Chemistry · Structure Of Atom · NEET
Because ejection depends on the FREQUENCY of each photon, not on brightness. Red light photons have low energy (low frequency). Even if you send billions of them (very bright), one single red photon still cannot push out one electron. Blue light photons have high frequency, so each one carries enough energy to eject an electron — even if there are only a few of them (dim). One electron is knocked out by exactly one photon. This is why frequency, not intensity, decides IF electrons come out.
Threshold frequency (ν₀, read 'nu-zero') is the minimum frequency of light needed to just barely knock an electron out of a metal. At this frequency the electron leaves with zero speed. If the light's frequency is below ν₀, no electron ever comes out. If it is above ν₀, electrons come out AND move with extra speed. Each metal has its own ν₀.
Work function is the minimum energy needed to remove one electron from the metal surface. It is written as W₀ = hν₀ (Planck's constant × threshold frequency). Think of it as the 'entry fee' the electron must pay to escape. Metals like caesium have a low work function (easy to eject), so they work even with visible light.
No — this is the most common trap. Increasing intensity (brightness) only means MORE photons per second, so MORE electrons are ejected per second (higher current). But the SPEED (kinetic energy) of each electron stays the same. To make electrons faster, you must increase the FREQUENCY of the light, not the brightness.
The energy of the incoming photon (hν) splits into two parts: the energy used to free the electron (work function hν₀) and the leftover energy that becomes kinetic energy. So: hν = hν₀ + ½mv². This gives KE = ½mv² = hν − hν₀ = h(ν − ν₀). If ν is below ν₀, there is no leftover energy, so no electron escapes.
If light were only a wave, then a bright (high-energy) beam of any colour should slowly build up enough energy to free electrons. But that never happens below ν₀. Instead, ejection is instant and depends on each photon's own energy. This shows light comes in tiny packets (photons), each carrying energy E = hν. So light behaves like a stream of particles here — this is the 'dual nature' of light.
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.
Albert Einstein explained it in 1905 using Planck's photon idea (E = hν). He won the Nobel Prize for this, not for relativity.
No. If the frequency is above threshold, electrons are ejected instantly (within about 10⁻⁹ s). There is no waiting time even for very dim light. This is another proof of the particle nature of light.
A photoelectron is simply the name for an electron that has been ejected from a metal surface by light in the photoelectric effect.
Alkali metals like potassium, sodium and caesium have low work functions, so ordinary visible light can eject their electrons. Most other metals need higher-frequency UV light.
They are directly related: W₀ = hν₀. A higher work function means a higher threshold frequency is needed to start ejecting electrons.
