Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

When a ray is traveling from air to water

  1. May 14, 2008 #1

    Air

    User Avatar

    I can't seem to get my head around refraction so on this thread I will ask many questions to understand the concept (bit by bit). These two questions to start of with. Could someone help me with them?

    • When a ray is traveling from air to water, does it refract away or toward the normal?
    • Is water more dense than air?
     
  2. jcsd
  3. May 14, 2008 #2
    1.) the ray will refract toward the normal. Light travels slower in water than in air, since water has a larger index of refraction (n = c/v). The part of the wavefront that reaches the air-water interface first will travel slower than the rest of the wavefront. This is what causes the light ray to bend toward the normal.

    2.) Water is more dense than air. This is part of the reason why water has a higher index of refraction than air.
     
  4. May 14, 2008 #3

    Air

    User Avatar

    Ah, I see. What about the same material but different form, e.g: Is hot air more dense than cold air?
     
  5. May 14, 2008 #4
    cold air is more dense than hot air, which is why your car engine runs more efficiently in colder weather...because the compression is higher.

    But yeah, index of refraction is a function of temperature. This explains why you see mirages (puddles of water) on the road on a hot sunny day. The air is hotter near the road and gets colder as you go up. This temperature gradient refracts sunlight to the point where it appears as if it is coming from the road, causing the appearance of a mirage.
     
    Last edited: May 14, 2008
  6. May 14, 2008 #5

    DavidSnider

    User Avatar
    Gold Member

    Hot air is less dense than cold air. Hot air refracts less, but the mixture of hot air and cold air is what creates the 'wavy' effect.
     
  7. May 14, 2008 #6

    Air

    User Avatar

    So, is it always that a ray is always refracted toward the normal in a more dense material and away from the normal in less dense material? There no exception to this principle?
     
  8. May 14, 2008 #7

    DavidSnider

    User Avatar
    Gold Member

    Interesting question. According to wikipedia:

    "In general, the refractive index of a glass increases with its density. However, there does not exist an overall linear relation between the refractive index and the density for all silicate and borosilicate glasses. A relatively high refractive index and low density can be obtained with glasses containing light metal oxides such as Li2O and MgO, while the opposite trend is observed with glasses containing PbO and BaO as seen in the diagram at the right."
     
  9. May 15, 2008 #8

    Andy Resnick

    User Avatar
    Science Advisor
    Education Advisor

    Intead of 'density', use 'index of refraction': as DavidSnyder pointed out, it's possible to have a dense low refractive index glass.

    Then, as a ray crosses a boundary between a low index material n1 and high index material n2, if the ray goes from n1 to n2 it will refract towards the normal, and vice-versa.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?