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Given a scientific passage and an answer, generate a question for the given answer.

Passage: Materials that eject electrons when illuminated with light, such as potassium, are called photoemissive. Not all materials are photoemissive, nor are all light sources capable of initiating electron emission from a given substance. For example, in Figure above , we see that 700 nm light will not initiate electron ejection, while 550 nm light will. Answer: photoemissive
Output:
What are materials that eject electrons when illuminated with light called?