water waves

Question

when water waves pass a log in the water, a smooth area of water is created behind the log.  when the same water waves pass a blade of grass growing in the water there is no smooth area of water created. Why?

 

 

a.  the length of the water waves is short in comparison to the size of the blade of grass

b.  the length of the water waves is short in comparison to the size of the log

c.  a shadow area is created by the blade of grass but no shadow area is created by the log

d.  the log diffracts the water waves more than the blade of grass

 

thanks

Answer 1

  Answers a & b are opposites, which is often a hint in a multiple choice question that one of them is the correct answer - not always, but often.  In this case, c & d seem to describe the opposite (impossible) situation than what is described in the question - the smooth area of water behind the log would be a shadow; if the water was diffracting MORE as it passed around the log, it would have a smaller shadow, not a larger one.

 

So, there are 2 ways to decide whether the correct answer is a or b.

1.  (hard) Look up the relationship between the wavelength, width of opening and amount of diffraction.

2.  (easy) Visualize the situation.  Is the distance between the wave crests (wavelength) smaller than the size of the log or the blade of grass?