I think here is what you have searched for
$(document).ready(function(){
var opacityOfNight = 0;
$('.turnNightBtn').click(function(){
if (opacityOfNight == 1){
opacityOfNight = 0;
} else {
opacityOfNight += 0.05;
if(opacityOfNight > 1) {
opacityOfNight = 1;
}
}
$('.night_img').css({
opacity : opacityOfNight
});
});
});