Key Fact
Photon properties
Energy E = hν = hc/λ. Momentum p = h/λ = E/c. Mass = 0 (rest mass). Travels at c. Number of photons = I·A/(hν).
-- NCERT Class 12 Physics, Ch. 11, p. 285Particle Nature Light
8 MCQs1 revision card9-step worked example
Source: NCERT Dual Nature of Matter and RadiationPYQ coverage: NEET 2024Official key: NTA-verifiedLast updated: 25 Sep 2026
Particle Nature Light, explained for NEET
Turn up the brightness of a red lamp. Many aspirants then say each photon carries more energy. It does not. Brightness is photon count; colour is photon energy. Getting that split wrong costs marks on questions that never mention a metal or a stopping potential.
In the photon picture, a beam of monochromatic radiation of frequency ν is a stream of quanta, each carrying energy E = hν = hc/λ and momentum p = h/λ = E/c. NCERT Class 12 Physics, Chapter 11 (Dual Nature of Radiation and Matter), page 281 sets out the properties that follow. A photon's energy depends only on frequency, never on how intense the beam is. Intensity fixes how many photons cross unit area per second, so a brighter beam of the same colour is more photons of identical energy.
Three further properties on that page are examinable on their own. Photons are electrically neutral, so a light beam passes undeflected through electric and magnetic fields. Photons have zero rest mass and always travel at c in vacuum, which is why p = mv cannot be used on them — p = E/c is the relation that holds. And in a photon–particle collision, total energy and total momentum are conserved, but the number of photons is not: a photon can be absorbed outright or a new one created.
Two neighbouring topics sit next to this one and are handled separately. The photoelectric effect asks what happens when these photons strike a metal; matter waves ask whether particles show the reverse behaviour. This lesson is only the photon itself.
NEET asks this as short numerical work — photon energy from wavelength, photon momentum from energy, photon emission rate from source power — plus one-line property recall.
Watch-out: E = hc/λ and p = h/λ differ by a factor of c. Writing one where the other belongs gives an answer off by 10⁸ with correct-looking algebra, and such a value is usually sitting in the options.
Can you answer these Particle Nature Light MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
A narrow beam of light crosses a region carrying strong electric and magnetic fields and emerges undeflected. In the photon picture, this happens because a photon:
Show answer and why every option is right or wrong
Answer: D. Photons carry no electric charge, so neither field exerts a force on them. This property is listed among the photon properties in NCERT Class 12 Physics, Chapter 11 (Dual Nature of Radiation and Matter), page 281.
Why A is wrong: A is wrong because deflection in these fields depends on charge, not on how large the momentum is; an uncharged object of any momentum is undeflected.
Why B is wrong: B is wrong because transit time only affects how much a charged particle deflects, never whether it deflects at all — a charged photon would still bend measurably.
Why C is wrong: C is wrong because the photon's charge is exactly zero, not merely small; NCERT states photons are electrically neutral.
MCQ 2Easy RecallPractice
For a monochromatic beam of fixed frequency, increasing the intensity of the beam increases:
Show answer and why every option is right or wrong
Answer: A. Intensity is fixed by photon flux; the energy per photon depends only on frequency. NCERT Class 12 Physics, Chapter 11 (Dual Nature of Radiation and Matter), page 281 states that intensity determines the number of photons crossing unit area per second.
Why B is wrong: B is wrong because photon energy is hν, set entirely by frequency — the same colour made brighter delivers more photons of identical energy, not larger quanta.
Why C is wrong: C is wrong because every photon travels at c in vacuum irrespective of frequency or intensity.
Why D is wrong: D is wrong because the beam is stated to be monochromatic; brightening a source does not shift its frequency.
MCQ 3Easy RecallPractice
In a collision between a photon and an electron, which of the following is NOT necessarily conserved?
Show answer and why every option is right or wrong
Answer: C. Total energy, total momentum and total charge are conserved, but a photon may be absorbed or a new one created, so photon number is not a conserved quantity. This is stated in NCERT Class 12 Physics, Chapter 11 (Dual Nature of Radiation and Matter), page 281.
Why A is wrong: A is wrong because total energy is conserved in a photon–particle collision; NCERT lists it explicitly as conserved.
Why B is wrong: B is wrong because total momentum is conserved — a photon carries momentum h/λ and contributes to the balance.
Why D is wrong: D is wrong because the photon is neutral and the electron's charge is unchanged, so total charge is trivially conserved.
MCQ 4Direct ApplicationPractice
Light of wavelength 6.63 × 10⁻⁷ m travels in vacuum. Taking h = 6.63 × 10⁻³⁴ J·s and c = 3.00 × 10⁸ m/s, the energy of one photon of this light is:
Show answer and why every option is right or wrong
Answer: B. E = hc/λ = (6.63 × 10⁻³⁴ × 3.00 × 10⁸)/(6.63 × 10⁻⁷) = 3.00 × 10⁻¹⁹ J. The relation E = hν = hc/λ is given in NCERT Class 12 Physics, Chapter 11 (Dual Nature of Radiation and Matter), page 281.
Why A is wrong: A is wrong because it comes from multiplying by the wavelength (hcλ) instead of dividing by it; the energy must rise as wavelength falls, so λ belongs in the denominator.
Why C is wrong: C is wrong because 6.63 × 10⁻³⁴ / 6.63 × 10⁻⁷ = 1.00 × 10⁻²⁷ is the photon's momentum h/λ in kg·m/s, not its energy — the factor c has been dropped.
Why D is wrong: D is wrong because c/λ = 4.52 × 10¹⁴ is the frequency in Hz; the factor h is missing and the unit does not even match.
MCQ 5Direct ApplicationPractice
A photon travelling in vacuum has energy 3.3 × 10⁻¹⁹ J. Taking c = 3.0 × 10⁸ m/s and h = 6.6 × 10⁻³⁴ J·s, its momentum is:
Show answer and why every option is right or wrong
Answer: A. For a photon, p = E/c = (3.3 × 10⁻¹⁹)/(3.0 × 10⁸) = 1.1 × 10⁻²⁷ kg·m/s. The relation p = hν/c = h/λ = E/c appears in NCERT Class 12 Physics, Chapter 11 (Dual Nature of Radiation and Matter), page 281.
Why B is wrong: B is wrong because it multiplies energy by c instead of dividing; dimensionally E × c is not a momentum at all.
Why C is wrong: C is wrong because E/h = 5.0 × 10¹⁴ is the photon's frequency in Hz, carrying the wrong quantity and the wrong unit.
Why D is wrong: D is wrong because E/c² = 3.7 × 10⁻³⁶ has the dimensions of mass, not momentum — the photon's rest mass is zero and this quantity is not its momentum.
MCQ 6Direct ApplicationPractice
A monochromatic source radiates 1.00 W of power at wavelength 4.42 × 10⁻⁷ m. Taking h = 6.63 × 10⁻³⁴ J·s and c = 3.00 × 10⁸ m/s, the number of photons the source emits per second is:
Show answer and why every option is right or wrong
Answer: D. Each photon carries E = hc/λ = 4.50 × 10⁻¹⁹ J, so the rate is n = P/E = 1.00/(4.50 × 10⁻¹⁹) = 2.22 × 10¹⁸ per second. Power as photon flux times photon energy follows from the photon description in NCERT Class 12 Physics, Chapter 11 (Dual Nature of Radiation and Matter), page 281.
Why A is wrong: A is wrong because 4.50 × 10⁻¹⁹ is the energy of a single photon in joules, quoted as if it were the emission rate; the power still has to be divided by it.
Why B is wrong: B is wrong because c/λ = 6.79 × 10¹⁴ Hz is the frequency of the light, which is not the number of photons the source emits.
Why C is wrong: C is wrong because it divides the power by h/λ (the photon momentum, 1.50 × 10⁻²⁷ kg·m/s) instead of by the photon energy — the factor c has been dropped from E = hc/λ.
MCQ 7CalculationPractice
Photon X has three times the wavelength of photon Y, both travelling in vacuum. The ratio of their energies (X : Y) and the ratio of their momenta (X : Y) are respectively:
Show answer and why every option is right or wrong
Answer: B. Both E = hc/λ and p = h/λ vary as 1/λ, so tripling the wavelength divides each by 3, giving 1 : 3 for energy and 1 : 3 for momentum. Both relations are stated in NCERT Class 12 Physics, Chapter 11 (Dual Nature of Radiation and Matter), page 281.
Why A is wrong: A is wrong because it treats energy and momentum as proportional to λ; both are inversely proportional to it, so the longer-wavelength photon is the weaker one.
Why C is wrong: C is wrong because it inverts only the momentum ratio, as though p were proportional to λ — but p = h/λ carries the same 1/λ dependence as the energy.
Why D is wrong: D is wrong because it applies a 1/λ² dependence to the energy; E = hc/λ is first power in 1/λ, not second.
MCQ 8Concept TrapPractice
A student reasons that since a photon carries momentum, it must possess some rest mass, and computes that mass from p = mv with v = c. The reasoning fails because:
Show answer and why every option is right or wrong
Answer: C. A photon has zero rest mass yet carries momentum p = hν/c = h/λ; the mass–velocity product is simply the wrong expression to use on it. NCERT Class 12 Physics, Chapter 11 (Dual Nature of Radiation and Matter), page 281 gives the photon's momentum and states that its rest mass is zero.
Why A is wrong: A is wrong because a photon does carry momentum h/λ — that is precisely why radiation exerts pressure on a surface it strikes.
Why B is wrong: B is wrong because momentum is perfectly well defined for a photon; only the non-relativistic formula for it breaks down.
Why D is wrong: D is wrong because the photon's rest mass is exactly zero as a matter of principle, not an unresolved measurement; a massive photon could not travel at c.
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Particle Nature Light: quick recall before you leave
How do you solve a Particle Nature Light question? A worked example
- 1
Given
A monochromatic laser of power P = 3.00 × 10⁻³ W emits light of wavelength λ = 5.00 × 10⁻⁷ m in vacuum. Take h = 6.63 × 10⁻³⁴ J·s and c = 3.00 × 10⁸ m/s.
- 2
Required
(a) The momentum of one photon of this light. (b) The number of photons emitted per second. (c) The total momentum carried away by the beam in one second.
- 3
Concept
In the photon picture the beam is a stream of identical quanta. Each carries energy hc/λ and momentum h/λ. Source power is the energy delivered per second, so dividing power by the energy of a single photon gives the emission rate. Multiplying that rate by the momentum of one photon gives the momentum the beam carries per second.
- 4
Formula
p = h/λ; E = hc/λ; n = P/E; p_total (per second) = n·p.
- 5
Substitution
p = (6.63 × 10⁻³⁴)/(5.00 × 10⁻⁷)
E = (6.63 × 10⁻³⁴ × 3.00 × 10⁸)/(5.00 × 10⁻⁷)
n = (3.00 × 10⁻³)/E - 6
Calculation
p = 1.326 × 10⁻²⁷ kg·m/s
E = (1.989 × 10⁻²⁵)/(5.00 × 10⁻⁷) = 3.978 × 10⁻¹⁹ J
n = (3.00 × 10⁻³)/(3.978 × 10⁻¹⁹) = 7.541 × 10¹⁵ per second
p_total = (7.541 × 10¹⁵)(1.326 × 10⁻²⁷) = 1.000 × 10⁻¹¹ kg·m/s
The one-second interval is an exact defined interval and contributes no significant figures; the three data values (P, λ, and the constants h and c as quoted) each carry three significant figures, which is what limits the answers to three. - 7
Final answer
(a) p = 1.33 × 10⁻²⁷ kg·m/s. (b) n = 7.54 × 10¹⁵ photons per second. (c) p_total = 1.00 × 10⁻¹¹ kg·m/s per second. Cross-check: n·p must equal P/c = (3.00 × 10⁻³)/(3.00 × 10⁸) = 1.00 × 10⁻¹¹, which it does — the photon count cancels out entirely.
- 8
Common trap
E = hc/λ and p = h/λ sit one factor of c apart, and a beam problem asks for both in the same breath. Using h/λ as the photon energy in step (b) returns 2.26 × 10²⁴ photons per second — an answer off by 10⁸ that is reached with flawless arithmetic, and such values are routinely placed in the options. Fix the two expressions by their units before substituting: joules need the c, kg·m/s does not.
- 9
Similar NEET-style question
A 10.0 mW laser of wavelength 6.63 × 10⁻⁷ m falls normally on a perfectly absorbing surface. Find the number of photons striking the surface per second and the force it experiences. (Answers: 3.33 × 10¹⁶ per second; 3.33 × 10⁻¹¹ N.)
What to remember before solving Particle Nature Light questions
Which Particle Nature Light formulas do you need for NEET?
1 formula — click to collapse
Photon energy and momentum
Energy and momentum of a photon. h = 6.626e-34 J*s.
| Symbol | Quantity | SI Unit |
|---|---|---|
| E | energy | J |
| nu | frequency | Hz |
| lambda | wavelength | m |
| p | momentum | kg*m/s |
| h | Planck constant | J*s |
Valid when
- Vacuum (c=speed of light)
- Photon = EM quantum
More in Dual Nature of Matter and Radiation: 2 exam traps and mistakes · 2 formulas · 2 question patterns from its other lessons.
Particle Nature Light questions from past NEET papers
1 question from NEET 2024. Answers verified against NTA official keys. — click to collapse
NEET 2024Revised key
1A and B only
2A, B, C and D only
3A, C and D only
4A, B, D and E only
NTA Answer: Option 2(revised_final)
All 11 past-paper questions from Dual Nature of Matter and Radiation →
Sources
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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