1. Wavefronts and Snell’s Law of Refraction
Summary Lecture 17
Wavefronts and Snell’s Law of Refraction
Smaller velocity
Wavefronts closer together
Change in direction

2. Refraction (nair ~ 1, nWater = 1.33 …) 1 2
Normal 1
Medium 1 (n1)
Medium 2 (n2) 2
Caused by the difference in the speed
of light in medium 1 and 2
Index of Refraction n = c/v
(nair ~ 1, nWater = 1.33 …)
Snell’s Law of Refraction
n1 sin 1 = n2 sin 2

3. Example
A ray of light passes from air (n = 1.0) into glass (n = 1.52) and then into Jell-O. The incident ray makes a 58.0o angle with the normal as it enters the glass and a 36.4o angle with the normal in Jell-O. What is the index of refraction in Jell-O?
Glass Jell-O
58.0o
36.4o

4. Total Internal Reflection
going from a medium with large n to
a medium with small n
“refracted” away from normal n2 1 2 3 4 qc n1
Critical Angle: sin C = n2/n1
 > C
Total Internal Reflection

5. Lenses
Lenses refract light in such a way that an image of the light source is formed.
Two prisms cause incoming parallel light rays to change direction toward the principal axis
Think of a lens as a combination of many prisms with “optimized” shape (spherical instead of flat)
With a convex lens parallel light converges to the focal point.

6. Ray Tracing for Lenses Conventions Focal Length
Converging Lenses (convex)
Diverging Lenses (concave)
Conventions
Focal Length
f is positive for converging lens
f is negative for diverging lens

7. Image Characterization Type: the image is real if di is positive
Conventions
Focal Length
f is positive for converging lens
f is negative for diverging lens
Object Distance
do is positive if object is to the left of the lens
do is negative if object is to the right of the lens
Image Distance
di is positive if object is to the right of the lens
di is negative if object is to the left of the lens
Object and Image Size
ho, hi are positive if above the principal axis
Image Characterization
Type: the image is real if di is positive
Orientation: the image is upright if hi and ho have the same sign
Size: the image is reduced if | hi | < | ho |

8. Thin Lens Equation
Magnification Equation