CBSE 2025 · Region 2 · Set 1 · Q30 · 4 marks

When a photon of suitable frequency is incident on a metal surface, photoelectron is emitted from it. If the frequency is below a threshold frequency $\displaystyle \left(v_{\mathrm{o}}\right)$ for the surface, no photoelectron is emitted. For a photon of frequency $\displaystyle v\left(v>v_{\mathrm{o}}\right)$, the kinetic energy of the emitted photoelectrons is $\displaystyle \mathrm{h}\left(v-v_{\mathrm{o}}\right)$. The photocurrent can be stopped by applying a potential $\displaystyle \mathrm{V}_{\mathrm{o}}$ called 'stopping potential' on the anode. Thus maximum kinetic energy of photoelectrons $\displaystyle \mathrm{K}_{\mathrm{m}}=\mathrm{e} \mathrm{V}_{\mathrm{o}}=\mathrm{h}\left(v-v_{\mathrm{o}}\right)$. The experimental graph between $\displaystyle \mathrm{V}_{\mathrm{o}}$ and $\displaystyle v$ for a metal is shown in figure. This is a straight line of slope $\displaystyle \mathrm{m} . \quad$

(i)

The straight line graphs obtained for two metals

(A)

coincide each other.

(B)

are parallel to each other.

(C)

are not parallel to each other and cross at a point on $\displaystyle v$-axis.

(D)

are not parallel to each other and do not cross at a point on $\displaystyle v$-axis.

(ii)

The value of Planck's constant for this metal is

(A)

$\displaystyle \frac{\mathrm{e}}{\mathrm{m}}$

(B)

$\displaystyle \frac{1}{\mathrm{me}}$

(C)

me

(D)

$\displaystyle \frac{\mathrm{m}}{\mathrm{e}}$

(iii)OR(iii)

The intercepts on $\displaystyle v$-axis and $\displaystyle \mathrm{V}_{\mathrm{o}}$-axis of the graph are respectively :

(A)

$\displaystyle v_{\mathrm{o}}, \frac{\mathrm{h} v_{\mathrm{o}}}{\mathrm{e}}$

(B)

$\displaystyle v_{\mathrm{o}}, \mathrm{~h} v_{\mathrm{o}}$

(C)

$\displaystyle \frac{\mathrm{h} v_{\mathrm{o}}}{\mathrm{e}}, v_{\mathrm{o}}$

(D)

$\displaystyle \mathrm{h} v_{\mathrm{o}}, v_{\mathrm{o}}$

(iv)

The momentum of a photon is $\displaystyle 5.0 \times 10^{-29} \mathrm{~kg} . \mathrm{m} / \mathrm{s}$. Ignoring relativistic effects (if any), the wavelength of the photon is

(A)

$\displaystyle 1.33 \mu \mathrm{~m}$

(B)

$\displaystyle 3.3 \mu \mathrm{~m}$

(C)

$\displaystyle 16.6 \mu \mathrm{~m}$

(D)

$\displaystyle 13.3 \mu \mathrm{~m}$

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.