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Light refraction


Waves travel at different speeds in different materials, and this leads to the phenomenon of refraction.

Light travels more slowly in glass than it does in air, or in a vacuum. 

Sound waves and light waves change speed when they pass across the boundary between two substances with different optical densities, such as air and glass. 

This causes them to change direction and this effect is called refraction.




There is one special case you need to know. Refraction doesn't happen if they cross the boundary at an angle of 90° - in that case they carry straight on.

YouTube Video






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 The normal


 Is a line drawn perpendicular to the boundary at the point of incidence.
 
The incident ray


 Is a ray that shows the direction that light travels as it approaches the boundary
 
The angle of incidence


 The angle that the incident ray makes with the normal line

 The refracted ray


 Is a ray that shows the direction that light travels after it has crossed over the boundary.

 The angle of refraction

 

The angle that the refracted ray makes with the  normal line.



.Snell's law (also known as the Snell–Descartes law and the law of refraction) is a formula used to describe the relationship between the angles of incidence and refraction, when referring to light or other waves passing through a boundary between two different isotropic media, such as water, glass, or air.

Snell's law states that the ratio of the sines of the angles of incidence and refraction is equivalent to the ratio of  velocities in the two media, or equivalent to the reciprocal of the ratio of the index of refraction of these two media.

\frac{\sin\theta_1}{\sin\theta_2} = \frac{v_1}{v_2} = \frac{n_2}{n_1}        
 nmaterial = index of refraction of the material
check this interactive link Snell's law


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  The Direction of Bending


 
Light Traveling from a Fast to a Slow Medium
(less dense to more dense)

 Light Traveling from a Slow to a Fast Medium
(more dense to less dense)
 
 
 

The light bends towards the normal line .
 

The light bends away from the normal 


The angle of incidence is more than
 the angle of refraction. 

i > r 

Note that

Refraction doesn't happen if they cross the boundary at
an angle of 90° - in that case they carry straight on.



The angle of incidence is less than
 the angle of refraction.  

i < r 

Note that

Refraction doesn't happen if they cross the boundary at 
an angle of 90° - in that case they carry straight on.







Refraction of Light through a Glass Block.

When a ray of light enters a glass block at an angle other
than the normal, it changes speed, wavelength
and direction as shown in the diagram.
In going from a less dense medium (air) to a more dense
medium (glass), light bends towards the normal.

This means that i > r  (the angle i is greater than the angle r).
In going from a more dense to a less dense medium (glass to air),
light bends away from the normal.
How much the light bends depends on its colour.
The change in angle of the light ray is the same
when it  enters and leaves the glass.
If the incident ray had continued without changing direction,
then the emergent ray would be parallel to the incident ray
Subpages (1): Refraction and Sight
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