The Naked Scientists

The Naked Scientists Forum

Author Topic: why can i see my reflection in a puddle?  (Read 6151 times)

paul.fr

  • Guest
why can i see my reflection in a puddle?
« on: 09/09/2007 18:38:11 »
why and how see my reflection in a puddle or other body of water?


 

lyner

  • Guest
why can i see my reflection in a puddle?
« Reply #1 on: 10/09/2007 12:55:58 »
When ANY wave hits ANY interface between two 'media' (materials), some of the wave is transmitted and some is reflected. For light traveling through transparent substances, it is the difference between the refractive indices (this often relates to the difference in their densities) that tells you how much will be reflected. A big difference means more reflection.  This is why you can hardly see glass, when it is under water - the refractive indices are almost the same and there's hardly any reflection from the glass.

As for seeing yourself in a puddle; The amount of light reflected depends on the angle it strikes the water; hitting at right angles gives about 5% reflection - so the bottom of the puddle needs to be quite dark because you wouldn't see a clear reflection with a bright background.
For a  'grazing angle' the reflection approaches 100% - you can get very strong reflections of  a low Sun on a smooth lake - but you can't see yourself under those conditions because you are looking straight into the water; the angle's wrong.
Of course, the water needs to be flat and still.
 

another_someone

  • Guest
why can i see my reflection in a puddle?
« Reply #2 on: 10/09/2007 13:41:08 »
You are also ignoring the murkiness of the water.

This will at very least help to darken the background.
 

Offline lightarrow

  • Neilep Level Member
  • ******
  • Posts: 4586
  • Thanked: 7 times
    • View Profile
why can i see my reflection in a puddle?
« Reply #3 on: 10/09/2007 16:15:47 »
As for seeing yourself in a puddle; The amount of light reflected depends on the angle it strikes the water; hitting at right angles gives about 5% reflection -
Are you sure about 5%?
Using the formula |(n-1)/(n+1)|2 gives, for water ( n = 1.333):
|(1.333-1)/(1.333+1)|2 = (0.333/2.333)2 = 0.02 = 2%
 

Offline JimBob

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 6564
  • Thanked: 7 times
  • Moderator
    • View Profile
why can i see my reflection in a puddle?
« Reply #4 on: 11/09/2007 03:15:01 »
If I were not a responsible person, Paul, I would say that you can see your reflection because you are not a vampire. However, being a responsible, I must agree that it is the refractive index contrast that causes your reflection. Sometimes, when there is a stiff breeze blowing, the ripples on the water, viewed at the correct angle will flash color because the difference in refractive indices are dynamically oscillating. (Is that a redundancy? I think it is. But ... yawn)
 

paul.fr

  • Guest
why can i see my reflection in a puddle?
« Reply #5 on: 11/09/2007 07:53:56 »
Thank you all (even Jim) for the answer.
 

lyner

  • Guest
why can i see my reflection in a puddle?
« Reply #6 on: 15/09/2007 15:12:01 »
Yes, Lightarrow - your sums must be right!
I was remembering the mu of glass as more like 1.55! Old age, I think.
 

The Naked Scientists Forum

why can i see my reflection in a puddle?
« Reply #6 on: 15/09/2007 15:12:01 »

 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums
 
Login
Login with username, password and session length