The opposite of Dense, Light is a liquid that tries to move up. Light attempts to push particles below it, causing it to climb and giving the illusion of lightness. Light is a default component of Gas. Just like Dense, Light has the same physical density and weight as the other materials.
Light can be used to simulate convection, make pipes that pump water, or placed on top of Gas which will cause chaotic mixing because the heavier liquid is trying to climb on top of the lighter gas. Light can be used to make airships or clouds or flames that flick and move realistically. Light + Dense can be combined to make a special material.