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To explain this , he divided the incident wavefront at the obstacle into what we now call " Fresnel zones " , such that the secondary waves from each zone were spread over half a cycle when they arrived at the observation point .
 Fresnel
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Fresnel