1. Reflection and Refraction of Light

2. Geometrical Optics – Using a Ray Approximation
Light travels in a straight-line path in a homogeneous medium until it encounters a boundary between two different media
The ray approximation is used to represent beams of light
A ray of light is an imaginary line drawn along the direction of travel of the light beams

3. Wave Fronts and Rays, cont.
“Far field”
“Near field”

4. Reflection of Light
A ray of light, the incident ray, travels in a medium
When it encounters a boundary with a second medium, part of the incident ray is reflected back into the first medium
This means it is directed backward into the first medium

5. Specular Reflection
Specular reflection is reflection from a smooth surface
The reflected rays are parallel to each other
All reflection in this text is assumed to be specular

6. Diffuse Reflection
Diffuse reflection is reflection from a rough surface
The reflected rays travel in a variety of directions
Diffuse reflection makes the road easy to see at night

7. Specular and Diffuse Reflection

8. Law of Reflection The normal is a line perpendicular to the surface
It is at the point where the incident ray strikes the surface
The incident ray makes an angle of θ1 with the normal
The reflected ray makes an angle of θ1’ with the normal

9. Law of Reflection, cont.
Incident and reflected ray are in the same plane.
The angle of reflection is equal to the angle of incidence
θ1=θ1’
1’ 1

10. Refraction of Light
When a ray of light traveling in a transparent medium encounters a boundary leading into a second medium, part of the ray is reflected and part of the ray enters the second medium
The ray that enters the second medium is bent at the boundary
This bending of the ray is called refraction

11. Refraction of Light, cont.
The incident ray, the reflected ray, the refracted ray, and the normal all lie on the same plane
The angle of refraction, θ2, depends on the properties of the medium

12. Following the Reflected and Refracted Rays
Ray  is the incident ray
Ray  is the reflected ray
Ray  is refracted into the lucite
Ray  is internally reflected in the lucite
Ray  is refracted as it enters the air from the lucite

13. Refraction, cont. Speed of light=constant?
Yes, but only in ONE medium!
v=c (light velocity)
Air
Index of refraction
v=c/n
Water (optically denser than air)

14. Index of refraction
The index of refraction defines the velocity of light in the optically denser medium  c/n.
Speed of light in vacuum (air)
Index of refraction
Speed of light in a medium (e.g. water)

15. Index of Refraction, cont.
For a vacuum and air, n = 1
For other media, n > 1
n is a unitless ratio

16. Frequency Between Media
As light travels from one medium to another, its frequency does not change
Both the wave speed and the wavelength do change
The wavefronts do not pile up, nor are created or destroyed at the boundary, so ƒ must stay the same

17. n=l0/ln Change of Wavelength
Wavelength of a medium with the refractive index n
n=l0/ln
Vacuum wavelength

18. Refraction Details
Light may refract into a material where its speed is lower
The angle of refraction is less than the angle of incidence
The ray bends toward the normal

19. Refraction Details, cont.
Light may refract into a material where its speed is higher
The angle of refraction is greater than the angle of incidence
The ray bends away from the normal

20. Snell’s Law of Refraction
n1sinθ1=n2sinθ2
θ1 is the angle of incidence
30.0° in this diagram
θ2 is the angle of refraction

21. Substance Refractive index Air 1.00029 Water 1.33 Ethyl alcohol 1.36
Some indices of refraction for various substances at 590 nm:
Substance
Refractive index
Air
Water
1.33
Ethyl alcohol
1.36
Fused quartz
1.46
Glycerine
1.47
Glass
Oil
1.50
Zircon
1.92
Diamond
2.42

22. Example and Application
A digital information on a DVD consists of a series of pits that are “read” by a laser beam. The surface of a DVD is shown on the right side.