Transcript Light and Optics - Net Start Class

Mirrors & Reflection
Light
We see objects because of reflected light
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Travels far and fast (3 x 108 m/s)
Travels in a straight lines, called rays
Reflection Review
Law of reflection: θi = θr
 Light rays bounce off a
mirror at the same angle at
which they arrive
 We always define angles
relative to the normal (the
line perpendicular to the
mirror (or lens)
Types of Mirrors
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Plane Mirror –
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Concave Mirror –
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A flat mirror that reflects light in a regular way
Reflects light from inner surface
Light rays are reflected so that they “come together” at a
point; a converging mirror
Convex Mirror –
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Reflects light from outer surface
Light rays are reflected so that they “go apart” and never
come to a point; a diverging mirror
Object vs. Image
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Object – the source of light rays
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Ex: you when you look in the bathroom mirror
Image – reproduction of object formed with
lenses or mirrors.
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The image is formed at the intersect of the
reflected rays.
Kinds of Images
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Real images
 Formed by converging light rays
 Can be projected on a screen
 Inverted orientation
Virtual images
 Formed by diverging light rays
 Cannot be seen on a screen
 Erect orientation
Reflection & Mirrors
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A mirror reflects rays of light so that they change
their path
Mirrors can create a virtual image
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Image appears behind the mirror and is reversed
The light rays are reflected back to your eye at an equal
but opposite angles
Incident ray – the ray that comes from the object
and hits the mirror
Reflected ray – the ray that bounces off the mirror
Objects, Images & Plane Mirrors
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Plane mirror – flat, smooth surface that
reflects light in a regular way
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Ex: your bathroom mirror
Object
Image
Things to know about Plane Mirrors
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Object size = Image size
Object distance = Image distance
Orientation = Upright
Forms a virtual image
Image is reversed (left to right)
Drawing Ray Diagrams – Plane
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A ray striking perpendicular to the surface
(parallel to normal) will reflect perpendicular
to the surface; the reflected ray is extended
behind the mirror
A ray striking at any angle will reflect so that
θi = θr; the reflected ray is extended behind
the mirror to form the image
Refraction & Lenses
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A lens uses refraction to cause light to come
together or spread apart
Refraction – The bending of light as a result
of light crossing a boundary between two
different media
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EX. Glass, Plastic, Water
Lens – a transparent optical device that is
used to converge or diverge light rays (bend
light)
Lens Types
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Convex Lens (converging lens) – bend
parallel light rays passing through them
inward toward the focal point
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Thicker in the center than the edges
Light travels slower in the thick center
Focal length (f) is always positive
Things to know about Convex lenses…
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If the object is outside the focal point, it is
real & inverted
If the object is at AT the focal point, no
image is formed
If the object is inside the focal point, the
image is upright & virtual.
Let’s take a look…
The lens equation
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Mathematical prediction of image location
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1/f = 1/di + 1/do
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f = focal length
di = image distance
do = object distance
Conventions to Know:
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Focal Length
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Object Distance:
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f is positive for convex lenses
do is positive for REAL objects
do is negative for a virtual object*
Image Distance
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di is positive for a real image (image on other side
of lens)
di is negative for a virtual image (image on the
same side of the lens)
Example 1
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An object is placed 35 cm from a convex lens
with a focal length of 20 cm.
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How far is the image from the lens?
What type of image is formed?
Example 2
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A virtual image is formed 20 cm from a
convex lens having a focal length of 20 cm.
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How far is the object from the lens?
What is the orientation of the image?
Drawing Ray Diagrams - Lenses
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2)
3)
Incident light rays parallel to the principal axis
of a lens are refracted through the focal point
(F)* (ray 1)
Incident rays that pass through the secondary
focal point (F’) are refracted parallel to the
principal axis (ray 2)
Incident rays passing through the center of
lens are not refracted (ray 3)
F’
F
F’
F
F’
F
F’
F