The symbol y/xi , where i = 1, 2, ..., n, represents a partial derivative. It means to take the derivative with respect to xi only and treat all other variables as constants. Let's use the above expression to calculate the absolute uncertainty of the wavelength . = f(d, ) = d sin n , where n= constant (4.4) = | f d d| | f | = | sin n d| | d cos n |