Dual Nature of Radiation and Matter PYQs on the CBT Screen

Question 1 · NEET 2026 · Q9 · +4 / −1

Match List I with List II:

List I	List II
P) $E = h\nu$	I) de Broglie wavelength
Q) Diffraction and Interference	II) Particle nature of light
R) $\lambda = h/p$	III) Wave nature of light
S) Compton effect	IV) Energy of photon

Choose the correct answer from the options given below:

Answer and explanation

Answer: (4)

$E = h\nu$ gives the energy of a photon (IV); diffraction and interference demonstrate the wave nature of light (III); $\lambda = h/p$ is the de Broglie wavelength of a particle (I); and the Compton effect confirms the particle nature of light (II). Hence P-IV, Q-III, R-I, S-II, which is option (4).

Question 2 · NEET 2026 · Q42 · +4 / −1

For a metal of work function $6.6\ \text{eV}$ , which of the following wavelengths of incident radiation does not give rise to the photoelectric effect? (Take Planck's constant as $6.6 \times 10^{-34}\ \text{J s}$ )

Answer and explanation

Answer: (3)

Photoemission requires photon energy $\dfrac{hc}{\lambda} \geq W_0$ , i.e. wavelength $\lambda \leq \dfrac{hc}{W_0}$ . Here $\dfrac{hc}{W_0} = \dfrac{6.6\times10^{-34}\times3\times10^{8}}{6.6\times1.6\times10^{-19}} \approx 187.5\ \text{nm}$ . Any wavelength longer than this cannot eject electrons; $200\ \text{nm} > 187.5\ \text{nm}$ , so it produces no photoelectric effect.

Question 3 · Re-NEET 2026 · Q8 · +4 / −1

A beam of light falls on a metal surface such that photo-electrons are generated. If power of the light source starts to decrease linearly with time $t$ , then variation of the photocurrent $I$ and magnitude of the stopping potential $|V|$ with time is best represented by:

Answer and explanation

Answer: (1)

Photocurrent is set by the NUMBER of photons striking the surface per second, so it is proportional to the source power and falls linearly with $t$ . Stopping potential, by contrast, depends only on the maximum kinetic energy of the emitted electrons, $eV_0 = h\nu - \phi_0$ , which is fixed by the FREQUENCY and the work function - neither of which changes here. So $|V|$ stays flat while $I$ decays. NCERT Class 12, Dual Nature of Radiation and Matter.

Why the other options are wrong:

- (2) treats stopping potential as intensity-dependent, the single most common photoelectric misconception
- (3) swaps the roles entirely, tying current to frequency and stopping potential to intensity
- (4) assumes neither quantity responds to the source, ignoring that current tracks photon flux

Question 4 · Re-NEET 2026 · Q18 · +4 / −1

A ray of light with wavelength $\lambda$ is incident on three different photoelectric cells namely 1, 2 and 3. The threshold wavelength of these photoelectric cells are $\lambda_1$ , $\lambda_2$ , and $\lambda_3$ , respectively and the magnitude of stopping potentials of these cells are $V_1$ , $V_2$ and $V_3$ , respectively. The relation between $\lambda$ and threshold wavelengths are $\lambda_1 < \lambda$ , $\lambda_2 > \lambda$ and $\lambda_3 \gg \lambda$ . The correct option is:

Answer and explanation

Answer: (1)

Emission occurs only when the incident wavelength is SHORTER than the threshold wavelength. Cell 1 has $\lambda_1 < \lambda$ , so the photons are too weak and $V_1 = 0$ . For the other two, $eV_0 = hc\left(\frac{1}{\lambda} - \frac{1}{\lambda_{th}}\right)$ ; a LARGER threshold wavelength means a smaller work function and hence a larger stopping potential. Since $\lambda_3 \gg \lambda_2$ , the term $1/\lambda_3$ is nearly zero and $V_3 > V_2$ . NCERT Class 12, Dual Nature of Radiation and Matter (Einstein's photoelectric equation).

Question 5 · Re-NEET 2026 · Q19 · +4 / −1

A photon and an electron, each of 20 eV energy, move in free space. The ratio of linear momentum of electron $p_e$ to that of photon $p_{Ph}$ , $\frac{p_e}{p_{Ph}}$ is: [Take speed of light = $3 \times 10^8\ \text{ms}^{-1}$ , charge of electron = $-1.6 \times 10^{-19}$ C and mass of electron = $9 \times 10^{-31}$ kg]

Answer and explanation

Answer: (3)

A photon's momentum is $p = \frac{E}{c} = \frac{20 \times 1.6 \times 10^{-19}}{3 \times 10^8} \approx 1.07 \times 10^{-26}\ \text{kg m s}^{-1}$ . A non-relativistic electron of the same kinetic energy has $p = \sqrt{2mE} = \sqrt{2(9 \times 10^{-31})(3.2 \times 10^{-18})} = 2.4 \times 10^{-24}\ \text{kg m s}^{-1}$ . The ratio is therefore about 225 - a massive particle carries far more momentum than a photon of the same energy, because $p \propto \sqrt{E}$ for matter but $p \propto E$ for light. NCERT Class 12, Dual Nature of Radiation and Matter.

Question 6 · NEET 2020 · Q143 · +4 / −1

An electron is accelerated from rest through a potential difference of V volt. If the de Broglie wavelength of the electron is 1.227 × 10⁻² nm, the potential difference is

Answer and explanation

Answer: (3)

For an electron, $\lambda$ (nm) = 1.227/ $\sqrt{V}$ . Setting $\lambda = 1.227 \times 10^{-2}$ nm gives $\sqrt{V}$ = 100 and V = 10 $^{4}$ V. Chapter: Dual Nature of Radiation and Matter.

Question 7 · NEET 2020 · Q146 · +4 / −1

Light of frequency 1.5 times the threshold frequency is incident on a photosensitive material. What will be the photoelectric current if the frequency is halved and intensity is doubled?

Answer and explanation

Answer: (3)

The new frequency is 0.75ν $_{0}$ , below the threshold ν $_{0}$ . No electrons are emitted irrespective of the doubled intensity, so the photoelectric current is zero. Chapter: Dual Nature of Radiation and Matter.

Dual Nature of Radiation and Matter, on the exam screen.