Why are all underwater lenses blue?

Most photos or movies taken underwater are blue, which is due to the selective absorption of seawater-some colors of light are absorbed, while others cannot spread. White light is a mixture of red, orange, yellow, green, blue, indigo and violet light with different frequencies. When it shines on an object, some colors of light waves are absorbed and some colors are reflected. What we usually call the color of an object is the color of the light reflected by the object (or itself). For example, a person wearing a red shirt mainly reflects red light, and most other colors are absorbed. In air, light waves of different colors travel together at the same speed, but not in water. Water absorbs some wavelengths of light, but not all of it. It absorbs red light better. Sunlight shot into the sea less than 20 feet from the air, and all the red light was absorbed. Then there are orange and yellow lights, which also travel less than 40 and 70 feet in seawater. After 75 feet, only blue light is left, and all other colors of light are absorbed, which is why all underwater photos taken with sea light are blue. After using special light sources, underwater photographers can also have a panoramic view of the colorful underwater world. When shooting, they put the light source as close as possible to the object to shorten the distance of light propagation under water, reduce selective absorption and expose the true color of the object.