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In astronomical observations, signals observed from the distant stars are generally weak. If the photon detector receives a total of 3.15 × 10^{–18} J from the radiations of 600 nm, calculate the number of photons received by the detector.

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#### Solution

From the expression of energy of one photon (E),

`"E" = "hc"/lambda`

λ = wavelength of radiation

h = Planck’s constant

c = velocity of radiation

Substituting the values in the given expression of E:

`"E" = ((6.626 xx 10^(-34) " Js")(3xx10^(8) ms^(-1)))/((600xx10^(-9) " m"))`

Energy of one photon = 3.313 × 10^{–19} J

Number of photons received with 3.15 × 10^{–18} J energy

`= (3.15 xx 10^(-18) " J")/(3.313 xx 10^(-19)" J")`

= 9.5

`~~10`

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