What is the missing word?
Substances that stop giving off light once the source of light illuminating them is removed, are called_____ substances.

Explanation

To solve the question, we need to identify the term that describes substances that stop emitting light once the source of illumination is removed.

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks for a specific term that describes substances that emit light only while being illuminated by an external light source.

2. **Identifying the Characteristics**: We need to focus on the properties of these substances. They emit light only in the presence of an external light source and stop emitting light when that source is removed.

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