Uncategorized - NEET Physics Chapterwise MCQs & PYQs

NEET Uncategorized MCQs & PYQs

Practice NEET Uncategorized Questions

Question 71:

easy

27. A 5 watt source emits monochromatic light of wavelength $5000 \mathring{A}$ . When placed 0.5 m away, it liberates photoelectrons from a photosensitive metallic surface. When the source is moved to a distance of 1.0 m, the number of photoelectrons liberated will be reduced by a factor of (2007)

The intensity of light $I$ is inversely proportional to the square of the distance $r$ from a point source, so $I \propto \frac{1}{r^2}$ . When the distance is doubled from $0.5 m$ to $1.0 m$ , the intensity becomes $\frac{1}{4}$ of its initial value. Since the number of photoelectrons liberated is directly proportional to intensity, it will also be reduced by a factor of 4.

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Question 72:

easy

28. When photons of energy $h\nu$ fall on an aluminum plate (of work function $E_0$ ), photoelectrons of maximum kinetic energy K are ejected. If the frequency of the radiation is doubled, the maximum kinetic energy of the ejected photoelectrons will be: (2006)

From Einstein's photoelectric equation, the initial maximum kinetic energy is $K = h\nu - E_0$ . When the frequency is doubled, the new incident energy is $2h\nu$ . The new maximum kinetic energy is $K' = 2h\nu - E_0$ . This can be rewritten as $K' = h\nu + (h\nu - E_0) = h\nu + K$ .

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Question 73:

easy

45. Which of the following statement is correct? (1997)

The photoelectric current is directly proportional to the intensity of incident light, provided the frequency of incident light is greater than the threshold frequency.

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Question 74:

easy

51. The cathode of a photoelectric cell is changed such that the work function changes from $W_1$ to $W_2$ ($W_2 > W_1$). If the current before and after changes are $I_1$ and $I_2$, all other conditions remaining unchanged, then (assuming $h\nu > W_2$) (1992)

Saturation photoelectric current depends only on the intensity of the incident light (number of photons per second) and is independent of the work function of the cathode material, provided the incident frequency is above the threshold. Therefore, $I_1 = I_2$.

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Question 75:

easy

84. In a discharge tube at $0.02 mm$, there is formation of (1996)

At a very low pressure of about $0.02 mm$ of Hg in a discharge tube, the Crookes dark space expands to fill the entire tube.

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Question 76:

easy

20. Out of the following which one is not a possible energy for a photon to be emitted by hydrogen atom according to Bohr’s atomic model? (2011 Mains)

Energy levels of H-atom are $-13.6 eV$, $-3.4 eV$, $-1.51 eV$, $-0.85 eV$, etc. Possible photon energies are differences between these: $E_3-E_2 = 1.89 \approx 1.9 eV$, $E_4-E_3 = 0.66 \approx 0.65 eV$, $E_{\infty}-E_1 = 13.6 eV$. There is no transition corresponding to $11.1 eV$.

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Question 77:

easy

24. The ionisation energy of the electron in the hydrogen atom in its ground state is 13.6 eV. The atoms are excited to higher energy levels to emit radiations of 6 wavelengths. Maximum wavelength of emitted radiation corresponds to the transition between: (2009)

Number of spectral lines emitted is $\frac{n(n-1)}{2} = 6 \Rightarrow n = 4$. Maximum wavelength corresponds to the minimum energy difference. For transitions among levels up to $n=4$, the transition $4 \rightarrow 3$ has the minimum energy difference and thus the maximum wavelength.

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