Plot[
 Sin[x]/x
 , {x, -2 π, 2 π}
 , Filling -> {1 -> {Axis, {Transparent, Automatic}}}
 , AxesLabel -> {"Distance from Slit", "Amplitude"}
 , PlotLabel -> "Single Slit Difraction"
 ]

Plot[
 Sin[x]/x
 , {x, -2 π, 2 π}
 , Filling -> {1 -> {Axis, {Transparent, Automatic}}}
 , AxesLabel -> {"Distance from Slit", "Amplitude"}
 , PlotLabel -> "Single Slit Difraction"
 ]

OR

Plot[
 Sin[x]/x
 , {x, -2 π, 2 π}
 , Filling -> Axis
 , FillingStyle -> {Transparent, LightBlue}
 , AxesLabel -> {"Distance from Slit", "Amplitude"}
 , PlotLabel -> "Single Slit Difraction"
 ]

Documentation for Filling