E.m. wave of audible frequency cannot be directly propagated over a long distance because

Options

A. they have vary small energy content
B. the length of antenna required for transmission of these wave is too large
C. both ( a) and (b)
D. neither (a) nor (b)

Answer

To solve the question "E.m. wave of audible frequency cannot be directly propagated over a long distance because," we need to analyze the options provided and understand the underlying physics principles.

### Step-by-Step Solution:

1. **Understanding Audible Frequencies**:
- Audible frequencies range from 20 Hz to 20 kHz. These frequencies are relatively low compared to radio frequencies used in communication systems.

2. **Energy Content of Waves**:
- The energy of a wave is given by the equation \( E = h \nu \), where \( E \) is energy, \( h \) is Planck's constant, and \( \nu \) is the frequency of the wave.
- Since audible frequencies have a low frequency (\( \nu \)), the energy content of these waves is also low.

3. **Implications of Low Energy**:
- Low energy content means that the waves cannot travel long distances without losing their strength. This necessitates the use of modulation techniques to superimpose higher frequency waves, which carry more energy.

4. **Antenna Size Considerations**:
- The height of the antenna required for transmission is related to the wavelength (\( \lambda \)) of the wave. The relationship is given by \( \text{Height} \approx \frac{\lambda}{4} \).
- For audible frequencies, since the frequency is low, the wavelength is large (as \( \lambda = \frac{c}{\nu} \), where \( c \) is the speed of light). This results in a very large antenna size, which is impractical for use.

5. **Conclusion**:
- Both reasons provided in the options are valid. The low energy content and the large size of the antenna required make it impractical to propagate electromagnetic waves of audible frequency over long distances.

6. **Final Answer**:
- Therefore, the correct option is **C. Both A and B**.