From the physical perspective that the light is some kind of material consists of some extreme small particles,and will travel in the straight line in the relatively same condition,what will be the exact description for this condition is complex for me.So if the condition changes the light will also change it route suddenly that is why when we see the fish somewhere in the river,but we can not catch it at the same point.
And another problem,eye can not emit light(is it true?),it is the sun that emit the light which travels in the straight line in the air then changes its direction in the water so get to the obstacle(fish) thus changes its direction almost backward,then get into the human’s eyes.Now the eyes are only be viewed as a receiver.
The earth is surrounded by the atmosphere,so when sun light get to the earth ground,it really changes direction maybe for several times or more.
The light is made up of particles,why changes it direction in different conditions,atmosphere is also made up of various particles many times larger than the light one.I think for sometimes part of the atmosphere is relatively stable that means various particles are distributed uniformly everywhere in this part.So this part will have the best quality as the one which are made up of the same particles.Then view this condition as the same condition then the light will travel in the straight line.
Another problem when the light can travel through the air the water but why it change its direction when getting on the fish then backwards to eyes?I think maybe the particles which consist of the air and water are distributed in such a way that the light particles can get through.Comparing the same volume of air and fish,fish is much heavier than the air so the fish is made up by more more particles means these particles attach to each other more closely under the attractive force.As these particles give no space for the light particles to pass then light have to go in other direction.
