Photoelectric Effect: Experimental Observations Class 12 Physics | Dual Nature of Radiation and Matter

Understand it

The main observations are that emission is practically instantaneous, no photoelectrons are emitted below the threshold frequency, photoelectric current increases with intensity when frequency is suitable, and maximum kinetic energy depends on frequency rather than intensity. Stopping potential is the minimum retarding potential needed to stop the fastest photoelectrons from reaching the collector.

Example

For a given metal, red light may fail to emit electrons if its frequency is below threshold, while violet light may emit electrons even at low intensity because each photon has higher energy.

Formula and rule

Important relations: eV0 = Kmax, where e is electronic charge in C, V₀ is stopping potential in V, and Kmax is maximum kinetic energy in J.

Threshold condition is nu ≥ nu0.

Intensity affects saturation current when frequency is above threshold, while frequency affects stopping potential and Kmax.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

In an experiment, stopping potential is 1.5 V.

1The maximum kinetic energy is Kmax = eV0 = (1.6 × 10⁻¹⁹ C)(1.5 V) = 2.4 × 10⁻¹⁹ J = 1.5 eV.
2This is the energy of the fastest emitted photoelectrons.

Method you can reuse

Check whether incident frequency is above threshold, identify whether intensity or frequency is changed, connect intensity with number of photoelectrons and current, connect frequency with Kmax and stopping potential, then interpret the relevant graph or observation.

Sketch it in your answer

Draw the photoelectric circuit with evacuated tube, photosensitive cathode, collector anode, variable battery, microammeter, and incident radiation. For graphs, label photoelectric current on the y-axis and collector potential on the x-axis; stopping potential is the negative potential where current becomes zero.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

What happens to stopping potential when the frequency of incident light is increased for the same metal?

Think first, then show answer

The stopping potential increases because higher frequency photons have greater energy. After overcoming the same work function, the emitted photoelectrons have larger maximum kinetic energy, so a larger retarding potential is needed to stop them.

Common wrong answer

Wrong: Higher intensity always gives electrons higher speed. Correct: Higher intensity gives more emitted electrons per second if frequency is sufficient; higher frequency gives greater maximum kinetic energy.

Don't mix this up

A common error is to say that increasing intensity increases maximum kinetic energy. Intensity increases the number of photons per second, so it can increase photoelectric current, but it does not increase energy of each photon if frequency is unchanged.

Exam tip

When answering graph questions, always name the axes and state what changes with intensity and what changes with frequency. This avoids mixing current, stopping potential, and kinetic energy.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Often asked through observation-based questions, current-potential graphs, intensity-frequency comparisons, and assertion-reason items on instantaneous emission and threshold frequency.

1
Enough for 1 mark
The photoelectric effect is the emission of electrons from a metal surface when electromagnetic radiation of frequency greater than or equal to a certain threshold frequency falls on it.
2
Add for 2 marks
Example: For a given metal, red light may fail to emit electrons if its frequency is below threshold, while violet light may emit electrons even at low intensity because each photon has higher energy.
3
Add for 3 marks
The main observations are that emission is practically instantaneous, no photoelectrons are emitted below the threshold frequency, photoelectric current increases with intensity when frequency is suitable, and maximum kinetic energy depends on frequency rather than intensity.

Use the concept graph around this idea instead of revising it in isolation.

Electron Emission and Work Function

Electron emission is the liberation of electrons from the surface of a metal when they receive enough energy to overcome the surface binding. The minimum energy needed by an electron to just escape from a metal surface is called the work function, usually denoted by phi0.

Einstein's Photoelectric Equation

Einstein's photoelectric equation states that the energy h nu of an incident photon is used partly to overcome the work function phi0 of the metal and the remaining energy appears as the maximum kinetic energy of the emitted photoelectron.

Photon: Particle Nature of Light

A photon is a discrete packet, or quantum, of electromagnetic radiation that carries energy E = h nu and momentum p = h/λ while moving with the speed of light in vacuum.

de Broglie Hypothesis: Wave Nature of Matter

The de Broglie hypothesis states that every moving material particle is associated with a matter wave whose wavelength is λ = h/p, where p is the momentum of the particle.

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Photoelectric Effect: Experimental Observations

Understand it

See it worked out

Check yourself

Write it in the exam

Concept practice is coming soon

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Photoelectric Effect: Experimental Observations

Understand it

See it worked out

Check yourself

Write it in the exam

Related concepts

Concept practice is coming soon

Help improve this page