Why fish under water see a circle surrounded by darkness

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SUMMARY

The discussion centers on the optical phenomenon observed by fish underwater, specifically the cone of light they perceive when looking up towards the surface of a pond. The light forms a bright field surrounded by darkness due to the principles of refraction and total internal reflection, governed by Snell's Law. The critical angle for light transitioning from water (n_w=1.33) to air plays a crucial role in determining the boundaries of this cone. Understanding these optical principles clarifies why fish experience this unique visual effect.

PREREQUISITES
  • Understanding of Snell's Law and refraction principles
  • Knowledge of critical angle calculations in optics
  • Familiarity with total internal reflection concepts
  • Basic physics of light behavior in different media
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  • Study the concept of critical angles and their calculations
  • Explore total internal reflection and its implications in various mediums
  • Investigate the behavior of light in water versus air
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Students of physics, optical engineers, and anyone interested in the behavior of light in different mediums, particularly in aquatic environments.

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A fish looking straight up to the surface of a pond receives a cone of light filled with images. This bright field is surrounded by darkness. Explain what is happening and compute the cone angle.

No given data but ##n_w=1.33##

No give equations but I anticipate Snell's law and maybe the critical angle equations will come into play.My attempts were to explain this by the light rays being bent in a way that the higher ##\theta_i## completely miss the fish eye. However that would not create the dark halo effect the problem states. I have looked for an answer using total internal reflection but that requires the light to be passing from a dense medium to a less dense. Total external reflection also has the stipulation the that the frequency of the light must be much greater than the resonant frequency of the atoms, and this would cause no light transmitted correct? Please help me understand this question a bit better. Thank you !
 
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"I have looked for an answer using total internal reflection but that requires the light to be passing from a dense medium to a less dense"

So what would this look like to you if you were underwater? Assume that there is only darkness from the direction of the water and all the light is coming from above the water.
 
outside of that cone, there is reflection only of the "darkness" from below.
 
Oh wow thank you I guess I was over thinking this.
 

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