What is photoelectric effect?

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Q. What is photoelectric effect?

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Concept understanding — Photoelectric Effect

The Photoelectric Effect: When Light Knocks Electrons Loose

Imagine you're throwing tennis balls at a wall covered in loose pebbles. If you throw hard enough, a pebble might get knocked off. That's the basic picture — but the photoelectric effect is the quantum version of this, and it completely shattered classical physics.

The Intuition

Light is made of tiny packets of energy called photons. Each photon carries a specific amount of energy, determined by its colour (frequency). When a photon hits a metal surface, it can transfer its energy to an electron inside the metal. If that energy is enough, the electron breaks free and flies out.

Think of electrons in a metal like people in a room with a high window. To escape, they need enough energy to reach the window sill. A photon is like a boost — but only if it gives enough energy in one shot. No amount of weak boosts (dim light) will work if each individual boost is too small.

The Precise Statement

E_{\text{photon}} = hf = \phi + K_{\text{max}}

Where:

$E_{\text{photon}} = hf$ is the energy of a photon (Planck's constant $h = 6.63 \times 10^{-34} \ \text{J·s}$ , $f$ is frequency)
$\phi$ is the work function — the minimum energy needed to remove an electron from that metal
$K_{\text{max}}$ is the maximum kinetic energy of the ejected electron

What Classical Physics Got Wrong

Before Einstein (1905), physicists thought light was a continuous wave. They expected:

Brighter light → more energy per electron → faster electrons
Any colour would eventually eject electrons if you waited long enough

But experiments showed the opposite:

Observation	Classical Prediction	Actual Result
Effect of intensity	Brighter light → faster electrons	Brighter light → more electrons, same speed
Threshold frequency	None — any light works eventually	Below a certain frequency, no electrons no matter how bright
Time delay	Electrons need time to absorb energy	Electrons appear instantly (within $10^{-9}$ s)

The Key Insight

Einstein said: light behaves like a stream of particles (photons), each with energy $hf$ . One photon interacts with one electron. If $hf < \phi$ , the electron cannot escape — period. If $hf > \phi$ , the excess energy becomes kinetic energy:

K_{\text{max}} = hf - \phi

This is why:

Increasing intensity (more photons) ejects more electrons, but each electron still gets the same energy per photon — so their speed doesn't change.
Below threshold frequency, even a trillion photons per second can't help — each one is too weak individually.

Important

The photoelectric effect proved that light is quantized — it comes in discrete packets. This was the birth of quantum mechanics. Einstein won the 1921 Nobel Prize for this, not for relativity.

A Worked Example

Note

Problem: A metal has work function $\phi = 2.0 \ \text{eV}$ . Light of frequency $f = 6.0 \times 10^{14} \ \text{Hz}$ shines on it. Find the maximum kinetic energy of ejected electrons. ( $h = 4.14 \times 10^{-15} \ \text{eV·s}$ )

Step 1: Photon energy

E = hf = (4.14 \times 10^{-15})(6.0 \times 10^{14}) = 2.48 \ \text{eV}

Step 2: Subtract work function

K_{\text{max}} = 2.48 - 2.0 = 0.48 \ \text{eV}

Step 3: Convert to joules if needed

0.48 \ \text{eV} \times 1.6 \times 10^{-19} = 7.68 \times 10^{-20} \ \text{J}

The electron escapes with this much kinetic energy.

Common Mistake to Avoid

Watch out

Students often think "more intense light means more energy per electron." Wrong. Intensity = number of photons per second. Each photon still has the same $hf$ . More photons = more electrons, but each electron gets the same energy kick.

The Big Picture

The photoelectric effect is your first encounter with wave-particle duality. Light, which we model as a wave for interference and diffraction, behaves as a particle when transferring energy to matter. This duality is central to all of quantum mechanics.

Final takeaway: Light ejects electrons only if each photon carries enough energy individually. The colour (frequency) determines whether ejection happens; the brightness (intensity) determines how many electrons get ejected.

"Photoelectric effect formula and Einstein equation" is among the most-searched Class 12 physics topics, and it is a core result of the Dual Nature of Radiation and Matter chapter in the NCERT/CBSE Class 12 Physics curriculum. Work function and threshold frequency questions built on this concept appear in nearly every JEE Main and NEET physics paper.

Why this formula?

Photoelectric Effect: Why the Key Formulas Hold

The photoelectric effect is a cornerstone of quantum physics. It showed that light behaves as particles (photons) , not just waves. Let's build the reasoning step-by-step.

1. The Core Idea: Energy Conservation

When a photon hits a metal surface, it transfers all its energy to a single electron inside the metal.

The photon's energy is $E = hf$ , where $h$ is Planck's constant and $f$ is the frequency of light.
The electron needs a minimum energy to escape the metal — this is called the work function, $\phi$ .

Why only one electron?

Einstein proposed that light is quantized into discrete packets (photons). A single photon cannot split its energy among multiple electrons — it interacts with one electron at a time.

2. The Photoelectric Equation

If the photon's energy is greater than the work function, the excess energy becomes the electron's kinetic energy after escape:

hf = \phi + K_{\text{max}}

Where:

$hf$ = energy of incident photon
$\phi$ = work function (minimum energy to remove electron)
$K_{\text{max}}$ = maximum kinetic energy of ejected electron

Why "maximum" kinetic energy?

Electrons inside the metal have different binding energies.
Some electrons are near the surface (loosely bound) → get maximum $K$ .
Others are deeper → lose energy in collisions before escaping → lower $K$ .

3. The Stopping Potential Connection

We measure $K_{\text{max}}$ using a stopping potential $V_s$ :

K_{\text{max}} = eV_s

Where $e$ is the electron charge. This is because:

An electric field opposing the electron's motion does work $eV_s$ to stop it.
At the stopping potential, the electron's kinetic energy is exactly balanced by the electric potential energy.

Combining:

hf = \phi + eV_s

This is the Einstein photoelectric equation in its most testable form.

4. Why the Threshold Frequency Exists

From the equation:

hf = \phi + eV_s

If $f$ is too low, $hf < \phi$ . Then:

The photon cannot supply enough energy to overcome the work function.
No electron is ejected, regardless of light intensity.

The threshold frequency $f_0$ is when $K_{\text{max}} = 0$ :

hf_0 = \phi \quad \Rightarrow \quad f_0 = \frac{\phi}{h}

Why intensity doesn't matter for ejection?

Intensity = number of photons per second.
Each photon still has energy $hf$ . If $hf < \phi$ , even a billion photons won't eject an electron — each photon is individually too weak.

5. Why Kinetic Energy Depends on Frequency, Not Intensity

From $K_{\text{max}} = hf - \phi$ :

Frequency $f$ directly determines $K_{\text{max}}$ .
Intensity only affects the number of electrons ejected (more photons → more electrons), not their individual energy.

This was the key experimental contradiction with classical wave theory:

Classical: Higher intensity = bigger wave amplitude = more energy to electrons.
Reality: Higher frequency = more energy per electron; intensity only changes current.

6. Summary of Key Relationships

Quantity	Formula	Why it holds
Photon energy	$E = hf$	Light is quantized (Planck-Einstein)
Work function	$\phi = hf_0$	Minimum energy to escape at threshold
Max kinetic energy	$K_{\text{max}} = hf - \phi$	Energy conservation per photon-electron
Stopping potential	$eV_s = hf - \phi$	Electric work balances kinetic energy
Threshold frequency	$f_0 = \phi/h$	Below this, no ejection possible

7. The Deeper "Why" — Particle Nature of Light

The photoelectric effect cannot be explained by classical wave theory because:

Waves spread energy over the whole wavefront — an electron would take time to absorb enough energy.
But experiments show instantaneous ejection (within $10^{-9}$ s).
Wave theory predicts kinetic energy should increase with intensity — it doesn't.

Einstein's photon model resolves all three:

Instantaneous — one photon, one interaction.
Frequency-dependent — photon energy is $hf$ .
Intensity-independent — more photons = more electrons, not more energy per electron.

Key takeaway: The photoelectric effect is a direct consequence of energy quantization — both light and electron binding energy are quantized. The formulas are simply conservation laws applied to this quantum world.

The photoelectric effect is the phenomenon in which electrons are ejected (emitted) from the surface of a metal when electromagnetic radiation of an appropriate, sufficiently high frequency falls on it. The surface that shows this behaviour is called a photosensitive surface. It was first observed by Heinrich Hertz in 1887 while studying spark discharges, and later investigated systematically using an evacuated tube containing an emitter (cathode) and collector (anode) plate.\n\nA key feature of the effect is that it only occurs above a THRESHOLD FREQUENCY $\nu_0$ specific to the metal being illuminated -- below this frequency, no electrons are emitted at all, no matter how intense the radiation; above it, electrons are emitted instantly, even for very feeble incident light. The emitted electrons are called photoelectrons, and their maximum kinetic energy depends on the frequency of the incident light and the emitter material (via Einstein's photoelectric equation $KE_{max}=h\nu-\phi_0$ ), never on the light's intensity. [!ANSWER] Photoelectric effect = light-induced electron emission from a metal surface, occurring only above a material-specific threshold frequency.

Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

Showing the 12 most recent of 113 on this concept.

CBSE 2026Set 55/2/11 markMCQ

\phi_0

›Reveal solution

The maximum kinetic energy of photoelectrons is determined by Einstein's photoelectric equation: it depends on both the photon energy (set by frequency $\nu$ ) and the work function $\phi_0$ of the material. The answer is (D).

Why the photoelectric effect reveals energy quantization

When light strikes a metal surface, electrons can be ejected—but not in the way classical wave theory predicted. Einstein's revolutionary insight was that light arrives in discrete packets (photons), each carrying energy $E = h\nu$ . An electron absorbs one photon entirely; if that energy exceeds the minimum needed to escape the metal (the work function $\phi_0$ ), the electron breaks free, and any leftover energy becomes kinetic energy.

This is fundamentally an energy-balance problem. The photon delivers a fixed amount of energy $h\nu$ . The electron must "pay" $\phi_0$ to escape. What remains is the maximum kinetic energy:

K_{\text{max}} = h\nu - \phi_0

Notice what this equation tells us: $K_{\text{max}}$ increases linearly with frequency $\nu$ and decreases with larger work function $\phi_0$ . Both parameters matter.

Step-by-step reasoning

The photon energy is $h\nu$ .

A single photon of frequency $\nu$ carries energy proportional to that frequency. Higher frequency means more energetic photons (ultraviolet photons pack more punch than red photons).
The work function $\phi_0$ is the escape barrier.

Different materials bind their electrons with different strengths. Cesium has a low work function (~2 eV), so even visible light can eject electrons. Platinum has a high work function (~6 eV), requiring ultraviolet light. This material property directly subtracts from the available kinetic energy.
Energy conservation gives $K_{\text{max}} = h\nu - \phi_0$ .

The electron that escapes with maximum kinetic energy is one that was initially at the Fermi level (loosest bound) and lost no energy to collisions on the way out. All the "profit" after paying $\phi_0$ goes into kinetic energy.
Intensity does not affect $K_{\text{max}}$ .

Intensity measures the number of photons arriving per unit time, not the energy of each photon. Brighter light ejects more electrons (higher photocurrent), but each electron still gets energy from just one photon. Doubling intensity doubles the electron count, not their individual speeds.

Watch out

A common mistake is thinking that brighter light (higher intensity) gives electrons more energy. Intensity affects how many electrons are emitted, not how fast each one moves. Only frequency controls the energy per photon.

Examining each option

Option	Claim	Verdict
(A)	Depends on $\phi_0$ only	Incorrect—ignores the photon energy $h\nu$
(B)	Depends on $\nu$ only	Incorrect—ignores the material's work function
(C)	Depends on $I$ only	Incorrect—intensity affects photocurrent, not $K_{\text{max}}$
(D)	Depends on both $\phi_0$ and $\nu$	Correct—both appear in Einstein's equation

The equation $K_{\text{max}} = h\nu - \phi_0$ makes it unambiguous: you need to know both the frequency of the light (which sets the photon energy) and the work function of the surface (which sets the escape cost).

Tip

If you plot $K_{\text{max}}$ versus $\nu$ , you get a straight line with slope $h$ and $y$ -intercept $-\phi_0$ . This was one of the key experimental confirmations of Einstein's theory and a direct way to measure Planck's constant.

✓Final answer

\phi_0

CBSE 2026Set 55/2/11 markMCQ
$\nu\,(>\nu_0)$
›Reveal solutionSolution
The Assertion is true because photocurrent depends on the number of photoelectrons, which increases with light intensity. The Reason is also true — stopping potential depends on frequency, not intensity. But the Reason does not explain the Assertion, since they involve different physical mechanisms. So the correct choice is (B).

The core physics: photoelectric effect and the two separate ideas

The photoelectric effect has a clean conceptual split that many students miss. There are two independent things happening when light hits a metal surface:

How many electrons come out — this is governed by the intensity of light (number of photons per second). More photons → more electrons ejected → larger photocurrent.

How much kinetic energy each electron has — this is governed by the frequency of light (energy per photon). Higher frequency → more energy per electron → you need a larger stopping potential to bring them to rest.

These two are completely separate. Intensity does not affect the energy of individual electrons; frequency does not affect how many electrons are ejected (above threshold). This is the key insight that makes the question straightforward.

Step-by-step reasoning

1. Understanding the Assertion (A)

On increasing the intensity of incident light of frequency $\nu > \nu_0$ on a photosensitive surface, the photocurrent increases.

When frequency is above the threshold $\nu_0$ , every photon that hits the surface can eject a photoelectron (assuming it's absorbed). Intensity means the number of photons per second per unit area. If you increase intensity, more photons arrive each second, so more electrons are knocked out. The photocurrent (rate of flow of charge) is directly proportional to the number of electrons emitted per second.
$\propto$
So the Assertion is true.

2. Understanding the Reason (R)

The stopping potential for a photosensitive surface increases with increase of frequency $\nu > \nu_0$ of incident light.

Stopping potential $V_s$ is the voltage needed to stop the most energetic photoelectrons. Einstein's photoelectric equation gives:
$K_{\text{max}} = h\nu - \phi_0$
where $\phi_0$ is the work function. Since $K_{\text{max}} = eV_s$ , we have:
$eV_s = h\nu - \phi_0 \quad \Rightarrow \quad V_s = \frac{h}{e}\nu - \frac{\phi_0}{e}$
This is a straight line with slope $h/e$ . As $\nu$ increases, $V_s$ increases linearly. So the Reason is also true.

Watch out

A common mistake is to think that increasing intensity increases stopping potential. It does not — stopping potential depends only on frequency (and the metal's work function). Intensity only changes the magnitude of photocurrent, not the energy of individual electrons.

3. Does the Reason explain the Assertion?

The Assertion talks about what happens when you change intensity. The Reason talks about what happens when you change frequency. These are different physical quantities affecting different aspects of the photoelectric effect. The Reason does not explain why photocurrent increases with intensity — it explains a completely separate phenomenon.

Therefore, both statements are true, but the Reason is not the correct explanation of the Assertion.

Tip

A quick way to check: if the Reason were the explanation, then increasing frequency should also increase photocurrent. But it doesn't — above threshold, changing frequency changes the energy of electrons, not their number (assuming constant intensity). This mismatch immediately tells you the Reason cannot explain the Assertion.

✓Final answer
$The correct option is (B) : Both (A) and (R) are true, but (R) is not the correct explanation of (A).$
CBSE 2026Set 55/3/11 markMCQ
$200$
›Reveal solutionSolution
The fastest photoelectron’s kinetic energy is found from Einstein’s photoelectric equation: $K_{\text{max}} = h\nu - \phi$ . Converting wavelength to frequency and using the given work function gives $K_{\text{max}} \approx 2.0$ eV, so the correct option is (D).

The photoelectric effect is a clean, direct application of energy conservation: a photon gives all its energy to an electron. The electron uses some of that energy to escape the surface (the work function $\phi$ ), and whatever remains shows up as kinetic energy. The fastest electron is the one that loses the least energy on its way out — so its kinetic energy is simply $h\nu - \phi$ .

Let’s walk through it.

Find the photon energy in eV. Wavelength $\lambda = 200$ nm $= 200 \times 10^{-9}$ m. Photon energy $E = h\nu = \frac{hc}{\lambda}$ . Use the handy constant $hc = 1240$ eV·nm (this is a standard shortcut for such problems). So

$E = \frac{1240\ \text{eV·nm}}{200\ \text{nm}} = 6.2\ \text{eV}.$
Tip

Memorise $hc \approx 1240$ eV·nm — it converts wavelength in nm directly to energy in eV without messing with SI units.

Apply Einstein’s photoelectric equation. The maximum kinetic energy of emitted photoelectrons is

$K_{\text{max}} = h\nu - \phi.$
Here $\phi = 4.2$ eV. So
$K_{\text{max}} = 6.2\ \text{eV} - 4.2\ \text{eV} = 2.0\ \text{eV}.$
Watch out

A common mistake is to forget that the work function and photon energy must be in the same units. Here both are in eV, so no conversion is needed — but if $\phi$ were given in joules, you’d have to convert everything to joules first.

Interpret the result. The “fastest” photoelectrons are those that suffer no energy loss due to collisions inside the metal. Their kinetic energy is exactly $K_{\text{max}}$ . So the value we just calculated is the answer.

✓Final answer
$\boxed{2.0\ \text{eV}}$
CBSE 2026Set 55/3/11 markMCQ
$Q. Assertion (A) : Photoelectric current depends upon the intensity of the incident radiation. Reason (R) : Stopping potential is independent of the intensity of the incident radiation. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Both Assertion (A) and Reason (R) are false.$
›Reveal solutionSolution
The photoelectric current depends on intensity (more photons → more electrons), while stopping potential depends only on frequency (photon energy minus work function). Both statements are true, but the reason does not explain the assertion — they are independent facts about the photoelectric effect. The correct option is (B).

The photoelectric effect is one of those rare topics where a single experiment cleanly separates two ideas that students often mix up: how many electrons get ejected versus how energetic each electron is. The assertion and reason here test exactly that distinction.

The core physics: When light hits a metal surface, each photon can knock out one electron — but only if the photon’s energy ( $hf$ ) exceeds the metal’s work function ( $\phi$ ). The number of electrons ejected (and hence the current) depends on how many photons arrive per second — that’s intensity. But the maximum kinetic energy of each ejected electron depends only on the photon’s frequency, not on how many photons there are.

Let’s walk through each statement carefully.

Assertion (A): Photoelectric current depends on the intensity of incident radiation.

Current is charge per second. Each ejected electron carries charge $e$ . So if more electrons leave the metal per second, the current increases. What controls the number of electrons per second? The number of photons striking the metal per second — that is, the intensity (for a fixed frequency).

Tip

Think of it like rain: intensity is how hard it’s raining (number of raindrops per second). More raindrops → more splashes (electrons). But each splash’s height depends on the size of the raindrop (frequency), not on how many are falling.

So (A) is true.

Reason (R): Stopping potential is independent of the intensity of incident radiation.

Stopping potential $V_0$ is the voltage needed to stop the most energetic photoelectrons. Einstein’s photoelectric equation gives:

$eV_0 = hf - \phi$
Here $f$ is the frequency of light, $\phi$ is the work function. Notice: intensity does not appear anywhere in this equation. Whether you shine a dim light or a bright light of the same colour, the stopping potential stays the same.

Watch out

A common mistake: students think brighter light gives more energetic electrons. It doesn’t — it gives more electrons, each with the same maximum energy. Intensity changes current, not stopping potential.

So (R) is also true.

Now the critical question: Does (R) correctly explain (A)? The assertion says current depends on intensity. The reason says stopping potential does not depend on intensity. These are two separate, independent facts about the photoelectric effect. The reason does not cause the assertion, nor does it explain why current depends on intensity.

Current depends on intensity because more photons → more photoelectrons.

Stopping potential is independent of intensity because photon energy depends only on frequency. They are both true, but the reason is not the explanation for the assertion.

Important

In assertion-reason questions, always check the logical link: does the reason directly account for why the assertion is true? Here, the two facts are parallel consequences of the photon model, not cause and effect.

✓Final answer
$The correct option is (B) — both statements are true, but the reason does not explain the assertion.$
CBSE 2026Set DS1 mark
$Q. The work function of a metal is 3.3 eV. Calculate the minimum frequency of photon that will emit the photoelectron.$
›Reveal solutionSolution

The minimum (threshold) frequency is $\nu_0 = W/h \approx 8.0\times10^{14}$ Hz.

Concept. Photoelectrons are emitted only when the incident photon's energy at least equals the work function $W$ of the metal. The minimum (threshold) frequency corresponds to a photon whose whole energy is used up just to free the electron, with no kinetic energy left:

$h u_0 = W \;\Rightarrow\; u_0 = \frac{W}{h}.$

Calculation. $W = 3.3\text{ eV} = 3.3\times1.6\times10^{-19} = 5.28\times10^{-19}$ J.

$u_0 = \frac{5.28\times10^{-19}}{6.6\times10^{-34}} \approx 8.0\times10^{14}\ \text{Hz}.$

✓Final answer
$\nu_0 \approx 8.0\times10^{14}$
CBSE 2026Set ANNUAL1 markMCQ
$Q. When light falls on a metal surface, the maximum kinetic energy of the emitted electrons depends upon$
(a) the time for which light falls on the metal
(b) the frequency of the incident light
(c) the intensity of the incident light
(d) the velocity of the incident light
›Reveal solutionSolution

Maximum kinetic energy of photoelectrons depends only on the frequency of incident light (and the metal's work function), never on intensity or time.

By Einstein's photoelectric equation,

$KE_{max} = h\nu - \phi_0$

where $h$ is Planck's constant, $\nu$ is the frequency of the incident light, and $\phi_0$ is the work function of the metal (a fixed property of the metal surface). Increasing the intensity of light (at fixed frequency) increases the number of photoelectrons emitted per second (photocurrent), not their maximum kinetic energy. Exposure time and the (irrelevant) velocity of light play no role in this energy balance — only the photon energy $h\nu$ matters.

✓Final answer
$(b) the frequency of the incident light.$
CBSE 2026Set ANNUAL1 markMCQ
$Q. The work function of four metals P, Q, R and S are 2.3 eV, 3.2 eV, 4.25 eV and 5.15 eV respectively. Among these the metal having lowest threshold frequency will be$
(a) R
(b) S
(c) Q
(d) P
›Reveal solutionSolution

Threshold frequency is directly proportional to work function (nu0 = W/h), so the metal with the smallest work function has the lowest threshold frequency.

Einstein's photoelectric equation gives the threshold frequency as nu0 = W/h, where W is the work function and h is Planck's constant (a fixed constant). Since nu0 scales directly with W, the metal with the smallest work function has the smallest (lowest) threshold frequency. Among P (2.3 eV), Q (3.2 eV), R (4.25 eV), S (5.15 eV), P has the lowest work function, so P has the lowest threshold frequency.

✓Final answer
$(d) P.$
CBSE 2026Set ANNUAL1 mark
$Q. If the workfunction of a metal is 4.5 eV and the kinetic energy of the photoelectron emitted from its surface is 1.5 eV, then calculate the energy of a photon of the incident light.$
›Reveal solutionSolution

Einstein's photoelectric equation says the photon's energy is used partly to free the electron (work function) and partly becomes its kinetic energy; adding the two gives the photon energy.

Einstein's photoelectric equation: E_photon = W (work function) + KE_max (maximum kinetic energy of photoelectron).

Given W = 4.5 eV, KE_max = 1.5 eV:

E_photon = 4.5 + 1.5 = 6.0 eV

✓Final answer
$Energy of incident photon = 6.0 eV.$
CBSE 2026Set ANNUAL1 markMCQ
$Q. Photoelectric effect supports$
(a) wave nature of light
(b) particle nature of light
(c) dual nature of light
(d) polarization nature of light
›Reveal solutionSolution

Classical wave theory cannot explain the instantaneous emission or the frequency threshold seen in the photoelectric effect; only Einstein's photon (particle) picture of light does.

In the photoelectric effect, light falling on a metal surface ejects electrons. Key observations - that emission starts INSTANTLY (no time lag) even at very low intensity, that there is a definite THRESHOLD FREQUENCY below which no electrons are emitted no matter how intense the light, and that the maximum kinetic energy of emitted electrons depends on FREQUENCY (not intensity) - cannot be explained by the classical wave theory of light. Einstein explained these features by treating light as made of discrete quanta (photons) of energy E = h*nu, each photon interacting with a single electron (particle-like behaviour). This is why the photoelectric effect is the classic evidence for the particle nature of light, distinct from phenomena like interference/diffraction which show its wave nature.

✓Final answer
$(b) particle nature of light.$
CBSE 2026Set ANNUAL1 mark
$Q. Define work function of metal.$
›Reveal solutionSolution

Work function is the minimum energy needed to pull the least tightly bound electron out of a metal surface.

Electrons inside a metal are held back from escaping by attractive forces from the positive ions in the lattice. The work function phi_0 is defined as the minimum energy that must be supplied to an electron (typically the most energetic, least tightly bound one, at the Fermi level) so that it can just escape from the metal surface, with no kinetic energy left over. It is usually expressed in electron-volts (eV) and is related to the threshold frequency nu_0 for the photoelectric effect by phi_0 = h*nu_0, where h is Planck's constant.

✓Final answer
$The minimum energy needed to just eject an electron from a metal's surface (with zero leftover kinetic energy): phi_0 = h*nu_0.$
CBSE 2026Set ANNUAL1 markMCQ
$Q. Stopping potential is minimum for:$
(a) Yellow
(b) Blue
(c) Violet
(d) Red
›Reveal solutionSolution

Stopping potential increases with the frequency of incident light; red light has the lowest frequency among the options, so it gives the minimum stopping potential.

By Einstein's photoelectric equation, $eV_0 = h\nu - \phi_0$ , so stopping potential $V_0$ increases linearly with frequency $\nu$ . Among yellow, blue, violet and red light, red has the longest wavelength and hence the lowest frequency, giving it the smallest (minimum) stopping potential.

✓Final answer
$Stopping potential is minimum for red light (option d).$
CBSE 2026Set ANNUAL1 markMCQ
$Q. The work function of caesium metal is 2.14 eV. When light of frequency 6 \times 10^14 Hz is incident on the metal surface, photoemission of electrons occurs. What is the Stopping potential?$
(a) 34 V
(b) 3.4 V
(c) 340 V
(d) 0.34 V
›Reveal solutionSolution

Using Einstein's photoelectric equation, the stopping potential comes out to about 0.34 V.

Step 1 — photon energy:

$E = h\nu = (6.626\times10^{-34})(6\times10^{14}) = 3.976\times10^{-19}\ \text{J}$

Converting to eV: $\dfrac{3.976\times10^{-19}}{1.602\times10^{-19}} \approx 2.48\ \text{eV}$

Step 2 — Einstein's photoelectric equation:

$K_{max} = h\nu - \phi_0 = 2.48 - 2.14 = 0.34\ \text{eV}$

Step 3 — stopping potential: Since $K_{max} = eV_0$ , and $K_{max}$ is expressed in eV, the numeric value of $V_0$ in volts equals the numeric value of $K_{max}$ in eV:

$V_0 \approx 0.34\ \text{V}$

✓Final answer
$(d) Stopping potential \approx 0.34 V.$

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Q.What is photoelectric effect?

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