Experimental Study of the Photoelectric Effect ⚡

1. Quick Look at the Setup 🔍

  • An evacuated glass / quartz tube holds two plates: a photosensitive emitter C and a collector A. A quartz window lets ultraviolet (or visible) light reach C. :contentReference[oaicite:21]{index=21}
  • A battery lets you make A either positive (to pull electrons) or negative (to repel them) relative to C. You read the potential with a voltmeter and the current with a micro-ammeter. :contentReference[oaicite:22]{index=22}
  • You can vary three things independently: the light’s intensity, its frequency n, and the potential difference V between A and C. :contentReference[oaicite:23]{index=23}

2. Handy Terms & Formulas 🧮

  • Photoelectric current: flow of emitted electrons.
  • Saturation current: the maximum current when every emitted electron reaches A. :contentReference[oaicite:24]{index=24}
  • Stopping (cut-off) potential V0: the smallest negative potential on A that makes the current drop to zero. :contentReference[oaicite:25]{index=25}
  • Maximum kinetic energy of the fastest photoelectrons: \(K_{max}=eV_0\) :contentReference[oaicite:26]{index=26}
  • Threshold frequency n0: the minimum light frequency needed for any emission. :contentReference[oaicite:27]{index=27}

3. What Intensity Does 📈

Keep frequency and plate potential fixed. As you brighten the light, the photocurrent rises linearly. More light ⇒ more electrons per second. :contentReference[oaicite:28]{index=28}

4. What the Collector Potential Does 🪫→🔋

  • Increase the positive potential on A → current climbs until it saturates (all electrons collected). :contentReference[oaicite:29]{index=29}
  • Reverse the polarity and make A negative → current falls; at V0 it reaches zero because even the fastest electrons turn back. :contentReference[oaicite:30]{index=30}
  • Changing light intensity raises the saturation current but leaves V0 unchanged. :contentReference[oaicite:31]{index=31}

5. What Frequency Does 🎚️

  • For higher light frequencies, V0 shifts to more negative values. Greater frequency ⇒ larger electron energy. :contentReference[oaicite:32]{index=32}
  • A plot of V0 versus frequency is a straight line. :contentReference[oaicite:33]{index=33}
  • No electrons come out if the frequency is below n0—no matter how bright the light is. :contentReference[oaicite:34]{index=34}

6. Four Big Experimental Facts 📝

  1. Above the threshold frequency, photocurrent ∝ light intensity. :contentReference[oaicite:35]{index=35}
  2. Saturation current grows with intensity, but stopping potential stays the same. :contentReference[oaicite:36]{index=36}
  3. Stopping potential (and therefore \(K_{max}\)) rises linearly with frequency but ignores intensity. :contentReference[oaicite:37]{index=37}
  4. Emission starts instantly—within about \(10^{-9}\,\text{s}\)—even for very dim light. :contentReference[oaicite:38]{index=38}

7. Material Matters 🧑‍🔬

Each metal owns its personal threshold frequency. Alkali metals (Na, K, Cs, …) respond to visible light, while metals like zinc need ultraviolet light. :contentReference[oaicite:39]{index=39}

8. Key Equations & Graph Tips ✏️

  • Intensity graph: straight line through the origin (current vs. intensity).
  • Potential graph: S-curve that levels at saturation current; crosses zero at \(-V_0\).
  • Frequency graph: straight line; x-intercept gives n0; slope relates to electron charge. :contentReference[oaicite:40]{index=40}

9. NEET High-Yield Nuggets 🎯

  1. \(K_{max}=eV_0\) and how to find V0 from the current–voltage curve.
  2. Direct proportionality between photocurrent and light intensity (above n0).
  3. Linear rise of V0 with frequency; threshold frequency concept.
  4. Independence of \(K_{max}\) from intensity—crucial to reject the classical wave model.
  5. Instantaneous emission time (~\(10^{-9}\,\text{s}\)) showing particle-like interaction.

🌟 Keep practicing graphs and equations—you’ll ace those photoelectric questions!