1. Created by Stephanie Ingle Kingwood High School
Refraction & Lenses
Created by Stephanie Ingle
Kingwood High School

2. Snell’s Law
normal
incident ray
Air (n=1.0003) 1
n = index of refraction for medium (no units)
Boundary
reflected ray 2
Water (n= 1.33)
Angles are always measured from the normal, never the surface

3. Index of Refraction Light changes speed (v) as it enters a new medium
In a vacuum the speed of light (c) is 3.0 x 108m/s
The index of refraction (n) of a material is the ratio of the speed of light in a vacuum to the speed of light in the material.
Index of refraction has no units!

4. Critical Angle
The incident angle of light that causes refraction along the boundary between surfaces
The angle of refraction will always be 90o
Only possible when going from more optically dense (high index of refraction) to less optically dense medium (low index of refraction
Only possible when light speeds up as it passes through the boundary
n=1
r=900 c
n=1.5

5. Total Internal Reflection
When incident light strikes a boundary at an angle greater than the incident angle it does not cross the boundary into the new medium.
Instead, all of the light is reflected from the boundary back into the original medium according to the Law of Reflection.

6. Concave Lenses Thicker at the edges than in the center
Parallel rays of light from a far object will refract throught the lense and diverge as if they came from the focal point.
Concave lenses also called “diverging lenses”
Light may come in from either side of lens so there will be a focal point on both sides equal distances from the lens (assuming symmetrical lenses).

7. Convex Lenses Thicker in the center than at the edges
Parallel rays of light from a far object will refract through the lens and converge at the focal point.
Convex lenses also called “converging lenses”
Light may come in from either side of lens so there will be a focal point on both sides equal distances from the lens (assuming symmetrical lenses).

8. Calculations f = focal length do = object distance di = image distance
hi = image height
ho = object height
M = magnification

9. Interpreting Calculations
Focal length (f)
converging, then f = + diverging, then f = -
Image distance (di)
di=+ , then image is real
do= -, then image is virtual
Magnification (M)
M = +, image is erect
M = - , image is inverted

10. Ray Diagram Convex Lens Image is real, inverted, & reduced
Draw 3 rays from tip of object:
1) parallel, then through f
2) through f, then parallel
3) through the lens at the principal axis
Image is real, inverted, & reduced f f

11. Ray Diagram Convex Lens (Inside f)
Draw 3 rays from tip of object:
1) parallel, then through f
2) from same side f, through tip of object, then parallel
3) through the lens at the principal axis
image f f
object
Image is virtual, erect, & magnified

12. Ray Diagram Concave Lens Image is virtual, erect, & reduced
Draw 3 rays from tip of object:
1) parallel, then refracted ray from f on same side of lens
2) to lens along a line that would pass through f on the other side of lens, then parallel
3) through the lens at the principal axis
concave lens (axis)
object
image f f
Image is virtual, erect, & reduced