In a photoelectric effect experiment a light of frequency 1.5 times the threshold frequency is made to fall on the surface of photosensitive material. Now if the frequency is halved and intensity is doubled, the number of photo electrons emitted will be:

Solution

Physics sir · 4 steps

Check the frequency first: below threshold, no amount of intensity can pull out a single electron.

1. New frequency. The light starts at ν = 1.5 ν₀, where ν₀ is the threshold frequency. Halving it gives ν' = 0.75 ν₀, which is below the threshold.

   hν' = 0.75 hν₀ < hν₀ = φ

2. One photon, one electron. Each electron is freed by absorbing a single photon. Here every photon carries less energy than the work function φ, so no photon can free an electron.
3. Role of intensity. Doubling the intensity only doubles the number of photons arriving each second. Each photon is still too weak, so more of them still free nothing.
4. So. No photoelectrons are emitted. The number becomes zero, option (c).

The idea to keep. Frequency decides whether emission happens at all. Intensity decides how many electrons come out, but only once the frequency is above the threshold.

Answer from the NTA final answer key. Solution by PrepWiser.