The degree to which a substance diffracts light depends largely on its density. Warm water is less dense than cooler water; columns of hot water rise towards the surface, and cooler water sinks. The lines you see are the result of light being diffracted to differing degrees as it passes through cooler and hotter water. Warm water is less dense than cooler water; columns of hot water rise towards the surface, and cooler water sinks. The lines you see are the result of light being diffracted to differing degrees as it passes through cooler and hotter water.