Teacher:Generate an overlapping word between the given two sentences. When you find the overlapping words, they don't have to match exactly, e.g., "survival" and "survive" are valid overlapping words. Little words like "the" or "of" don't count! You must generate significant words which are not the stop words.
Teacher: Now, understand the problem? Solve this instance: Sentence1: a prism is used for refracting light. 
Sentence2: a prism is used for refracting wavelengths and photons.
Student:
prism