QNEET-2024-Ph-54: NEET 2024 Physics - Class 12 NEET-2024-Ph-54 Solution: The property which is not of an electromagnetic wave travelling …

Core Concept & Governing Formulae - Electromagnetic (EM) waves in free space are solutions of Maxwell’s equations with no charges or currents. - Key properties: (i) they are transverse; (ii) electric‑field energy density $u_E=\frac{1}{2}\varepsilon_0E^2$ equals magnetic‑field energy density $u_B=\frac{1}{2\mu_0}B^2$; (iii) propagation speed $c=\frac{1}{\sqrt{\mu_0\varepsilon_0}}$; (iv) radiation is produced only by accelerating charges, not by charges moving uniformly. Step 1: Test the statement “They are transverse in nature.” From Maxwell’s equations in vacuum, $\nabla\cdot\mathbf{E}=0$ and $\nabla\cdot\mathbf{B}=0$, leading to $\mathbf{E}\perp\mathbf{k}$ and $\mathbf{B}\perp\mathbf{k}$, where $\mathbf{k}$ is the propagation vector. Hence EM waves are transverse – the statement is true. Step 2: Test the statement “The energy density in electric field is equal to energy density in magnetic field.” For a plane wave, $E=cB$ and $u_E=\frac{1}{2}\varepsilon_0E^2$, $u_B=\frac{1}{2\mu_0}B^2$. Substituting $E=cB$ and $c=1/\sqrt{\mu_0\varepsilon_0}$ gives $u_E=u_B$. Thus the statement is true. Step 3: Test the statement “They travel with a speed equal to $\frac{1}{\sqrt{\mu_0\varepsilon_0}}$.” The wave equation derived from Maxwell’s equations yields the phase velocity $v=\frac{1}{\sqrt{\mu_0\varepsilon_0}}$, which is the speed of light $c$ in vacuum. Hence the statement is true. Step 4: Test the statement “They originate from charges moving with uniform speed.” Radiation from charges requires a non‑zero acceleration ($\mathbf{a}\neq0$). A charge moving at constant velocity ($\mathbf{a}=0$) does not emit EM waves; it merely carries its static field along. Therefore this statement is false. Final Conclusion & Correct Option The property that is **not** true for an electromagnetic wave travelling in free space is that it “originates from charges moving with uniform speed.” **Correct option: D**.