Refraction  is a change in the direction of a light wave caused by a change in its speed as the light wave passes at an angle from one medium to the next.  In the ray diagram example, a light ray is incident on the surface of water.  Some of the light is reflected and some of the light slows down as it enters the water and is refracted.

 

According to the law of reflection, the angle of reflection is identical to the angle of incidence.

 

The angle of refraction, however, is not the same as the angle of incidence. It is determined by  Snell's Law .  The simplest way to write Snell's Law is as follows:

 

, where  n  is the index of refraction

 

The simple form of Snell's Law assumes that the medium the light is leaving is air.  The refractive index of air is 1.0003, which is close to, and usually assumed to be the same as, the refractive index of a vacuum ( n = 1.000).  This is true because the speed of light in air is nearly identical to its speed in a vacuum so, as it passes from air into a vacuum, it is only refracted by a very small amount, making the ratio of the  equal to 1.  The amount of refraction in other materials, as light enters them from air or a vacuum, is much more and leads to higher indexes of refraction.  Therefore, the index of refraction is a relative measure of how much the light changes speed and is bent as it moves between two mediums.

 

Table 1: Absolute Refractive Indexes of Some Common Materials

 

Medium

Index of Refraction

vacuum

1.0000

air

1.0003

ice

1.31

water

1.33

quartz glass

1.47

crown glass

1.52

lucite (plexiglass)

1.52

diamond

2.42

 

refraction : a change in the direction of a wave due to a change in its speed

 

Snell's Law : for any angle of incidence greater than zero, the ratio of

 

sin ϑincidentsin ϑrefracted=n

 

refractive index : a ratio comparing the speed of light in a vacuum to its speed in a given medium

 


Try This
Use Table 1 (above) and the simulation below (instructions below, click the hamburger in the bottom right corner and select full screen) to determine which medium makes light refract the most as it enters the medium from air.  Does a higher index make the light bend more or less?


Once the simulation is open, do the following:

  • Check that material 1 is air (n=1) and that material 2 is water (n=1.33) 
  • Click the red button to turn on the laser and then drag the laser to vary the angle of the beam.  Note the way in which the light bends as it enters the denser (water) layer.  (Toward or away from the normal?) 
  • Change the substance so that water is now the top layer and air is the bottom layer. Vary the angle and note the way (toward or away from the normal), in which the light bends as it enters the air layer.  Repeat the procedure using different substances with different indexes of refraction. 
 

 

Self-Check

Answer the following self-check (SC) question then click the "Check your work" bar to assess your response.

 

SC 1. 

A laser is directed into a pool of water ( n water  = 1.33).  The beam contacts the air-water interface with an angle of incidence equal to 35.0°.  Using the simple form of Snell's Law, calculate the angle of refraction (relative to the normal) with which it travels through the water.  Show your calculations and label the ray diagram showing the angle of incidence and angle of refraction.  Verify your answer using the Light Refraction simulation.


  Self-Check Answer

Contact your teacher if your answers vary significantly from the answers provided here.

 

SC 1.

 

Given

 

Required

the angle of refraction,  n 2

 

Analysis and Solution

 

 

Paraphrase

The angle of refraction is 25.5°.