Question

Identify the correct statements among the following.
A. All electromagnetic waves, irrespective of their frequencies, travel at the same speed through vacuum.
B. A capacitor is fully charged by a d.c. source. When it is fully charged, the displacement current is zero.
C. Ultraviolet radiation can be used for viewing objects through haze and fog.
D. A moving charge in a circular orbit cannot produce electromagnetic waves.
Choose the correct answer from the options given below:

• A. A, B and C only
• B. B and C only
• C. A and B only
• D. C and D only

Hint:

Infrared, not UV, penetrates haze. Accelerated charges radiate.

Answer 1

The Correct Option is C

Step 1: Plan:
Sort the statements into true and false using the physical law behind each one.

Step 2: Speed of waves:
Maxwell's equations give one vacuum speed for all wavelengths. Radio waves and gamma rays travel equally fast. So A is true.

Step 3: Charging capacitor:
Displacement current is $\varepsilon_0 \, d\Phi_E/dt$. At full charge the field is constant, so this is zero. So B is true.

Step 4: Scattering:
Scattering goes as $1/\lambda^4$ (Rayleigh). UV has a short wavelength so it scatters heavily in haze. That makes UV a poor choice for seeing through fog, so C is false.

Step 5: Radiation from acceleration:
Circular motion needs a centripetal acceleration, and any accelerated charge emits radiation. So D is false.

Step 6: Result:
The true statements are A and B, which is the third option.

Final Answer:
A and B only are correct. \[ \boxed{A \text{ and } B \text{ only}} \]