1. Refractive Optics
Chapter 26

2. Refraction: Refractive Index
Index of refraction:
n depends on:
material
wavelength of light

3. Snell’s Law: Total Internal Reflection
If the angle of incidence is large enough, the angle of refraction increases to 90°
n = n2
n = n1

4. Snell’s Law: Total Internal Reflection
At that point, none of the light is transmitted through the surface. All of the light is reflected (total internal reflection). The angle of incidence for which this happens is called the critical angle.
n = n2
n = n1

5. Refractive & Frequency
The refractive index depends on the medium & the frequency of light.
Each frequency “color” propagates at a different speed and bends a different amount.

6. Snell’s Law: Total Internal Reflection
We can easily calculate the critical angle by
imposing the additional condition
on Snell’s Law:

7. Snell’s Law: Polarization
We can calculate the angle of incidence for light entering a more-dense medium from a less-dense medium so that the reflected and refracted rays are perpendicular:
n = n1
n = n2

8. Snell’s Law: Polarization
By inspection of our drawing, we see that the perpendicularity of the reflected and transmitted rays requires that:
n = n1
n = n2

9. Snell’s Law: Polarization
Substitute for q2:

10. Snell’s Law: Polarization
Substitute for q2:
angle-difference identity:

11. Snell’s Law: Polarization
qB is called Brewster’s angle.

12. Electromagnetic Wave

13. EM wave is … Light is an electromagnetic wave.
It consists of vibrations of electric field and magnetic field.
The electric field and magnetic field are perpendicular to each other and in phase.
EM wave is a transverse wave.
The speed of EM wave is 3 x 108 ms-1.

14. Electric Field Vector

15. Polarized Light Polarized Light
Vibrations lie on one single plane only.
Unpolarized Light
Superposition of many beams, in the same direction of propagation, but each with random polarization.

16. Representation . . . E E
Unpolarized
Polarized

17. Representation . . .
Polarized
Unpolarized

18. Polarization of Light

19. Selective Absorption Unpolarized Light
Vertical Component being Transmitted
Horizontal Component being Absorbed

20. Selective Absorption - Explanation

21. Polarizing Material I = I0 cos2q
A Polarizing material will only allow the passage of that component of the electric field parallel to the polarization direction of the material
I = I0 cos2q

22. Crossed Perpendicularly

23. Crossed at different angles . . .

24. Crossed at different angles . . .

25. Reflection

26. Glares

27. Sunglasses – Glare Reduction
Polarized lenses have the added benefit of filtering out reflected light, or glare, off surfaces such as water or pavement
Ideal for boating, fishing, driving or any other activity associated with intense glare
Reduces eyestrain and fatigue, while increasing contrast and visual acuity

28. How Do Polarized Lenses Work?
Light reflected from surfaces like a flat road or smooth water is generally horizontally polarized. This horizontally polarized light is blocked by the vertically oriented polarizers in the lenses.
The result: a reduction in annoying and sometimes dangerous glare.

29. Action of Polaroid Sunglass
Light reflected from surfaces like a flat road or smooth water is generally horizontally polarized. This horizontally polarized light is blocked by the vertically oriented polarizers in the lenses.
Vertically Polarized Light from Objects
Unwanted glares are usually horizontally polarized light

30. Glare and Polarization

31. Glare and Polarization

32. Snell’s Law: Longitudinal Focus Shift
Rays are converging to form an image:

33. Snell’s Law: Longitudinal Focus Shift
Insert a window: the focus is shifted rightward (delayed)

34. Snell’s Law: Longitudinal Focus Shift
The amount of the longitudinal focus shift:

35. Snell’s Law: Longitudinal Focus Shift
If an object is immersed in one material and viewed from another: “apparent depth”

36. Snell’s Law: Longitudinal Focus Shift
The longitudinal focus shift and apparent depth relationships presented:
are paraxial approximations. Even flat surfaces exhibit spherical aberration in converging or diverging beams of light.