A near U.V. photon of 300 nm is absorbe by a gas is red then remitted as two photons. One photon is red with wavelength 760 nm. Hence wavelength of the second photon is

Explanation

It may be noted that energy of photon which is emitted as sun of the energy of two photons.
Energy absorbed, hv=`hc//lambda`
According to available information , `(hc)/(lambda)=(hc)/(lambda_(1))+(hc)/(lambda_(2)) , (1)/(lambda)=(1)/(lambda_(1))+(1)/(lambda_(2)) , (1)/(lambda_(2))=[(1)/(lambda)-(1)/(lambda_(2))]`
Now `lambda=300nm , lambda_(1)=760 nm , lambda_(2)` can be calculated as :
`(1)/(lambda_(2))=[(1)/(300)-(1)/(760)]=(760-300)/(300xx760) (nm^(-1))`
`(1)/(lambda_(2))=(460)/(760xx300)(nm^(-1))` or `lambda_(2)=(760xx300)/(460)(nm)=496 nm`.