Physics · Dual Nature Of Radiation And Matter · NEET
In wave theory, the energy carried by a light wave depends on its intensity (amplitude squared) and the time you shine it. So a brighter light or a longer exposure means more energy delivered to the metal. Frequency (colour) does not decide how much energy an electron can absorb. This single idea is the root of all three failures.
Wave theory says: if you make the light bright enough, or wait long enough, electrons must eventually gain enough energy to escape any metal, no matter the colour. But experiments show that below a certain threshold frequency, even very bright light ejects zero electrons. Red light on many metals gives no photoelectrons however intense it is. Wave theory has no reason for this cut-off frequency.
On wave theory the light energy spreads over the whole surface, so a single electron must soak up energy slowly, like filling a bucket drop by drop. For weak light this should take seconds or even minutes before an electron collects enough energy. But experiments show photoelectrons appear almost instantly, in about 10 to the power minus 9 seconds, even for very dim light. Wave theory predicts a measurable delay that simply is not there.
Wave theory predicts that brighter light (higher intensity) should give faster, more energetic electrons, and that changing colour should not change their speed. Experiment shows the opposite: increasing intensity only increases the number of electrons, not their maximum kinetic energy, while increasing frequency raises the maximum kinetic energy. This link between KE and frequency is invisible to wave theory.
Einstein's photon idea. Light comes in packets (photons) of energy E = hf. One photon gives all its energy to one electron. This instantly explains the threshold (photon energy must beat the work function), the no-time-lag (energy is delivered in one lump), and why KE depends on f (KEmax = hf minus work function). That is the next topic, Einstein's photoelectric equation.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Very little. Wave theory correctly links brightness to the number of electrons in a loose way, but it fails on the three key facts: threshold frequency, no time lag, and kinetic energy depending on frequency. That is why the photon model replaced it for this phenomenon.
One, it cannot explain threshold frequency. Two, it wrongly predicts a time lag before emission. Three, it wrongly makes maximum kinetic energy depend on intensity instead of frequency.
Because wave theory says energy depends on intensity, so a very bright red beam should deliver enough energy over time to free electrons. In reality, if red light is below the threshold frequency, no electrons come out however bright it is. This contradiction exposes wave theory.
For weak light, wave theory predicts an electron would need seconds to minutes to soak up enough energy. Experiments show emission within about 10 to the power minus 9 seconds, essentially instant, which wave theory cannot explain.
Yes. NEET often asks which observation cannot be explained by wave theory, or asks you to match intensity with number of electrons and frequency with kinetic energy. Knowing the three failures F-T-E scores easy marks.
