Refraction

2

REFRACTION

NOTE

the bending of light at the boundary of two transparent substances (or “media”)

occurs because different materials slow the speed of passing light at different rates

e.g. the speed of light in glass is less than the speed of light in air

Light travels fastest in a vacuum at 3.00 x 108 ms-1.

We’ll assume air is approximately the same as a vacuum.

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LIGHT SPEED IN DIFFERENT MEDIA

FYI

 

 

 

MEDIUM SPEED (km/s)

air 300 000ice 229 000

water 226 000vegetable oil 204 000

glass 197 000ruby 170 000

diamond 124 000

  

4

INDEX OF REFRACTION (n)

NOTE

number which indicates how much the speed of light will decrease when entering a certain medium

i.e. it is impossible for light to move at top speed when particles get in the way

the larger the “n”, the more the medium decreases the speed of light

since speed of light is fastest in the vacuum of space (where there are very few particles), n = 1.00 (no units)

Index of Refraction (n) =speed of light in vacuum (c)

speed of light in medium (v)

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n FOR SELECTED MEDIA

NOTE

vacuum 1.00air

1.0003CO2 gas 1.0005water 1.33alcohol 1.36Pyrex glass 1.47Plexiglas 1.49table salt 1.51flint glass 1.61sapphire 1.77cubic zirconia 2.16diamond 2.42gallium phosphide 3.50

Index of Refraction Trends

• The materials in which light travels fastest are the least optically dense materials.

• The materials in which light travels slowest are the most optically dense materials.

• As the index of refraction value increases, the optical density increases, and the speed of light in that material decreases.

7FYI

LESS DENSE

MORE DENSE

TOWARDS NORMAL

When light travels from a less dense medium to a more dense medium it travels towards the normal

8FYI

LESS DENSE

MORE DENSE

AWAY FROM NORMAL

When light travels from a more dense medium to a less dense medium it bends away from the normal

“Broken Pencil”

• What happens when a pencil is moved across the middle of a glass of water?

• Why is this phenomenon observed? • How does the refraction of light cause

the pencil to appear fatter and shifted to the side?

Broken Pencil Conclusion

• Answer: The light rays leaving the pencil are broken at the water surface. Our eye is unaware of this, so it sees the rays running straight forward. So we think the pencil lies in the extension of the straight line from our eyes

Snell’s Law

• Snell was a wave theorist and he discovered that the bending of light followed a pattern:

• Where i is the angle of incidence and R is the angle of refraction, v is the speed of light, λ is the wavelength and n is the index of refraction.

Examples

1) When light passes from air into water at an angle of 60 from the normal, what is the angle of refraction?

Examples

2) When light passes from water into a diamond at an angle of 45 from the normal, what is the angle of refraction?

Examples

3) A ray of light approaches a jar of honey at an angle of 30. If the angle of refraction is 19.5, what is the refractive index of honey?

Examples

4) The refractive index of the lens in the human eye is 1.41. If a ray of light goes from the air into the lens at an angle of 55, what is the angle of refraction?

Examples

5) A block of amber is placed in water and a laser beam travels from the water through the amber. The angle of incidence is 35 while the angle of refraction is 24. What is the index of refraction of amber?

Examples

6) When light passes from air into water at an angle of 30 from the normal, what is the angle of refraction?

Examples

7) A red laser beam travels from flint glass into lemon oil. The angle of incidence is 40 and the angle of refraction is 44.4. What is the refractive index of lemon oil?

Examples

8) In an experiment, a block of cubic zirconia is placed in water. A laser beam is passed from the water through the cubic zirconia. The angle of incidence is 50, and the angle of refraction is 27. What is the index of refraction of cubic zirconia?

Critical Angle and TIR

• A special angle exists when light moves from a more dense to a less dense medium.

• As the angle of refraction moves away from the normal, the critical angle yields an angle of refraction of 90 degrees!

• The is the property that allows total internal reflection to occur, which is used in fibre optics.

• If the critical angle is exceeded, the light ray reflects and obeys the law of reflection: i = r

Example

• Determine the critical angle of diamond (n = 2.42).

Let’s See it Again!

• http://www.youtube.com/watch?v=xwzw5GhpC_0&NR=1