The heights of objects or images are taken as negative when they are measured

Options

A. downwards, perpendicular to the principal axis.
B. along the direction of incident rays.
C. upwards, perpendicular to the principal axis.
D. None of these

Explanation

To solve the question regarding the heights of objects or images being taken as negative when measured, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Principal Axis**:
- The principal axis is an imaginary line that runs through the center of a lens or mirror. It serves as a reference point for measuring heights of objects and images.

2. **Defining Positive and Negative Directions**:
- In optics, the convention is that heights measured upwards (above the principal axis) are considered positive, while heights measured downwards (below the principal axis) are considered negative.

3. **Identifying the Measurement Direction**:
- When measuring the height of an object or image, if the measurement is taken downwards from the principal axis, it will be negative. Conversely, if the measurement is taken upwards, it will be positive.

4. **Conclusion**:
- Therefore, the heights of objects or images are taken as negative when they are measured downwards perpendicular to the principal axis.

### Final Answer:
The heights of objects or images are taken as negative when they are measured downwards perpendicular to the principal axis.

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