Empty Array causing an error

I have a large query and part of it depends upon a returned Array.  If there is no data returned by the query, the Array is empty, obviously.  But My code so far has an area that is looking for the the second part of the Array Data:

<?php
$a = false;
  $b = false; 
  if($wave[0] <= $wave_max) $a = true; 
   if($wave[1] >= $wave_min) $b = true; 
   			
   	$wavelength = '';
   				
   	if($a && $b) $wavelength = true;

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So if the return is empty, I get an error on this line:  if($wave[1] >= $wave_min) $b = true; , the second part of the array.
The error message is:
A PHP Error was encountered
Severity: Notice
Message: Undefined offset: 1
And refers to that line