Mammoth Memory

Part reflection part refraction

Again you don't need this in an exam it is just interesting to know.

 

When a ray of light travels from a fast medium, like air to water at different angles of incidence (degree to the normal) you will always get some surface boundary reflection and you will always get some refraction. It is just in different proportions depending on the angle. See below:

(Please note: In nearly all questions in exams at basic level the reflected is not taken into account)

Ray of light travelling along the normal line from air into water.

At no angle bright beam.

(but some is reflected backwards 3%)

Percentages reflected and refracted as a ray of light travels from air to water at thirty degrees to the normal line.

  Light ray in `=` `30^circ` To the normal  
  Reflected ray `=` `-30^circ` To the normal  
      Dim reflection (5%)  
  Refracted ray `=` `23^circ` To the normal  
    `=` Bright refraction  

 

Percentages reflected and refracted as a ray of light travels from air to water at sixty degrees to the normal line.

 

  Light ray in `=` `60^circ` To the normal  
  Reflected ray `=` `-60^circ` To the normal  
      Dim reflection (10%)  
  Refracted ray `=` `42^circ` To the normal  
    `=` Bright refraction  

 

A ray of light entering water from air at 85 degrees will be 70% reflected at the surface.

 

  Light ray in `=` `85^circ` To the normal  
  Reflected ray `=` `-85^circ` To the normal  
      Bright reflection (70%)  
  Refracted ray `=` `50^circ` To the normal  
    `=` Dim refraction  

 

If you were to draw the amount of light refracted at the boundary when light travels from air to water you would get the following graph:

Graph showing the percentage reflected as the angle of incidence increases.

Light travelling from air to water.

 

Light travelling at a water surface at angles of incidence less than 50 degrees mostly penetrate the water. Above 50 degrees the percentage of light that is reflected increases steeply.

So if you stand at a lake edge and look down near your feet (i.e. steep angle of incidence) you will see details in the water and on the bottom of the pool. 

As you look out across the lake (i.e. at a shallow angle of incidence) you can clearly see the reflection of the trees in the lake and little detail in the waters bottom.

 

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