*Question T0C05:
Why do exposure limits vary with frequency?*  

*Correct Answer is  D: The human body absorbs more RF energy at some frequencies than at others*  

### AI Response: (may contain errors)  

The correct answer to why exposure limits vary with frequency is:

**D: The human body absorbs more RF energy at some frequencies than at others.**

### Explanation of the Correct Answer:
- The human body interacts with electromagnetic fields differently depending on the frequency of the RF energy. 
- At certain frequencies, the body can absorb more energy due to resonance effects, which can lead to increased heating and potential biological effects. 
- For example, frequencies in the range of 30 MHz to 300 MHz are particularly effective at being absorbed by the body, leading to stricter exposure limits in these ranges.

### Why the Other Choices are Incorrect:
- **A: Lower frequency RF fields have more energy than higher frequency fields.**
  - This statement is misleading. Energy is not solely determined by frequency; it also depends on the amplitude of the RF field. Lower frequencies do not inherently have more energy than higher frequencies.

- **B: Lower frequency RF fields do not penetrate the human body.**
  - This is not entirely accurate. While lower frequencies may penetrate less deeply than higher frequencies, they can still penetrate to some extent. The penetration depth varies with frequency, but it does not mean they do not penetrate at all.

- **C: Higher frequency RF fields are transient in nature.**
  - This statement is incorrect. The transient nature of RF fields is not a function of frequency. Both lower and higher frequency fields can be steady or transient depending on the source and modulation.

### Useful Formulas or Rules of Thumb:
- The specific absorption rate (SAR) is a measure of the rate at which energy is absorbed by the body when exposed to an RF field. It is often used to evaluate exposure limits:
  
  $$SAR = \frac{P_{abs}}{m}$$

  where:
  - $P_{abs}$ = power absorbed in watts
  - $m$ = mass of the absorbing tissue in kilograms

- The frequency-dependent absorption can be summarized by noting that the body’s absorption characteristics change, leading to different exposure limits across the RF spectrum. 

Understanding these concepts will help you grasp why exposure limits are frequency-dependent and prepare you for related questions on the exam.