Electromagnetic Waves — NEET UG practice

1. Comparing the given field $E_x=E_0\sin(kz-2\pi\times10^6 t)$ with the standard form $E_0\sin(kz-\omega t)$, the wave travels in the +z direction (since the space and time terms have opposite signs when written as $kz - \omega t$), so option A is correct.

2. To find the speed of the wave in the medium, use the fact that speed reduces by a factor of $\sqrt{\varepsilon_r}$ compared to speed in vacuum: $v_m=\frac{c}{\sqrt{\varepsilon_r}}=\frac{3\times10^8}{\sqrt{9}}=\frac{3\times10^8}{3}$

3. Calculating: $v_m=10^8\text{ m/s}$ This matches option B, so B is correct.

4. From the given expression, $\omega=2\pi\times10^6$, so the frequency is: $f_0=\frac{\omega}{2\pi}=10^6\text{ Hz}$

5. Using the wave relation $\lambda=v/f$, the wavelength inside the medium is: $\lambda_m=\frac{v_m}{f_0}=\frac{10^8}{10^6}$

6. Calculating: $\lambda_m=100\text{ m}$ But option C claims the wavelength is 300 m, which does not match this correct value of 100 m, so option C is the incorrect statement.

7. For option D, in an electromagnetic wave the magnetic field has the same phase as the electric field and its amplitude equals $E_0/v$ (although the option should technically write $E_0$ instead of $B_0$ in the numerator, the relation as a form/structure is otherwise consistent with EM wave theory).

Hence, the incorrect choice is option C.

1. Microwaves are produced by special electronic devices called klystron or magnetron valves, so A matches IV.

2. Visible light is produced when electrons in atoms jump from a higher energy level to a lower one, emitting light in the process, so B matches I.

3. Gamma rays originate from the radioactive decay of atomic nuclei, so C matches II.

4. Infra-red rays are associated with the vibration of atoms and molecules (thermal motion), so D matches III.

5. Combining these gives A-IV, B-I, C-II, D-III.

Hence, the answer is option C.

1. Microwaves are used to heat and warm food (this is exactly how a microwave oven works), so P matches II.

2. UV rays have the ability to kill bacteria and are used to purify water, so Q matches I.

3. Gamma rays are highly energetic and penetrating, making them useful in treating cancer cells (radiotherapy), so R matches IV.

4. Radio waves have long wavelengths suited for AM and FM broadcasting, so S matches III.

5. Putting it all together gives P-II, Q-I, R-IV, S-III.

Hence, the answer is option D.