Physics · Dual Nature Of Radiation And Matter · NEET
When the frequency of light is above the threshold and the plate voltage is kept fixed, the photoelectric current is directly proportional to the intensity of the incident light. More intensity means more photons per second, so more electrons are knocked out per second, so more current. Memory hook: Intensity = how many photons, so it controls the NUMBER of electrons (current), never their energy.
Above the threshold frequency and at a fixed collector voltage, photoelectric current rises as a straight line through the origin with intensity, because more intense light delivers more photons per second and each photon frees one electron.
Your doubts, answered
Does increasing intensity increase the photoelectric current?
Yes, as long as the light frequency is above the threshold frequency and the collector voltage is fixed. Doubling the intensity doubles the number of photons hitting the metal per second, so about double the number of electrons are emitted per second, so the current doubles. In short, current is directly proportional to intensity.
Does intensity change the energy (speed) of the emitted electrons?
No. The maximum kinetic energy of a photoelectron depends only on the frequency of light and the work function, given by KEmax = h*f - work function. Intensity only changes how MANY electrons come out per second (the current), not how FAST each one moves. So intensity has no effect on stopping potential either.
Why is photoelectric current proportional to intensity?
In the photon picture, intensity means the number of photons arriving per second. Each single photon can eject at most one electron (one-to-one). So more photons per second means more ejected electrons per second, which means more current. This is a straight line through the origin on a current versus intensity graph.
What is the effect of intensity on the saturation current?
The saturation current is the maximum current, reached when the collector voltage is positive enough to collect every emitted electron. Since intensity sets the number of electrons emitted per second, the saturation current is directly proportional to intensity. A brighter beam gives a higher flat saturation level.
If the frequency is below threshold, does high intensity still produce current?
No. Below the threshold frequency, each photon has too little energy to free even one electron, no matter how many photons arrive. So even a very intense beam gives zero photoelectric current. Intensity only matters ONCE the frequency is above threshold. This is a favourite NEET trap (see NEET 2020).
⚠️ The NEET trap ✗ More intense light gives electrons more energy, so it raises the stopping potential and speeds up the electrons. ✓ More intense light gives MORE electrons per second (higher current and higher saturation current), but each electron has the same maximum kinetic energy. Stopping potential stays the same because it depends on frequency, not intensity. 🧠 Intensity controls the NUMBER of electrons (current). Frequency controls the ENERGY of electrons (stopping potential). Never mix the two.
Real NEET questions
2025
Which of the following options represents the variation of photoelectric current with the intensity of light (frequency kept above threshold)?
A · A curve bending toward the intensity axis (saturating)
B · A straight line through the origin ✓
C · A horizontal line parallel to the intensity axis
D · A straight line with a negative intercept on the current axis
Solution: Step 1: Frequency is fixed and above threshold, and collector voltage is fixed. Step 2: Intensity = number of photons per second, and each photon can eject one electron. Step 3: So number of electrons per second (the current) is directly proportional to intensity: I(current) proportional to intensity. Step 4: A direct proportion is a straight line passing through the origin. Hence option B.
2020
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 the intensity is doubled?
A · one-fourth
B · zero ✓
C · doubled
D · four times
Solution: Step 1: Original frequency = 1.5 * f0 (above threshold). Step 2: New frequency = (1.5 * f0) / 2 = 0.75 * f0. Step 3: 0.75 * f0 is BELOW the threshold frequency f0, so no photon has enough energy to eject an electron. Step 4: Below threshold, intensity does not matter, so no electrons are emitted. Photoelectric current = zero. The doubled intensity is a distractor. Answer: zero.
Solved Dual Nature Of Radiation And Matter NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Is photoelectric current directly proportional to intensity?
Yes, provided the frequency is above the threshold frequency and the accelerating voltage is fixed. The graph of current versus intensity is a straight line through the origin.
Does intensity affect stopping potential?
No. Stopping potential depends only on the frequency of light and the work function of the metal. Changing intensity does not change the maximum kinetic energy, so the stopping potential stays the same.
What does intensity physically mean in the photon model?
Intensity is the amount of light energy falling per unit area per second. In the photon model, at a fixed frequency this equals the number of photons arriving per second, since each photon carries fixed energy E = h*f.
Why does saturation current increase with intensity?
Saturation current is reached when all emitted electrons are collected. More intensity emits more electrons per second, so the maximum collectable current (saturation current) is higher. Saturation current is proportional to intensity.
Can very intense light cause emission below the threshold frequency?
No. Below threshold frequency, each individual photon lacks the energy to free an electron. Piling up more low-energy photons does not help, so the current stays zero regardless of intensity.
