## Core Concept
The head mirror used in ENT-OPD (Ear, Nose, and Throat Outpatient Department) is a crucial tool for examining patients. It provides a clear view of the patient's ear, nose, and throat by reflecting light onto these areas. The focal length of the head mirror is essential for determining its usability and effectiveness in clinical settings.

## Why the Correct Answer is Right
The correct focal length of the head mirror used in ENT-OPD is **20 cm**. This specific focal length allows for an optimal reflection of light onto the area being examined, providing a clear and magnified view. The head mirror works on the principle of **reflection**, where light from a source (usually a lamp) is reflected onto the patient's ear, nose, or throat. A focal length of 20 cm is considered ideal because it enables the examiner to have a good view of the area while keeping the head mirror at a comfortable distance from both the patient and the examiner.

## Why Each Wrong Option is Incorrect
- **Option A:** 10 cm - This focal length is too short, making it impractical for use in ENT examinations as it would require the mirror to be very close to the patient and might not provide a sufficiently broad view.
- **Option C:** 30 cm - Although this might seem reasonable, a focal length of 30 cm might be too long, potentially reducing the magnification and clarity of the view obtained.
- **Option D:** 40 cm - This is even longer and would likely provide a less magnified view, making it less suitable for detailed examinations.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the head mirror's focal length of **20 cm** is standard for ENT examinations. This standardization ensures that the mirror can be used effectively across different clinical settings for examining the ear, nose, and throat.

## Correct Answer Line
**Correct Answer: B. 20 cm.**