1. Reflection, refraction and optical fibre
Light
Reflection, refraction and optical fibre
12/08/03
SW Abingdon and Witney College

2. SW Abingdon and Witney College
Light rays
Light and other forms of electromagnetic radiation travel as waves
The waves go out in straight lines from the origin
We can think of rays of light going out
Laser
12/08/03
SW Abingdon and Witney College

3. SW Abingdon and Witney College
Speed of light
The speed of light depends on the material the light is travelling through
In a vacuum the speed of light is about 300,000 kilometres a second
When we talk about ‘the speed of light’ this is what we usually mean
Light travels more slowly through air, water, glass, diamond etc.
12/08/03
SW Abingdon and Witney College

4. SW Abingdon and Witney College
Index of refraction
The index of refraction of a material is the ratio of the speed of light in a vacuum to the speed of light in the material
So it is a measure of how much the material slows down light: its optical density
Speed of light in vacuum
Index of refraction =
Speed of light in material
12/08/03
SW Abingdon and Witney College

5. SW Abingdon and Witney College
Slowing light down
The bigger the index of refraction of a material, the more the material slows light down
Air: slightly more than 1 ( )
Water: about 1.3
Glass: about 1.5 but varies depending on what impurities are in the glass
Diamond: about 2.4
12/08/03
SW Abingdon and Witney College

6. Reflection and refraction
A ray of light hits the boundary between two materials
Part of it is reflected back
Part of it crosses the border into the other material, but it is refracted: it changes direction
Refracted
Incident
Reflected
12/08/03
SW Abingdon and Witney College

7. SW Abingdon and Witney College
Reflection
When light is reflected the angle of incidence 1 is equal to the angle of reflection 2
Incident ray
Reflected ray 1 2
Normal
12/08/03
SW Abingdon and Witney College

8. Refraction glass to air
Air has a smaller index of refraction than glass
When light passes from glass to air the ray bends away from the normal line
Refracted ray
Air 3
Glass 1
Incident ray
12/08/03
SW Abingdon and Witney College

9. SW Abingdon and Witney College
Critical angle
The incident ray hits the boundary at the critical angle
The refracted ray travels along the boundary
Air
Refracted ray
Glass
Incident ray
Critical
angle
12/08/03
SW Abingdon and Witney College

10. Total internal reflection
The incident angle is bigger than the critical angle
The light cannot escape
It all reflects back into the glass
Air
Reflected ray
Incident ray
Glass
12/08/03
SW Abingdon and Witney College

11. SW Abingdon and Witney College
Optical fibre
Optical fibre is designed so that light is reflected back into the core by total internal reflection and is not lost into the cladding
12/08/03
SW Abingdon and Witney College

12. SW Abingdon and Witney College
Optical fibre
The core material must have a higher index of refraction than the cladding material
The light ray must enter the core at an angle greater than the critical angle for the two materials
This will allow total internal reflection so that the light stays inside the core
12/08/03
SW Abingdon and Witney College

13. SW Abingdon and Witney College
Numerical aperture
The NA measures the range of angles that will be totally internally reflected
12/08/03
SW Abingdon and Witney College

14. SW Abingdon and Witney College
End
12/08/03
SW Abingdon and Witney College