why is water reflective?



why is water reflective?

In: Physics

It’s only reflective if there’s darkness behind it. Same reason mirrors work, they’re just a piece of glass with dark material behind them.

Its surface is smooth. More specifically, it’s smooth on the scale of the wavelength of visible light. Obviously it can be rough at larger scales, with waves and ripples due to wind and other disturbances.

ELI5: if you’ve ever rubbed a balloon on your shirt and then held it close to your hair, you can see your hair get pulled towards the balloon without them even touching. This is because you took electrons from your shirt and put them on the balloon. When you hold the balloon near your hair, the electrons in the balloon pull on your hair because your hair also has electrons and nuclei. This is a good way to visualize how electric charges interact with each other. The “pull” is called an electric field. Light is made up of very tiny waves that pull up down up down as they travel, so when light waves hit a surface, they can feel the charges in the new material and decide to travel in one new direction. Since the light is traveling in the same direction to start, and since they all feel the new charge in the same way, they all decide to change direction at the same angle. The only time you can see reflections is when the surface is smooth because the light doesn’t scatter when it changes direction. Water is a liquid so its surface is smooth.

ELI10: Light travels as waves made of an electric field and magnetic field, and materials are made of atoms that have positive and negative charges (nucleus and electrons), so when light waves travel through air and hit the surface of a new material such as glass, the electric charges in the glass make the waves change direction. If the surface of the glass is smooth, the angle of incoming light matches the angle of reflected light, so you can see objects in your house. If the surface is rough, the reflected waves scatter in many directions so you can’t make out objects.
The surface of liquids are smooth. There might be ripples because of wind, but if you zoom in enough the atoms on the surface of the liquid look smooth. Water molecules have electrons and nuclei arranged differently than those in air, so the same thing happens as with glass and air.

Light travels through water at a much slower speed than through air. This means light bends when it travels from water into air, or from air into water. Because of this, even though water is transparent, you can still see it because of how it distorts light. The same thing is true of glass. If a material transmits light in the same way as air, then it would be completely invisible.

Anyway, because light bends when it moves between air and water, some light also bounces off the surface of the water if it strikes the water at a very narrow angle. This is why it is easy to see deep into water if you look straight down, but if you look towards the horizon, the water becomes more reflective. The further away you look, the narrower the angle, and the more reflective the water becomes. The same is true of glass windows. Look at them from the side, and its like a mirror. Stand straight in front of them, and you can see right through.

It also helps both reflection and transparency if the surface is very smooth. Because of surface tension, water will naturally settle into a very smooth surface, unless its disturbed by some outside force. Smooth surfaces tend to not “refract” or scatter light, which cases them to appear opaque.