jQuery Effect fadeTo() Method

Gradually change the opacity of all <p> elements:

$("button").click(function(){
$("p").fadeTo(1000, 0.4);
});

Definition and Usage

The fadeTo() method gradually changes the opacity, for selected elements, to a specified opacity (fading effect).

$(selector).fadeTo(speed,opacity,easing,callback)

Parameter	Description
speed	Required. Specifies the speed of the fading effect. Possible values: milliseconds "slow" "fast"
opacity	Required. Specifies the opacity to fade to. Must be a number between 0.00 and 1.00
easing	Optional. Specifies the speed of the element in different points of the animation. Default value is "swing" Possible values: "swing" - moves slower at the beginning/end, but faster in the middle "linear" - moves in a constant speed Tip: More easing functions are available in external plugins
callback	Optional. A function to be executed after the fadeTo() method is completed To learn more about callback, visit our jQuery Callback chapter

fadeTo() - Using the callback parameter
How to use the callback parameter when fading in and out an element.