Question 81:
easySurface area of the nucleus (\(X^8\)) is \(A_0\), then surface area of nucleus (\(Y^{64}\) ) will be
Nuclear radius is given by \(R \propto A^{1/3}\), so the surface area is \(S \propto R^2 \propto A^{2/3}\). Thus, \(\frac{S_2}{S_1} = \left(\frac{64}{8}\right)^{2/3} = 8^{2/3} = 4\), meaning the surface area of the nucleus \(Y^{64}\) is \(4A_0\).
Question 82:
easyFor H-atom, Lyman spectral lines must lie in:
The Lyman series corresponds to transitions from outer orbits to the first orbit (\(n=1\)) of hydrogen. These emissions are of high energy and fall in the ultraviolet region.
Question 83:
easyIf the kinetic energy of a particle is increased to 16 times, the percentage decrease in de Broglie wavelength of particle is
Using \(\lambda = \frac{h}{\sqrt{2mK}}\), when kinetic energy \(K\) becomes \(16K\), the new wavelength becomes \(\lambda' = \frac{\lambda}{4}\). The percentage decrease is \(\frac{\lambda - \lambda/4}{\lambda} \times 100\% = 75\%\).
Question 84:
easyThe stopping potential in the photoelectric experiment is \( 1.6\text{ V} \). The maximum kinetic energy of photoelectrons emitted is
The maximum kinetic energy of photoelectrons is given by \( K_{\text{max}} = e V_0 \). Substituting the values: \( K_{\text{max}} = 1.6 \times 10^{-19}\text{ C} \times 1.6\text{ V} = 2.56 \times 10^{-19}\text{ J} \).
Question 85:
easyChoose the incorrect statement among the following.
Nuclides with the same atomic number \( Z \) but different neutron numbers \( N \) are called isotopes, not isotones. Thus, option C is incorrect.
Question 86:
easyAssertion (A): A particle at rest breaks into two particles of different masses. They fly off in different directions. Their de Broglie wavelengths will be different.
Reason (R): Their speed will be different.
When a particle at rest decays, conservation of momentum dictates that the two resulting particles must have equal and opposite momenta \( |p_1| = |p_2| \). Since the de Broglie wavelength is \( \lambda = h/p \), both particles must have the same wavelength, making the assertion that they are different false.
Question 87:
easyAssertion (A): Two photons having equal wavelengths have equal linear momentum.
Reason (R): When monochromatic light shows its photon character, each photon has a linear momentum \(p = \frac{h}{\lambda}\).
The linear momentum of a photon is given by \(p = h/\lambda\). If two photons have equal wavelengths \(\lambda\), then their linear momenta \(p\) must also be equal. The Reason (R) correctly states the formula and explains Assertion (A).
Question 88:
easyAssertion (A): If the accelerating potential of a X-Ray tube is increased then the characteristic wavelength decreases.
Reason (R): The cut-off wavelength for a X-Ray tube is given by \(\lambda_{\text{min}} = \frac{hc}{eV}\), where \(V\) is accelerating potential.
Characteristic wavelength depends on target material, not accelerating potential. However, if 'characteristic wavelength' in (A) refers to 'cut-off wavelength', then \(\lambda_{\text{min}} = hc/(eV)\) implies increasing \(V\) decreases \(\lambda_{\text{min}}\). Under this interpretation, (A) is true, and (R) is true and explains (A).
Question 89:
easyAssertion (A): A photon and an electron both have energy \(50\text{ eV}\). Both have different wavelengths.
Reason (R): Wavelength depends on energy and not on mass.
For a photon, wavelength is \(\lambda_p = hc/E\). For an electron, de Broglie wavelength is \(\lambda_e = h/\sqrt{2mE}\). Since their formulas are different and \(\lambda_e\) depends on mass \(m\), their wavelengths will be different for the same energy. So (A) is true. Reason (R) is false because an electron's de Broglie wavelength depends on its mass.
Question 90:
easyAssertion (A): In photoelectric effect, cathode or emitter plate is usually coated with barium oxide, barium sulphide or strontium oxide.
Reason (R): Coating prevents cathode from erosion.
Cathodes are coated with materials like barium oxide to lower their work function, enhancing photoemission efficiency. So (A) is true. Coatings can indeed prevent erosion, so (R) is also true. However, preventing erosion is not the primary reason for choosing these specific low work function materials, so (R) is not the correct explanation for (A).