46413
As the extent of the common tangent was limited by the extent of the initial wavefront , the repeated application of Huygens 's construction to a plane wavefront of limited extent ( in a uniform medium ) gave a straight , parallel beam .
 Huygens
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Huygens