1. Refraction of Light
Section 11.1
SNC2D

2. But what happens when another medium is introduced?
Light travels in straight lines and at a constant speed as long as it’s travelling in the same medium.
Medium – (plural:media) is a material that is being used.
But what happens when another medium is introduced?
For example ... What happens if you are riding in a golf cart on pavement, and then you hit some mud or gravel?
- The front wheels suddenly slow down, but the back wheels keep going and the golf cart twists around.
- This is similar to when light travels from one medium into a different medium ... Both its speed and direction may change.

3. What is Refraction?
Refraction is the bending or change in direction of light when it travels from one medium to another
Example: from air to water.
Refraction is used in designing and building lenses.
The bending of light is due to different media slowing light down by different amounts. The more light slows down, the more it is refracted.

4. How Light Refracts
As light enters a slower medium, the light waves are compressed. If light strikes a medium at an angle, the part of the light wave that enters the medium first will slow down first.

5. Refraction
This effect is commonly seen when looking at something in the water. It appears like the light is coming straight to us when really it is bending.

6. What Causes Refraction?
Light travels at different speeds in different media.
It is impossible for light to move at top speed when particles get in the way. The particles in a medium slow down the passage of the waves, which results in light travelling more slowly through a block of glass than through air.

7. The Speed of Light Speed of light in different materials:
Vacuum (empty space where there are very few particles) = 3.00x108 m/s or 300 million m/s
Air – slightly less than vacuum but use the same speed = 3.00x108 m/s
Water – 2.26x108 m/s

8. The Speed of Light
Example: The light ray strikes the Plexiglas at an angle.
(a) As the light enters the Plexiglas, it slows down and refracts.
(b) When the light leaves the Plexiglas and enters the air, it speeds up and refracts again.

9. The Speed of Light
Notice that light does not refract inside the block. Light refracts only at the boundary when it is entering or leaving a medium.

10. Reflection and Refraction
incident ray
reflected ray
NOTE: The angles of the refracted light rays are usually measured from the normal, drawn at 90° to the surface where the light ray crosses between the two media.

11. So, when light travels between two media at angles other than 90°, it bends due to its change in speed.

12. RULES FOR REFRACTION
1. Incident ray, refracted ray and the normal all lie in the same plane. Incident ray and refracted ray are on opposite sides of the line that separates the two media.
Note: Angle of refraction is defined as the angle between the refracted ray and the normal.
incident ray
Medium 1
Medium 2

13. RULES FOR REFRACTION
2. If the change in medium causes a decrease in speed, then the light bends towards the normal. 3. If the change in medium causes an increase in speed, then the light bends away from the normal. Refer to diagram in handout.

14. The Index of Refraction
Light travels fastest in a vacuum.
The refractive index of the speed of light in a vacuum is assigned a value of 1.00.
The larger the refractive index, the more the medium decreases the speed of light.

15. The Index of Refraction
So, the index of refraction for a medium is defined as the ratio of the speed of light in a vacuum to the speed of light in that medium.
Mathematically, it is written as:
n is the index of refraction
c is the speed of light in a vacuum (in m/s)
v is the speed of light in a given medium (in m/s)
n has no units (it’s a dimensionless quantity) since m/s in c and v cancel each other out.

16. Index of Refraction Values
Media
Index of refraction (n)
Vacuum
1.00 (exactly)
Air
CO2 gas
Water
1.3333
Ethyl Alcohol
1.362
Glycerol
1.470
Carbon disulfide
1.632
Glass (crown)
1.52
Ruby
1.54
Glass (flint)
1.65
Zircon
1.92
Diamond
2.42

17. Example Problem 1
The speed of light in sodium chloride (NaCl)1.96x108 m/s. The speed of light in a vacuum is 3.00×108 m/s. Calculate the index of refraction. G S R P A
vNaCl = 1.96 × 108 m/s
c = 3.00 × 108 m/s
n = 3.00 x 108 m/s
1.96 x 108 m/s
n = 1.53
Refractive index n = ?
Therefore the index of refraction for NaCl is 1.53
n = c v

18. Example Problem 2
Calculate the speed of light in olive oil, if the index of refraction of olive oil is 1.48. G S R P A
c = 3.00x108 m/s
n olive oil = 1.48
v = 3.00x108 m/s
1.48
v = 2.03x108 m/s
v olive oil = ?
Therefore, the speed of light in olive oil is 2.03x108 m/s.
v = c n
n = c v

19. Dispersion
The refraction of white light into separate wavelengths, or colours.
Recall that white light includes all the wavelengths of visible light.
The order of the colours of light in a spectrum are: red, orange, yellow, green, blue, indigo, violet [ROY G BIV].
Note that blue light bends more than red. So blue light travels slower than red light.

20. Homework Read pg. 449-456 Answer # 1-4 on pg. 455