Demystifying the Photoelectric Effect: Einstein's Equation & Key Graphs

Modern Physics is the ultimate scoring zone for NEET aspirants. It carries a massive weightage of 12-15%, and the questions are mostly direct formula-based. Among all Modern Physics topics, the Photoelectric Effect is NTA's absolute favorite.

In this quick guide, we will cover the core formulas, the concept of stopping potential, and the graphs you must memorize.

1. The Core Concept: Work Function

The photoelectric effect is the emission of electrons when light (photons) hits a metal surface. However, electrons won't just jump out for any light. The metal holds onto them with a minimum binding energy called the Work Function ( $\Phi$ or $W_0$ )
.

$\Phi = h\nu_0 = \frac{hc}{\lambda_0}$

Where:

$h$
= Planck's constant ( $6.63 \times 10^{-34} \text{ J s}$ )
$\nu_0$
= Threshold frequency (Minimum frequency required)
$\lambda_0$
= Threshold wavelength (Maximum wavelength allowed)

2. Einstein's Photoelectric Equation

Albert Einstein won the Nobel Prize for this exact equation. It is simply the conservation of energy applied to a photon and an electron.

Energy of incident photon = Work Function + Maximum Kinetic Energy of the emitted electron.

$E = \Phi + K_{max}$

This is most frequently written in NEET exams as:

$K_{max} = h\nu - h\nu_0 = hc \left( \frac{1}{\lambda} - \frac{1}{\lambda_0} \right)$

Stopping Potential ( $V_0$ )

NTA loves asking questions about Stopping Potential. It is the minimum negative (retarding) voltage applied to the anode with respect to the cathode that completely stops even the most energetic photoelectrons from reaching it.

The work done by the stopping potential equals the maximum kinetic energy of the electrons:

$K_{max} = eV_0$

By substituting this into Einstein's equation, we get the master formula for solving 90% of NEET numericals on this topic:

$eV_0 = h\nu - \Phi$

4. The 3 Golden Rules (and Graphs)

When dealing with theoretical or graphical questions, remember these three immutable rules:

Intensity determines Number: Increasing the intensity of light (keeping frequency constant) increases the number of emitted photoelectrons (saturation current), but does NOT change their kinetic energy.
Frequency determines Energy: Increasing the frequency of incident light (above the threshold) increases the maximum kinetic energy of the electrons and increases the magnitude of the stopping potential.
Time Lag:
The photoelectric emission is an instantaneous process ( $< 10^{-9}$ seconds). There is no time lag!

Pro Tip for Calculation:
In the exam hall, doing calculations with Planck's constant is a waste of time. Memorize the shortcut for photon energy in Electron-Volts (eV) when wavelength is in Angstroms (Å): $E (\text{in eV}) = \frac{12400}{\lambda (\text{in Å})}$

Test Your Knowledge!

Can you apply Einstein's equation to a real numerical? Jump into our Mock Test Arena and try solving the Modern Physics module. Your dream medical college is just a few correct formulas away!

1. The Core Concept: Work Function

2. Einstein's Photoelectric Equation

Stopping Potential (V0V_0V0​)

4. The 3 Golden Rules (and Graphs)

Test Your Knowledge!

Stopping Potential ( $V_0$ )