx10^(-19)J)/(1.6xx10^(-19)J//eV)`
=3.19eV
(ii) For yellow-green light ,
`lambda_(2)=550nm` (average wavelength)
Incident photon energy,
`E_(2)=(1.989xx10^(-25)Jm)/(550xx10^(-9)m)`
`=3.62xx10^(-19)J`
=2.26eV
(iii) For red light
`lambda_(3)=760nm` (higher wavelength end)
Incident photon energy,
`E_(3)=(1.989xx10^(-25)Jm)/(760xx10^(-9)m)`
`=2.62xx10^(-19)J`
=1.64eV