Download Refraction_and_Lenses

Refraction & Lenses
Created by Stephanie Ingle
Kingwood High School
Snell’s Law
incident ray
normal
Air (n=1.0003)
1
Boundary
2
reflected ray
Water (n= 1.33)
n = index of
refraction for
medium (no
units)
• Angles are always measured from the normal,
never the surface
•
n sin  n sin
1
1
2
2
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!
c
n v
Critical Angle
• The incident angle of light
n=1
that causes refraction along
r=900
the boundary between
surfaces
c
n=1.5
• 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
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.
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).
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).
Calculations
1 1 1
f do di
h
i

d
i
M 
ho do
f = focal length
do = object distance
di = image distance
hi = image height
ho = object height
M = magnification
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
Ray Diagram
Convex Lens
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
Ray Diagram
Draw 3 rays from tip of object:
1) parallel, then through f
Convex Lens (Inside 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
Ray Diagram
Draw 3 rays from tip of
object:
Concave Lens
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
Image is virtual, erect, & reduced
f
f