Refractive index

 

    

Refraction of light is the bending of light, when light refracts it bends. The refractive index of a substance is a measure of how fast light travels in that substance compared to its speed in a vacuum. It is expressed as a ratio of the speed of light in vacuum relative to the speed of light in the substance.

refractive index= speed of light in a vacuum / speed of light in substance.

The refractive index of water is 1.33, meaning that light travels 1.33 times faster in a vacuum than in water

Dense substances tend to slow light more as it passes through them.

 

When light passes from one substance to another, with a different refractive index, it bends. This bending of light gives us the effects such as a rainbow and a colour spectrum.

If it goes from a substance with a low refractive index to one with a higher refractive index it bends away from the surface, as shown on the right.
If it goes from a substance with a high refractive index to one with a lower refractive index it bends towards the surface.

If both substances have the same refractive index then light does not bend.

  
     
It is the difference in refractive index between air and water that gives us the bent nail illusion shown on the right and also makes fish in the water appear closer to the surface than they really are.  

 

 

When light passes through substances each with a different refractive index we can see the objects as light bends. However if there is no difference in the refractive index of each substance light passes straight through and gives the illusion that there is nothing present.

View the video on the left which clearly shows this magic trick. There is little difference between the refractive index of corn oil and pyrex glass. As you can see the test tube is invisible immersed in the oil. The magic trick shown on the left can be performed right in front of spectators. As you can see the glass completely disappears from view when placed in oil but not when placed in water. Explain why.

Conduct you own magic trick.

 

  

Given that the refractive index of air is close to 1.00 and that of water is 1.333 explain using refraction how this trick works. Click to see the video on the left. Notice how the coin appears out of nowhere.

Explain how this is done.

Solution
   

A rod is placed in a beaker with oil and water,

Which substance refracts light more? Explain

At what angle do we not see refraction?

A beam of light passes from a medium with refractive index of 1.33 to a medium with refractive index 1.58. What will be the path the beam takes through the green medium shown on the right?

Explain why

What is refractive index?  
What happens to the speed of light when it travels through very dense liquids?  

A beam of light passes from a medium with refractive index of 1.33 to a medium with refractive index 1.28. What will be the path the beam takes through the green medium shown on the right?

Explain why

A beam of light passes from a medium with refractive index of 1.58 to a medium with refractive index also of 1.58. What will be the path the beam takes through the green medium shown on the right?

Explain why

Do all colours that make up white light refract the same when they pass through a thick medium?
Explain how a rainbow forms.
What is the difference between refraction of light and reflection of light?