Chemistry · Structure Of Atom · NEET
n = P / E, where P is the light power in watts (joules per second) and E is the energy of one photon in joules. If a time t is given instead of per second, use n = (P x t) / E. Since E of one photon = hc/lambda, the full formula is n = (P x t x lambda) / (h x c). The answer n is a plain number (no units) because it is a count of photons.
Power (in watts) is the total light energy released each second, measured in joules. Each photon carries a tiny fixed energy, also in joules. So dividing total energy by the energy of ONE photon tells you how many photons make up that total. It is like sharing a big pile of money into equal coins to count the coins.
A bulb wastes most of its rated power as heat. Only the given percentage becomes light. So first multiply the rated power by that fraction. For a 150 W bulb that converts 8% to light, useful light power = 150 x 8/100 = 12 W = 12 J per second. Use only this 12 J (not 150 J) when you divide by the energy of one photon. Forgetting to take the percentage is the most common mistake.
The 2026 paper directly gave energy of one photon = 4.42 x 10^-19 J, so you do NOT need hc/lambda. You only had to (1) take 8% of 150 W to get 12 J, then (2) divide: 12 / (4.42 x 10^-19) = 2.71 x 10^19 photons per second. When wavelength is given instead, first compute E = hc/lambda yourself.
Yes. 1 watt = 1 joule per second. So a 100 W bulb releases 100 J of energy every second. If the question asks photons per second, watt equals joules per second, so you can use the wattage value directly as energy per second (after adjusting for efficiency if given).
Use E = hc/lambda. Here h = 6.626 x 10^-34 J s, c = 3 x 10^8 m/s, and lambda is the wavelength in metres. Convert nm to metres first: 400 nm = 400 x 10^-9 m = 4 x 10^-7 m. Then E of one photon = (6.626 x 10^-34 x 3 x 10^8) / (4 x 10^-7).
A bulb is rated at 150 watt, converting 8% of its energy into light. If the energy of one photon is 4.42 x 10^-19 J, how many photons are emitted by the bulb per second?
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: [c = 3 x 10^8 m/s]
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Use h = 6.626 x 10^-34 J s in NEET calculations. Keep c = 3 x 10^8 m/s. These give the energy of one photon as E = hc/lambda.
More photons. Brightness (higher power) means a larger NUMBER of photons per second. The energy of each photon depends only on its wavelength (colour), not on how bright the bulb is. Same colour = same energy per photon.
Multiply by 10^-9. So 400 nm = 400 x 10^-9 m = 4 x 10^-7 m. Always put lambda in metres before using E = hc/lambda, or the answer will be wrong by powers of ten.
It appears almost every year as a direct one-step numerical (NEET 2026 asked the 150 W bulb). It combines power, efficiency and photon energy, so it tests three ideas in one question. It is an easy, high-return mark if you do the efficiency step correctly.
Multiply power by time to get total light energy in joules, then divide by the energy of one photon. n = (P x t) / E. Per-second questions simply use t = 1 s.
