Chemistry · Structure Of Atom · NEET
E is the energy of ONE photon (one packet of light). h is Planck's constant (6.626 × 10⁻³⁴ J·s). ν (nu) is the frequency of the light in per second (Hz). So a photon of higher frequency carries more energy. This is the single most important formula in this chapter.
They are the SAME formula. Use E = hν when the question gives you frequency (ν). Use E = hc/λ when the question gives you wavelength (λ), because ν = c/λ. Here c = 3 × 10⁸ m/s. Just pick the one that matches the data given.
A quantum is the general name for the smallest fixed packet of any energy. When that energy is light (electromagnetic radiation), the quantum is called a photon. So every photon is a quantum, but 'quantum' is the wider word Planck used first.
Less energy. Energy and wavelength are inversely related (E = hc/λ). So longer wavelength = lower frequency = LESS energy. Shorter wavelength (like UV or X-rays) = higher frequency = MORE energy. Students often flip this, so remember: small λ, big E.
h = 6.626 × 10⁻³⁴ joule·second (J·s). The unit is energy × time. In many NEET questions you round it to 6.63 × 10⁻³⁴ J·s. Learn this number by heart; it appears in E=hν, de Broglie, and Heisenberg's principle.
First find the total light energy given out per second (in joules). Then divide by the energy of one photon: Number of photons = Total energy ÷ E(one photon). If a bulb gives 12 J of light per second and one photon is 4.42 × 10⁻¹⁹ J, then photons = 12 ÷ (4.42 × 10⁻¹⁹) = 2.71 × 10¹⁹ per second.
Classical physics said energy can be released in any amount, smoothly. But this could not explain black-body radiation. Planck fixed it by saying energy is released only in whole-number multiples of hν (E = nhν, where n = 1, 2, 3...). Energy is quantised, not continuous. This was the start of quantum theory.
A particular station of All India Radio, New Delhi, broadcasts on a frequency of 1368 kHz. The wavelength of the electromagnetic radiation emitted by the transmitter is: [Speed of light, c = 3 × 10⁸ m s⁻¹]
A bulb is rated at 150 watt, converting 8% of its energy into light. If the energy of one photon is 4.42 × 10⁻¹⁹ J, how many photons are emitted by the bulb per second?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. Because c = νλ, replacing ν with c/λ turns E = hν into E = hc/λ. Use whichever matches the data (frequency or wavelength) given in the question.
The joule (J). In atomic problems you may also see the electron-volt (eV), where 1 eV = 1.602 × 10⁻¹⁹ J.
Energy is absorbed or emitted only in whole-number multiples of one quantum. n is a positive whole number (1, 2, 3...). You cannot have half a quantum. This is what 'energy is quantised' means.
Blue light. Blue has a shorter wavelength and higher frequency than red, so by E = hc/λ its photons carry more energy.
It is the base for the photoelectric effect, Bohr's model, and atomic spectra. Many NEET numericals ask you to find photon energy, frequency, wavelength, or the number of photons using E = hν and E = hc/λ.
