mirror and lens - my blogpositive, in front of mirror) • virtual (d i negative, behind the mirror)...
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Mirror and Lens
Thursday, March 26:
Quiz on Mirrors
Thursday, April 2:
Test on Mirrors and Lenses
Reflection
Reflection occurs when a wave reaches a boundary and it bounces back into the first media as it strikes that boundary.
It may be:
Total - all of the wave’s energy reflects back. Ex: light hitting aluminum, chrome, other metals
Partial – some of the wave’s energy reflects back. Ex: light waves hitting water or glass
Law of Reflection
Waves bounce back at the same angle at which they arrived. Direction of waves are shown using straight line rays. Incident and reflected rays make equal angles perpendicular to a surface called the normal.
Angle of incidence = angle of reflection (as measured from the normal)
Normal
Diffuse Reflection Normal reflection is called specular reflection.
Diffuse reflection is when light is reflected in
many directions from a rough surface.
Most things around us are seen by diffuse reflection, such as the light which reflects from a book you read, because the surface of ordinary paper as seen through a microscope in is actually quite rough.
Terminology
An object is a source of diverging
light rays. The object can send out its
own light rays (like a light bulb) or can
reflect light.
The term upright mean “right side up”
and inverted means “upside down”.
Mirrors
3 main types:
Plane (flat mirror like in the bathroom)
Concave (inwardly curved like make-up and
shaving mirrors)
Convex (outwardly curved like car and
grocery store mirrors)
Plane Concave Convex
How Parallel Light Rays
Reflect
When parallel light rays are incident on a
mirror, they can reflect with 3 possible
options:
Remain parallel (only happens with a plane
mirror)
Converge (all rays eventually meet at a
single point) happens with a concave mirror
Diverge (all rays spread away from each
other) happens with a convex mirror
Parallel, Converge, or
Diverge?
Parallel Converge Diverge
Luminous vs. Illuminated
Luminous Objects
give off light
Illuminated Objects
reflect light
Characteristics of an Images
For all images, you must be able to determine the following relative to the object:
Size • Reduced
• Same Size
• Enlarged or Magnified
Orientation • Upright
• Inverted
Type of Image • Real – light rays converge
• Virtual – light rays diverge
Plane Mirror Objects seen appear to be
somewhere behind mirror. Eye sees the reflected light and extrapolates its path back to a point behind the mirror where it appears to originate.
What you see is called a VIRTUAL IMAGE.
Plane mirrors cannot produce a REAL IMAGE because parallel light rays that strike the mirror always reflect parallel to each other.
Reflected light rays must intersect in order to form a REAL IMAGE.
Reflection is
on top of
incident light
For a Plane Mirror…
object size = image size
Image orientation is the same as
object (no inversion). All mirrors
exhibit a left to right flip.
object distance in front of the mirror =
image distance behind the mirror
Convex vs. Concave Mirrors
Concave Mirrors
If object is placed beyond the focal point of concave mirror, all light rays from a single point on object intersect at a single point upon reflection.
Light rays converging on a single point in real space will produce a REAL IMAGE because the light rays appear to be radiating from that point as they continue onward.
Concave Mirrors also…
Can create a VIRTUAL IMAGE:
If object is placed closer to mirror than focal point, reflected light rays diverge. Observer would see a virtual image located somewhere behind the mirror because the light appears to originate from that point. Virtual images formed by concave mirrors are larger and farther away from the mirror than the object is.
NO IMAGE is created at all when an object is placed exactly at the focal point, the reflected light rays run parallel to each other. Ex: Instead, would see a wide beam of parallel light like that of flashlight or car headlights or a completely blurry object.
Convex Mirrors
Always produce virtual images
Virtual images formed by convex
mirrors are smaller and closer to the
mirror than the object is. The image is
always oriented the same as the
object.
Lens/Mirror Equation
(units of length MUST match)
f stands for focal length
di is the distance from the mirror/lens to the image
do is the distance from the mirror/lens to the object
f (di1 do
1)1
di ( f1 do
1)1
do ( f1 di
1)1
Image Height
Magnification of an image can be
found by a series of ratios between
the distances of objects and their
images and their respective heights
as shown below.
hi
hodi
do
Helpful Hints for Mirror Probs
In the formula, the numbers can tell you the characteristics of the image:
Size • Reduced ( |hi| < ho )
• Same Size ( |hi| = ho )
• Enlarged ( |hi| > ho )
Orientation • Upright(hi positive)
• Inverted (hi negative)
Type of Image • Real (di positive, in front of mirror)
• Virtual (di negative, behind the mirror)
Refraction of Light
Light changes direction (bends) as it crosses a
boundary between 2 media in which the light
moves at different speeds.
Amount of refraction of light depends on
properties of media (material type,
temperature or density) and angle at which it
hits the boundary.
Examples of Light Refraction
Pond or pool looks
shallower than it actually
is
Straw or spoon in a glass
appears bent
White light comes out of
prism as rainbow
Air above hot stove seems
to shimmer
Stars twinkle
More on Refraction of Light
Light waves travel faster in air than in water and slower in glass than water.
More dense = slower light
When light enters a different medium, speed changes and it bends.
Bending of light due to change in speed = REFRACTION
Fiber Optics
Fiber optics use
total internal
reflection.
Light is totally
internally reflected
over and over
many times due to
refraction.
Advantages of Fiber Optic
Technology
Used to get light to inaccessible places such as car engines, inside a patient’s body, and in communications transmitting telephone messages – replacing electrical circuits and microwave links in communication technology
Can carry more info in high frequencies of visible light than in lower frequency electrical current
Thin glass fibers replace bulky expensive copper cables – more practical in weight, size, cost
Lenses
A lens is made of transparent material, such
as glass or plastic, with an index of refraction
larger than that of air, causing light to bend
(refract) as it passes through it.
A lens has a curved surface on one or both
sides.
Plano-convex
Double-convex
Plano-concave
Double-concave
Types of Lenses
Convex vs. Concave Lenses
A convex lens causes parallel light rays to
eventually converge and a concave lens
causes parallel light rays to eventually diverge.
Convex Lens: Beyond 2F
Image is real, inverted, and reduced.
Check Line
Convex Lens: @ 2F
Image is real, inverted, and same size.
Check Line
Convex Lens: Between FP and 2F
Image is real, inverted, and enlarged.
Check Line
Convex Lens: @FP
No image is formed.
Check Line
Convex Lens: Between FP and Lens
The refracted light rays diverge. The image forms on the same side of the lens at the object. The image is virtual, upright, and enlarged.
Check Line
The Concave Lens
Rays diverge after they hit the lens. Image will
always be virtual, upright, and reduced.
Check Line
Applications that Use Lenses
Hand lenses/Magnifying glasses
Projectors
Refracting telescopes
Binoculars
Cameras
Microscopes
Corrective Eyeglasses and Contact lenses
Nearsightedness
Nearsightedness
(Myopia) occurs when
the eyeball is too long,
so focal length is too
short.
Image forms in front of
the retina; causes
distant objects to be
blurry.
Corrected by a concave
lens that forces light to
diverge to a farther point
on back of retina.
Farsightedness
Farsightedness (Hyperopia) occurs when the eyeball is too short so focal length is too long, also happens with aging as muscles holding the shape of lens relax and allow it to flatten.
Image forms behind wall of the retina; causes objects located close to the eye to become blurry.
Corrected by convex lens that forces light to converge at a closer point on the back of retina.
Mirror Image Chart
Object
Placement/Mirror
Size of image
compared to size of
object
Real or Virtual?
Sign of di? Upright or Inverted?
Sign of hi?
Plane Mirror same virtual so -di upright so +hi
Btwn Concave
Mirror and F larger virtual so -di upright so +hi
At F of Concave
Mirror NO IMAGE IS FORMED
Beyond F of
Concave Mirror same, smaller, or
larger real so +di inverted so -hi
Convex Mirror smaller virtual so -di upright so +hi
Lens Image Chart
Object
Placement/Lens
Size of image
compared to size of
object
Real or Virtual?
Sign of di? Upright or Inverted?
Sign of hi?
Btwn Convex Lens
and F larger virtual so -di upright so +hi
At F of Convex
Lens NO IMAGE IS FORMED
Beyond F of
Convex Lens same, smaller, or
larger real so +di inverted so -hi
Concave Lens smaller virtual so -di upright so +hi