Geometric Optics
•
Ray Model
•
•
assume light travels in straight line
uses rays to understand and predict reflection &
refraction
General Physics 2
Geometric Optics
1
Reflection
•
Law of reflection
•
•
the angle of incidence equals angle of reflection
angles are measured from normal
General Physics 2
Geometric Optics
2
Reflection
Diffuse reflection
General Physics 2
Specular reflection
Geometric Optics
3
Think-Pair-Share
•
What would be the appearance of the Moon if
it has
•
•
(a) a rough surface?
(b) a polished surface?
General Physics 2
Geometric Optics
4
Plane Mirrors
General Physics 2
Geometric Optics
5
Extra Credit
•
How large a mirror do you need to see your
whole body?
General Physics 2
Geometric Optics
6
Spherical Mirrors
By using ray tracing and the law of reflection, you
can figure out where the incoming rays are reflected.
General Physics 2
Geometric Optics
7
Focal Point & Focal Length
•
Parallel rays striking a concave mirror come
together at focal point
f = r/2
r = radius of sphere
f = focal length
General Physics 2
Geometric Optics
8
Image Formation
Real Image
General Physics 2
Geometric Optics
9
Mirror Equation & Magnification
General Physics 2
Geometric Optics
10
Plug & Chug
•
How far from a concave mirror (radius 23.0
cm) must an object be placed if its image is to
be at infinity?
General Physics 2
Geometric Optics
11
Convex Mirrors
•
Virtual Image
•
•
can’t detect on paper or
screen
Sign Conventions
•
•
•
object, image, or focal point
on reflecting side of mirror
has a positive distance
Anything behind mirror has
negative distance
image height is positive if
upright, negative if inverted
(relative to object)
General Physics 2
Geometric Optics
12
Refraction
•
index of refraction, n, where c = speed of light
in a vacuum and v = speed of light in that
medium
•
•
•
nair = 1
nglass = 1.5
Snell’s Law
General Physics 2
Geometric Optics
13
Indices of Refraction
General Physics 2
Geometric Optics
14
Total Internal Reflection
•
•
Incident angle where refracted angle (θ2) is 90 is the
critical angle
at incident angles greater than critical angle, light is
totally internally reflected
important for
fiber optic
technology
(endoscope)
General Physics 2
Geometric Optics
15
Think-Pair-Share
•
What is the focal length of a plane mirror?
What is the magnification of a plane mirror?
General Physics 2
Geometric Optics
16
Lens Activity
•
Use the equipment provided to make an
image of the light source on the screen.
General Physics 2
Geometric Optics
17
Thin Lenses
General Physics 2
Geometric Optics
18
Focal Length, Focal Plane and Power
•
•
f = focal length
Power
•
•
•
•
inverse of focal length
P = 1/f
measured in diopter (D)
1 D = 1 m-1
General Physics 2
Geometric Optics
19
Ray Tracing
General Physics 2
Geometric Optics
20
Thin Lens Equation
Magnification
Thin Lens Equation
General Physics 2
Geometric Optics
21
Sign Conventions
•
focal length
•
•
•
object distance
•
•
•
positive if the object is on the side of the lens from which the light is
coming (this is usually the case)
otherwise, it is negative (virtual object).
image distance
•
•
•
positive for converging lenses
negative for diverging lenses
positive if the image is on the opposite side of lens from where light is
coming
positive for real images, negative for virtual images
image height
•
•
positive if image is upright relative to object, negative for inverted
images
h0 is always positive
General Physics 2
Geometric Optics
22
Lens Activity
•
Find two different object-image distances that
create a real image. Describe any
differences in the image properties.
General Physics 2
Geometric Optics
23
Think-Pair-Share
•
Sunlight is observed to focus at a point 18.5
cm behind a lens. (a) What kind of lens is it?
(b) What is its power in diopters?
General Physics 2
Geometric Optics
24
Fermat’s Principle
•
When light travels between 2 points, its path
is the one that requires the smallest time
interval.
General Physics 2
Geometric Optics
25
Combination of Lenses
•
When adding two or more lenses in series,
the focal length of the combined lenses, f, is:
General Physics 2
Geometric Optics
26
Combining Lenses
•
Measure the focal length of the two double-convex lenses
individually.
Combine the lenses together and measure the combined
focal length.
Calculate the combined focal length using the equation.
•
Calculate the percent error of your measured value.
•
•
General Physics 2
Geometric Optics
27