Why does a glass rod or colorless gem having refractive index close to that of water becomes invisible in water? I couldn't get what happen that they tend to camouflage? Why can't our eye differentiate one(gem) from the other(water)?
You see a glass object because it both reflects and refracts light. When light traveling through air encounters a glass surface at an angle, some of the light reflects. The rest of the light keeps going, but it bends or refracts as it moves from the air to the glass.
When light passes from air into glass, it slows down. It’s this change in speed that causes the light to reflect and refract as it moves from one clear material (air) to another (glass). Every material has an index of refraction that is linked to the speed of light in the material. The higher a material’s index of refraction, the slower light travels in that material.
The smaller the difference in speed between two clear materials, the less reflection will occur at the boundary and the less refraction will occur for the transmitted light. If a transparent object is surrounded by another material that has the same index of refraction, then the speed of light will not change as it enters the object. No reflection and no refraction will take place, and the object will be invisible.
P.S-People fall of through glass door because of this
When a wave (light) reaches a change of medium some of the wave will be reflected and some will be transmitted (and refracted).
The fraction of the light which is reflected depends on the speed of the light (refractive indices) in the two mediums.
The closer the refractive indices are to one another the smaller the fraction of the light which is reflected and so less reflected light enters the eye.
Light will also be entering the eye from other sources around the change in medium and will have a larger chance of “swamping” the light reflected due to the change of medium when the difference in refractive index is smaller making it more difficult to observe the change in medium (gem).
The amount of ambient light is important as can be illustrated by the difference in observing your refection off the glass when looking out through a window when it is day as compared when it is night.
Another factor which affects the visibility of the change in medium is the angle of observation with the reflection from a glancing angle being more likely to be observed than that when looking along a normal to the surface.