Physics · Dual Nature Of Radiation And Matter · NEET
No. Intensity only increases the NUMBER of electrons (so the photocurrent goes up), not their energy. Maximum kinetic energy depends only on the frequency of light. Brighter light of the same colour throws out more electrons, but each electron leaves with the same maximum speed. This is one of the most tested traps in NEET.
Each photon carries energy E = h*nu. If the frequency is below the threshold, one photon does not carry enough energy to free an electron (E is less than the work function). Making light brighter only sends more such weak photons - one weak photon plus another weak photon do not add up on one electron. So current stays zero. This is why the 2020 NEET question answer was 'zero'.
Intensity decides the number (and hence the photocurrent), provided the frequency is already above threshold. Frequency decides the maximum kinetic energy. Keep the two separate: Intensity to Current, Frequency to Energy.
No. Emission is practically instantaneous, within about 10^-9 seconds, even for very dim light above the threshold. Wave theory predicted a long build-up delay, but experiments show none. This 'no time lag' is one of the five laws and is direct evidence for the photon (particle) picture of light.
No. Threshold frequency (nu_0) is a fixed property of the metal, set by its work function through W = h*nu_0. It does not change with how bright or dim the light is. Only changing the metal changes nu_0.
Light of frequency 1.5 times the threshold frequency is incident on a photosensitive material. What will be the photoelectric current if the frequency is halved and intensity is doubled?
Which of the following options represents the variation of photoelectric current with the intensity of light (shown on the x-axis)?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
There are five: (1) a minimum threshold frequency is needed for emission, (2) photocurrent is directly proportional to intensity, (3) maximum kinetic energy increases with frequency only, (4) no emission occurs below threshold whatever the intensity, and (5) emission is instantaneous with no time lag.
The intensity-versus-current law and the threshold-frequency law. NEET repeatedly tests that intensity controls current (number of electrons) while frequency controls kinetic energy, and that below threshold the current is always zero.
Laws about energy and threshold are explained by Einstein's photoelectric equation Kmax = h*nu - W. The intensity-current law comes from the fact that intensity equals the number of photons per second, and each photon frees at most one electron.
Work function W is the minimum energy to free an electron. Threshold frequency nu_0 is the frequency whose photon just supplies that energy: W = h*nu_0. They describe the same limit, one in energy units and one in frequency units.
