CBSE 2025 · Region 5 · Set 3 · Q30 · 4 marks

Einstein explained photoelectric effect on the basis of Planck's quantum theory, where light travels in the form of small bundles of energy called photons. The energy of each photon is $\displaystyle \mathrm{h} v$, where $\displaystyle v$ is the frequency of incident light and $\displaystyle \mathrm{h}$ is Planck's constant. The number of photons in a beam of light determines the intensity of the incident light. A photon incident on a metal surface transfers its total energy $\displaystyle \mathrm{h} \nu$ to a free electron in the metal. A part of this energy is used in ejecting the electron from the metal and is called its work function. The rest of the energy is carried by the ejected electron as its kinetic energy.

(i)

Which of the following graphs can be used to obtain the value of Planck's constant?

(A)

Photocurrent versus Intensity of incident light

(B)

Photocurrent versus Frequency of incident light

(C)

Cut-off potential versus Frequency of incident light

(D)

Cut-off potential versus Intensity of incident light

(ii)

(a)OR(b)

Red light, yellow light and blue light of the same intensity are incident on a metal surface successively. $\displaystyle \mathrm{K}_{\mathrm{R}}, \mathrm{K}_{\mathrm{Y}}$ and $\displaystyle \mathrm{K}_{\mathrm{B}}$ represent the maximum kinetic energy of photoelectrons respectively, then :

(A)

$\displaystyle \mathrm{K}_{\mathrm{R}}>\mathrm{K}_{\mathrm{Y}}>\mathrm{K}_{\mathrm{B}}$

(B)

$\displaystyle \mathrm{K}_{\mathrm{Y}}>\mathrm{K}_{\mathrm{B}}>\mathrm{K}_{\mathrm{R}}$

(C)

$\displaystyle \mathrm{K}_{\mathrm{B}}>\mathrm{K}_{\mathrm{Y}}>\mathrm{K}_{\mathrm{R}}$

(D)

$\displaystyle \mathrm{K}_{\mathrm{R}}>\mathrm{K}_{\mathrm{B}}>\mathrm{K}_{\mathrm{Y}}$

(iii)

When the frequency of the incident light is increased without changing its intensity, the saturation current :

(A)

increases linearly

(B)

decreases

(C)

increases non-linearly

(D)

remains the same

(iv)

Which of the following graphs shows the variation of photoelectric current I with the intensity of light?

(A)

(B)

(C)

(D)

(D)

DANCH

CBSE Class 12 Physics past-paper question from the 2025board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.