1.

The energy flux of sunlight reaching the surface of the earth is 1.388xx103W//m^(2). How many photons (nearly) per square metre are incident on the Earth per second ? Assume that the photons in the sunlight have an average wavelength of 550 nm.

Answer»

`2xx10^(10)`
`5xx10^(7)`
`2.25xx10^(17)`
`3.85xx10^(21)`

Solution :Given `lambda=550 nm=550 xx10^(-9)m`
ENERGY flux `phi=1.388xx10^(3)Wm^(-2)`
ALSO, `h=6.626xx10^(-34)Js`
and `c=3xx10^(8) ms^(-1)`
`:.` energy of each photon `E=(hc)/(lambda)`
`=(6.626xx10^(-34)xx3xx10^(8))/(550xx10^(-9))`
`=6.31xx10^(-19)J`
`:.` no. of photons per square metere of earth
`=(phi)/(E)=(1.388xx10^(3))/(3.61xx10^(-19))=3.85xx10^(21)`


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