Light
K Warne
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Light Rays Give evidence to show that light travels in straight lines
• Draw ray diagrams to represent the way in which light travels.
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Reflection of Light
• Light is reflected
(bounced) off
surfaces.
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Vision
• Objects are seen when
light reflects off them
into our eyes.
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Every day Refraction
Refraction is seen in a
number of everyday
circumstances.
• Water mirage on
roads.
• Distortion of
images viewed
through a glass
prism.
• Distortion of
objects viewed in
water.
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Passage of light through a glass block
A
B
AB is the incident ray.
The emerging ray is parallel to the
incident ray if the block is a rectangle.
Indicate incident rays
Indicate incident angles and
refracted angles on your
diagram.
i
r
Angle of incidence
Angle of refraction
Refracted ray
Emergent ray
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Passage of light through a glass block
On entering the glass (more
optically dense) block the light is
refracted TOWARDS the normal.
On exiting the glass (more
optically dense) block the
light is refracted AWAY from
the normal.
A
B
AB is the incident ray.
“i” is the incident angle i
rThe light ray is refracted
(bent) because :
• it enters a medium of
different optical density
• At an angle (not 90o)
BC is the REFRACTED
ray. “r” is the angle of
refraction.
C
D
CD is the
EMERGENT ray.
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Refraction of waves
• Waves ……………
on entering a …….
………. medium.
• If the wave strikes
this medium at ….
……….. it then
changes ………….
Normal line
90° to the
surface
The wave bends …………………………….. on entering a
more dense medium at an angle.
The waves bend …………………………….. when exiting a
more dense medium.
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Passage of light through a glass block
On entering the glass (more
optically dense) block the light
is refracted TOWARDS the
normal.
On exiting the glass (more
optically dense) block the
light is refracted AWAY
from the normal.
A
B
AB is the incident ray.
“i” is the incident angle i
rThe light ray is refracted
(bent) because :
• it enters a medium of
different optical density
• At an angle (not 90o)
BC is the
REFRACTED ray.
“r” is the angle of
refraction.
C
D
CD is the
EMERGENT ray.
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“Mirage” on Road
A “mirage” is simply an image of the sky that is
formed by the refraction of light through air of
varying densities due to temperature gradients.
Sky
Hot air
cold air
Light appears
to come from
road
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Real and apparent depth
Objects viewed in
water………………
……………………
……………………
……………………
…In reality the
object is not seen at
all - only the image!
Draw in the light rays - to show the formation of the image.
Label the object, image, real depth and apparent depth.
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Archer Fish• HW: Draw a ray diagram to show the refraction effects that must be taken into
account by an archer fish if it is to “shoot” down an insect outside the water
effectively.
• THE FISH MUST AIM LOWER THAN
• THE OBJECTAPPEARS! AIM
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Real and apparent depth – HW 2
• Draw the
position of the
image.
• Show by means
of light rays why
the object
appears
shallower than it
is.
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Total internal reflection
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Optical Fibres
• The light in a fiber-optic cable travels through the core (hallway) by constantly
bouncing from the cladding (mirror-lined walls), a principle called total
internal reflection. Because the cladding does not absorb any light from the
core, the light wave can travel great distances.
• However, some of the light signal degrades within the fiber, mostly due to
impurities in the glass.
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Triangular Prisim
ir
dN
i=angle of incidence
r=angle of refraction
d=angle of deviation
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Converging Lenses
Parallel light rays
Parallel to principle axis
Parallel light rays
Refracted towards a
centre point – we say
they converge to a point.
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Diverging Lenses
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Ray Diagrams – Centre line
Principle axis
In a thin lens the displacement is negligible. The rays are drawn
as if refracting from the center line.
.F
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Ray Diagrams - Optical Centre
Principle axis
Light rays passing along the
……………….. are ………
…………………… as they
strike both surfaces at 90°
Any ray
passing
through the
…………..
………….
can be
thought of as
passing
……………
…………….
In a thin lens the displacement is negligible.
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Ray Diagrams -Focal Point
…………………….
… ....
F ……...
………..
•
Any ray striking the lens ………… to the principle axis is
refracted through the …………….. point.
These values are the ………… for either side of a ………...………… lens.
Parallel to ………………… ………..
…………….
…….•••
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Diverging Lenses
F
F -…….
……….
……. …………..•
In a diverging lens the rays …………………. to have
originated at the ……………… point.
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Ray Diagrams
• Three rays can be used to find the image of any lense.
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Light PracticalPart I
Measure the angle of incidence when the angle of refraction is 90°
Part II
• Optical bench – measure: the distance from the object to the lens (Do ),
height of the object (Ho) and the distance from the image to the lens
(DI) and the height of the image (Ho ).
Write up method – and give a diagram!
RESULTS
Do (Ho) Di Hi (relative to Ho) .
1 >2F between 2F and F diminished (smaller)
2 At 2F
3 2F > x > F
4 At F
5 < F
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Light Practical
ObjectImage
Do Di
HoHi
Do greater than 2F
Do equal to 2F
Do less than 2F
F= 50cm
Ray diagrams p 39
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Light Practical
Object ImageDo Di
Ho
Object further than 2F
Image:
• Between F & 2F
• Inverted
• Real
• Diminished
F•
F•
2F•
2F•
HI
Magnification = = = 0.44Hi
Ho
1.9
4.3
Magnification = = = 0.5Di
Do
3.8
7.6
Do greater than 2F, Do equal to 2F, Do between 2F & F, Do equal to F, Do less than F
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Compound Microscope
F•2F F F 2F
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Eye - Defects
This is commonly
called short
sighted.
Object Far away
Distant objects
the lens is not
stretched
enough. (Too fat)
Rays are focused in
front of retina are
blurred.
Object close up are
focused clearly.
A diverging lens
would cause distant
rays to focus on the
retina and normal
sight is restored.
Short Sighted
Corrective glasses
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From World Book © 2002 World Book, Inc., 233 N. Michigan Avenue, Suite 2000, Chicago, IL
60601. All rights reserved. World Book i llustrations by Charles Wellek
From World Book © 2002 World Book, Inc., 233 N. Michigan Avenue,
Suite 2000, Chicago, IL 60601. All rights reserved. Nikon, Inc.
Eye vs Camera
ENLARGE
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Hi -
This is a SAMPLE presentation only.
My FULL presentations, which contain loads more slides (with all the gaps filled in) as well as
other resources, are freely available on my resource sharing website:
www.warnescience.net
(paste into your browser if link above does not work)
Have a look and enjoy!
Keith Warne