Photon Energy

Each photon carries energy given in NCERT by $E=\frac{hc}{\lambda }$, where $h=6.63\times 10^{-34}$ J·s is Planck's constant, $c=3\times 10^8$ m/s is the speed of light, and $\lambda$ is the wavelength. Substituting $\lambda =500\times 10^{-9}$ m gives $E=\frac{(6.63\times 10^{-34})(3\times 10^8)}{500\times 10^{-9}}=\frac{1.989\times 10^{-25}}{5\times 10^{-7}}=3.98\times 10^{-19}$ J. This tiny energy, about 2.48 eV, is typical of a single visible photon. The value $3.98\times 10^{-25}$ J is wrong because it drops six powers of ten by mishandling the wavelength conversion from nanometres. The value $6.63\times 10^{-34}$ J is wrong because it simply quotes Planck's constant without completing the calculation. The value $2.48\times 10^{-18}$ J is wrong because it is off by a factor of ten, likely from an arithmetic slip in the exponent. As stated in NCERT Class 12, Chapter 11, photon energy is inversely proportional to wavelength, so shorter wavelengths carry more energetic photons. It is worth stressing that this energy is an intrinsic property of a single quantum: doubling the beam power delivers twice as many such photons but never changes the energy each one carries. Expressed in electron-volts, the value 2.48 eV lies squarely in the visible range, which is why 500 nm light appears green to the eye and can readily excite molecular transitions in the retina. A magnitude check confirms that visible-light photons carry energies of order $10^{-19}$ J, matching the computed result.