light and shadokwng/spring2011/phy160/lecture materials/light_a… · start from simple - point...
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Human sensationFor us to see an object, the object has to send light
Light
j , j gto our eye.
Light
Sound
Smell
TouchTouch
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Where light comes from
• Some objects can radiate light by itself.
• Some objects reflect light from other sources.
If an object cannot radiate light itself, and there is no other li ht th bj t t blight source, the object cannot be seen.
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Start from simple - Point object
• A point object is an object small as a point (i.e. no size).
• An extended object is composed of
i t bj tmany point objects.
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Light from a point objectIf a point source can give out light by itself or if there isIf a point source can give out light by itself, or if there is
other light source:
The point object willThe point object will send out light rays in all directions.
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Properties of Light RaysProperties of Light Rays• A light ray is a perfect straight line. (ThisA light ray is a perfect straight line. (This
provides the geometric basis of the theory)• A light ray originates from a point object• A light ray originates from a point object.• A light ray extends indefinitely until it hits
( ) b t lan (opaque) obstacle.• For a point object to be seen, one of its light
rays has to hit the eye.
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Conditions for a point object toConditions for a point object to be seen
• Either the point object can radiate light by itself or there are other light sources.g
• There is no opaque obstacle blocking the straight line joining the point object and thestraight line joining the point object and the eye.
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An Example
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Seeing light ray?
• Light ray is a conceptual entity that we intend to represent it as a straight line in our drawing.U lik h i l i h li li h b• Unlike a physical straight line, a light ray cannot be seen from the sideway.
• When a light ray hits our eye, we will see the object thatWhen a light ray hits our eye, we will see the object that emit the light ray.
Light ray from a point source
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Light beam
A beam
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Light beam
• A beam is a bundle of light rays.
• A beam can be outlined by extreme
Beam from a flash light
rays.
Beam from a laserBeam from a laser
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Light beam
A beam
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Beam of Light
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Beam Scattered (or Reflected) byBeam Scattered (or Reflected) by Particles
Beam from a flash light
The original beam serves as a light source The particles in theThe original beam serves as a light source. The particles in thebeam will become lighted point object. What we are seeing arethe lighted particles outlining the beamthe lighted particles outlining the beam.
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In summaryA i t ith di t li ht• A point source can either radiate light rays from itself, or reflect light rays from other sources.
• A point source send out light rays in all directions.
• For the point source to be seen, one of these plight rays has to reach one of our eyes.
• Light ray travels in straight line.Light ray travels in straight line.
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Point source and a screen
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Point source and a screen
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With a mask in between
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E t RExtreme Rays
E tExtreme rays
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With a diff t hdifferent shape
hole
Extreme rays
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SummaryF i t lFor a point source only:
• Point source send out light rays in all directions.• Light rays travel in straight line.• Extreme rays formed by straight lines joining the point source to the
edge of the hole.• Only rays enclosed by the extreme rays can go through the hole and hitOnly rays enclosed by the extreme rays can go through the hole and hit
the screen.• Rays outside the region bound by the extreme rays will be blocked by
the mask.e as .• Image on screen is a projection of the hole: they have the same
orientation.
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Schematic Diagram
T’Extreme rays
T
Point sourceB
B’
Mask Screen
B
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Example: Moving point sourceExample: Moving point source towards the mask
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Example: Moving point sourceExample: Moving point source towards the mask
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Example: Moving point sourceExample: Moving point source towards the mask
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Extensive source
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Extensive source
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Different hole shape
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Different hole shape
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Summary
• An extensive source is formed by many many point source.
• Every point source project a spot of the same shape and orientation as the hole of the mask.
• The spot size determine how blur is the image on the screen.
• Projection on the screen is an inverted image of the extensive light source.
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Inverted imageInverted imageScreen
Mask ??
M
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Inverted imageInverted imageScreen
Mask
M
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Inverted imageInverted imageScreen
Mask
MMM
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Schematic diagramSpot size
Extensive source Mask
Screen
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Sh dShadowScreen
P i tPoint source
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ShadowF i t lFor a point source only:
• Point source send out light rays in all directions.• Light rays travel in straight line.• Extreme rays formed by straight lines joining the point source to the
edge of the object forming the shadow.• Rays enclosed by the extreme rays will be blocked by the object.Rays enclosed by the extreme rays will be blocked by the object.• Only rays outside the region bound by the extreme rays can pass and
hit the screen.