Transcript Light

Light
What is Light?
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Is light a particle or a wave?
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Yes – exhibits behaviors of BOTH
Light Can Act Like a Wave
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In 1801 Thomas Young an English scientist
did the Double slit experiment.
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Passed a beam of light through two narrow openings
and projected it onto a screen.
He found the light produced a striped pattern which
meant the light was constructively and destructively
interfering.
This meant that light is composed of waves.
Light can Behave Particle
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Other observations indicated that light can also
act like a particle:
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When light hits metal it knocks electrons off the
surface.
They found that red light cannot knock electrons off
metal no matter how bright it is.
If light were a wave then the brighter light should have
more energy.
Photons are light particles that contain certain
amounts of energy based on their frequency and
wavelength.
–
Blue light has a higher frequency and shorter wavelength
thus contains more energy than red light.
What is Light?
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The “particle” of light is called the photon. It
has no mass and no charge, but it does carry
energy.
For ‘optics’ lessons, we will focus on light as
a wave
What type of wave is light?
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Electromagnetic
Exhibits Behaviors of Waves
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Reflection
Refraction
Diffraction
Reflection
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Law of reflection
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Angle of ‘incidence’
is equal to angle of
‘reflection’
(as measured from
the ‘normal’ line)
More on Reflection
Reflection is Important
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Allows us to see as light bounces off objects
Important with mirrors
… more to come
EX 1
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Sitting in her parlor one night, Gerty sees the
reflection of her cat, Whiskers, in the living
room window. If the image of Whiskers
makes an angle of 40° with the normal, at
what angle does Gerty see him reflected?
Solution: Because the angle of incidence
equals the angle of reflection, Gerty must see
her cat reflected at an angle of 40°.
EX 2
Answer: 40°
Electromagnetic waves
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What are the characteristics of
electromagnetic waves?
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Travels at “speed of light”
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c = 3x108 m/s
Transverse wave
Requires no medium (can travel via a vacuum)
Electromagnetic Wave
Relationships
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Frequency and
wavelength are
inversely related via
wave speed
v  f
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And for
electromagnetic
waves, velocity is
equal to the speed
of light (c)
c  f
EX 3
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What is the frequency of infrared light that
has a wavelength of 8x10–6 m?
How Frequency/Wavelength relates to color
COLOR
Wave Characteristics
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Wavelength and
Frequency for
electromagnetic
waves determines
the ‘type’ of wave
The Color of light is Determined By its
Frequency and Wavelength
Seeing color
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The color an object appears depends on the colors of
light it reflects.
For example, a red book only reflects red light:
White
light
Only red light
is reflected
How The Eye Works
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Rods and Cones
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Rods (~120 million)
detect B&W images
Cones (~6 million)
detect color.
Each cone has
sensitivity to Red,
Green OR Blue
Green, Red and Blue Cones
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Cones have a
sensitivity to Green,
Red and Blue
–
Note that to a lesser
extent, our cones
can detect other
colors
Primary Colors
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Primary colors of light are Red, Green and Blue
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Additive Colors
Mixing lights adds
colors
Adding primary colors
gives ‘secondary’
color in between
Adding all three primary
colors gives white
Secondary Colors
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Secondary colors are
Magenta, Yellow and Cyan
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Colors that absorb
(subtractive) colors are
pigments
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Absorbs a primary color,
reflects two (2) primary colors
Primary “paint” colors
(pigments)
Adding secondary colors
gives ‘primary’ color in
between
Mixing all secondary colors
gives
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Black
MIRRORS
Reflection Definitions
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Object Distance:
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the distance from the mirror to the object. Positive ‘in front of mirror’
Image distance:
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the distance from the mirror to the image.
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Nature of image
– real - inverted and able to projected on a screen; in front of
mirror
– virtual - right-side-up and NOT able to be projected on a
screen; in ‘back’ of mirror
Size of Image
– enlarged – image is larger than object
– reduced – image is smaller than object
– true – image is same size as object
Orientation of Image
– upright – image points in same direction as object
– inverted - image points in opposite direction as object
Reflection on Plane Mirror
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Image forms where
all rays converge
Images are:
Virtual
 Upright
 Same Size
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(more) Reflections Definitions
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Focal Point:
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The point where parallel rays meet (or appear to
meet) after reflecting from a mirror. The distance
from the focal point to a mirror is called the focal
length.
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The focal length of a converging mirror is always
positive (in front of the mirror)
The focal length of a diverging mirror is always negative
Law of Reflection
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Reflected Rays bounce off at the same angle
as the Incident ray
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(as measured from the normal)
Image formation is where the rays cross
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i.e. Our eyes perceive the image at this location
Mirror Types
Plain/Flat
Converging/Concave
Diverging/Convex
Mirror Types
Reflected Rays
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Parallel Rays go through focal point
Lines through focal point reflect parallel
Rays through center reflect at incoming
angle
Optics Refresher
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Ray Model of Light
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Light is either absorbed
or reflected
When it is reflected, it is
reflected in all directions
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(expanding outwardly like
a sphere – similar to
sound)
We can think of this as
individual ‘rays’ of light
travelling in straight lines
in all directions
Reflection
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Reflection
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When light bounces off a surface.
Rough surfaces reflect light rays in many directions.
Diffuse reflection
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Incident ray
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Causes a blurry image or no image.
Ray hitting the surface
Reflected ray
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Ray bouncing off the surface
Reflection
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Normal
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Angle of Incidence
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Angle between incident ray and the normal.
Angle of reflection
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An imaginary line that is perpendicular to the surface
the light is reflecting from.
Angle between the reflected ray and the normal.
The angle of incidence equals the angle of
reflection.
About Diverging (convex) Mirror
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These
characteristics
always true for
diverging mirrors.
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Virtual image
Upright
Smaller
Terms of Mirrors
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Principal Axis (A)
– Optical axis
Center of Curvature (C)
- Center of sphere
Radius of Curvature (R)
– distance of C to mirror
Focal Point (F)
– Focal Length ( f )
– R=2*f
Equations for Curved Mirrors
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f = focal length
do = distance to
object
di = distance to
image
All distances are positive in front of
mirror and negative behind mirror.
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Magnification
hi = height of image
ho = height of object
Understanding Image
d
f
m
h
+
Real
-
virtual
+
Concave
-
convex
+
Upright
-
inverted
>1
enlarged
<1
reduced
=1
Same size
+
Upright
-
inverted
EX 4
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Wendy the witch is polishing her crystal ball. It is so shiny that she can see her reflection when she gazes into
the ball from a distance of 15 cm. a) What is the focal length of Wendy‘s crystal ball if she can see her reflection
4.0 cm behind the surface? b) Is this image real or virtual?
EX 5
EX 6
Answer: a) -10 cm
b) smaller
EX 7
Answer: -13.3 cm
EX 8
Answer: a) -36 cm
b) Yes, it would change the image distance
Drawing Ray Diagrams
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Helps predict characteristics of the image
Concave Mirror
Convex Mirrors
Image is virtual, upright and reduced
Ex 1
Convex
(diverting)
Mirror
1. Draw a ray parallel to the principal
axis.
2. Where this ray crosses the mirror,
draw a reflected ray through the
focal point.
3. Draw a ray toward the focal point.
4. Where this ray hits the mirror, draw
a reflected ray parallel to the
principal axis
5. Where the two reflected rays cross,
draw the reflected image.
6. Optional: Draw the midpoint ray
through the center of the mirror.
This ray will reflect at the same
angle as it came in (law of reflection)
Ex 2
Concave
(converging)
Mirror
Ex 3
Concave
(converging)
Mirror
When an object is
located at the focal
point of a concave
mirror NO image will
form.
Ex 4
Concave
(converging)
Mirror