Particulas completo

HTML

<canvas id='canvas' width=300 height=300>

JavaScript

var canvas = document.getElementById('canvas');
var c = canvas.getContext('2d');
var particulas = [];

function azar(min, max) {
    return Math.random() * (max - min) + min;
}





function dibujar() {
    var p = {
        x: 150,
        y: 150,
        vx: azar(-10, 10),
        vy: azar(-10, 10),
        ancho: 5
    }
    
    particulas.push(p);
    
    c.fillStyle='rgba(255, 255, 255, 0.2)';
    c.fillRect(0, 0, 300, 300);
    
    particulas.forEach(function(p) {
        c.fillStyle = 'blue';
        p.x += p.vx;
        p.y += p.vy;
        p.vy += 0.2;
        p.ancho = p.ancho * 0.95;
        c.fillRect(p.x, p.y, p.ancho, p.ancho);
    });
}

setInterval(dibujar, 30);