1. 1 - Light Beams, Rays & Shadows
2nd & 3th N.U.T.S. Workshops Gulu University Naples FEDERICO II University
1 - Light Beams, Rays & Shadows

2. Vision happens when light enters our eye
2nd & 3th NUTS Workshop ( Jan 2010)
How We See
Vision happens when light enters our eye
Without a source of light, there can be no vision at all;
Light is invisible unless it enters directly into our eye or is scattered by smoke, fog or some object into our eye!
1- Light Beams, Rays & Shadows

3. Luminous objects (… they produce light)
2nd & 3th NUTS Workshop ( Jan 2010)
Luminous objects
(… they produce light)
light from incandescence: the process of emitting light because of high temperature
Light from electrical discharge: the process of emitting light because of electricity passing through a gas
Light from fluorescence: the process of emitting visible light by a substance that has absorbed light of a differing, usually invisible, wavelength.
1- Light Beams, Rays & Shadows

4. Light diffusion is a key phenomenon when discussing “HOW WE SEE”
2nd & 3th NUTS Workshop ( Jan 2010)
Non-luminous Objects
(… illuminated objects)
Diffused beams
Any non mirroring, rough surface scatters or diffuse light, these diffuse beams of light travel and reach the eyes.
Light diffusion is a key phenomenon when discussing “HOW WE SEE”
1- Light Beams, Rays & Shadows

5. 2nd & 3th NUTS Workshop ( Jan 2010)
Point Light Source
A point light source emits light in all directions radially outwards
point-like is any object whose dimensions are smaller than all other distance relevant for the studied process (distance of observation, size of measuring instruments, etc.)
We know that the light gets dimmer as we move further away from the light source. (Think of the sun. It would be blinding if we were closer to the sun)
1- Light Beams, Rays & Shadows

6. 2nd & 3th NUTS Workshop ( Jan 2010)
Extended Light Source
Two point light sources look like this !
We can imagine an extended light source as a collection of point light sources
1- Light Beams, Rays & Shadows

7. Point and Extended Light Sources
2nd & 3th NUTS Workshop ( Jan 2010)
Point and Extended Light Sources
 a point light source
1 cm
5 m
1 m
5 m
an extended light source 
1- Light Beams, Rays & Shadows

8. Transparent and Opaque Objects
2nd & 3th NUTS Workshop ( Jan 2010)
Transparent and Opaque Objects
transparent object
opaque object
1- Light Beams, Rays & Shadows

9. Light Beam Physically we deal with Light Beams!
2nd & 3th NUTS Workshop ( Jan 2010)
Light Beam
Physically we deal with Light Beams!
Ray is a mathematical model, very useful in many representations, it is not a physical object.! What we can realized is a beam of light, not a ray
Very often Light Ray is used for Light Beam.
RAY IS A MATHEMATICAL MODEL, VERY USEFUL IN MANY REPRESENTATIONS, IT IS NOT A PHYSISCAL OBIECT
WHAT CAN BE REALISED IS A BEAM OF LIGHT, NOT A RAY
1- Light Beams, Rays & Shadows

10. Shadows appear when beams are blocked
2nd & 3th NUTS Workshop ( Jan 2010)
Shadows appear when beams are blocked
Point light source
Wall
Wall
2 point light sources A B
unblocked
umbra
Book
bright
penumbra
Beams that are NOT blocked by the book
Shadow
Book
Beams that ARE blocked by the book
blocked
What happens to the shadow if we move the book back from the wall ?
The two parts of the penumbra each get light from only one of the two bulbs. The umbra gets no light from either of the two bulbs. The bright region gets light from both of the bulbs.
1- Light Beams, Rays & Shadows 10

11. 2nd & 3th NUTS Workshop ( Jan 2010)
Umbra & Penumbra
Wall
2 point light sources A B
umbra
Book
bright
penumbra
The light from B doesn't reach this penumbra
The light from A doesn't reach this penumbra
We can extend the definition of the umbra and penumbra to exist in space even without a wall or screen!
1- Light Beams, Rays & Shadows

12. Shadows from Large Sources
2nd & 3th NUTS Workshop ( Jan 2010)
Shadows from Large Sources
Two light sources produce two shadows.
The region of overlap is the umbra.
The penumbra is reached by beams from one of the light source.
More shadows.
1- Light Beams, Rays & Shadows

13. Beams from this part of the sun DO reach the upper penumbra
2nd & 3th NUTS Workshop ( Jan 2010)
Solar Eclipse
All beams coming from point A on the sun between the two dashed rays are blocked by Moon
All beams coming from point B on the sun between the two dotted rays are blocked by Moon
The umbra gets no light from any portion of the sun
The umbra gets smaller not larger further behind Moon since the Sun is larger than Moon
The penumbra gets light from part of the sun
If you look back from the penumbra you can see part of the sun
For regions completely into the umbra there is a total eclipse of the sun.
For regions into the penumbra there is a partial eclipse of the sun
Beams from this part of the sun DO reach the upper penumbra A
Penumbra
Sun
Umbra
Beams from this part of the sun DON'T reach the upper penumbra because they are blocked by Moon
Moon B
1- Light Beams, Rays & Shadows 13

14. Solar Eclipse as Viewed from Space
2nd & 3th NUTS Workshop ( Jan 2010)
Solar Eclipse as Viewed from Space
1- Light Beams, Rays & Shadows

15. Map 1 of Solar Eclipse, Jan 15, 2010
2nd & 3th NUTS Workshop ( Jan 2010)
Map 1 of Solar Eclipse, Jan 15, 2010
1- Light Beams, Rays & Shadows

16. Map 2 of Solar Eclipse, Jan 15, 2010
2nd & 3th NUTS Workshop ( Jan 2010)
Map 2 of Solar Eclipse, Jan 15, 2010
1- Light Beams, Rays & Shadows

17. Map 3 of Solar Eclipse, Jan 15, 2010
2nd & 3th NUTS Workshop ( Jan 2010)
Map 3 of Solar Eclipse, Jan 15, 2010
1- Light Beams, Rays & Shadows

18. 2nd & 3th NUTS Workshop ( Jan 2010)
Medical Use of Shadows
X-rays were fist observed and documented in 1895 by Wilhelm Conrad Roentgen, a German scientist who found them quite by accident.
A week later, he took an X-ray photograph of his wife’s hand which clearly revealed her wedding ring and her bones.
1- Light Beams, Rays & Shadows

19. Shadows Tell Us: A) That light travels in straight lines.
2nd & 3th NUTS Workshop ( Jan 2010)
Shadows Tell Us:
A) That light travels in straight lines.
B) That something is blocking the light.
C) What direction the light is shining from.
1- Light Beams, Rays & Shadows

20. Extend Source and Object
2nd & 3th NUTS Workshop ( Jan 2010)
Extend Source and Object
An "extended object" consists of many small parts that can be treated as “points”. Each point on the object emits or reflects beams in all directions (unless the object is a mirror)
MANY reflected beams come from each point on Alex.
This is diffuse reflection
Incident beams from a frosted light bulb
Many beams from different points on a large frosted lightbulb hit Alex’s nose
1- Light Beams, Rays & Shadows

21. More Light Beams Reach the Nose, More Brighter It Is
2nd & 3th NUTS Workshop ( Jan 2010)
More Light Beams Reach the Nose, More Brighter It Is
Light source 2
This is why regions outside the penumbra and umbra are brighter
These regions get light rays from both point light sources
The more lights you turn on the brighter the reflected light from objects in the room
Light source 1
Reflected beam
from light 1
Reflected beam
from light 2
Your eye sees a brighter nose than with either light source alone
1- Light Beams, Rays & Shadows 21

22. 2nd & 3th NUTS Workshop ( Jan 2010)
Image?
Image : a reproduction or imitation of the form of a person or thing;
the optical counterpart of an object produced by an optical device (as a lens or mirror) or an electronic device. (Webster dict.)
Optical Device
A real image is formed on a screen when some beams from each point on the object reach the corresponding points on the screen and no other beams from other points on the object reach those points
1- Light Beams, Rays & Shadows

23. 2nd & 3th NUTS Workshop ( Jan 2010)
Virtual Image?
If we trace back, with straight lines, the beams coming out from the optical device it seams that beams coming from the same point of Alex are all coming from a different point.
Optical Device
We get a virtual image of the real Alex. It is virtual (fictitious) because there is no light energy there, no real beams reach it, and it cannot be seen by putting a screen at its position. It’s psychology of beams interpretation: we interpret all beams coming into our eye as traveling from a fictitious image in a straight line to our eye even if they are deflected beams!
1- Light Beams, Rays & Shadows

24. Pinhole Camera (using ray tracing)
2nd & 3th NUTS Workshop ( Jan 2010)
Pinhole Camera (using ray tracing)
A real image is formed on a screen when one or more rays from each point on the object reach the corresponding points on the screen and no other rays from other points on the object reach those points
Pinhole Camera
blocked rays
Image of light bulb
Light bulb
Notice that this image is upside down and left-right reversed.
A pinhole camera works by blocking beams
1- Light Beams, Rays & Shadows

25. Finding an image by using rays is called ray tracing.
2nd & 3th NUTS Workshop ( Jan 2010)
Ray Tracing
Finding an image by using rays is called ray tracing.
Trace rays from the object through the pinhole in the camera to find the image rather than trusting your intuition!
Is the image of Alex smaller or larger than the real Alex?
Smaller
Larger
Same size
Is the image of Alex smaller or larger than the real Alex?
Smaller
Larger
Same size
1- Light Beams, Rays & Shadows

26. 2nd & 3th NUTS Workshop ( Jan 2010)
Pinhole Size
Larger is the pinhole, brighter and less resolved is the image ! … optimal pinhole diameter (Lord Rayleigh):
1- Light Beams, Rays & Shadows

27. Pinhole Cameras in Gulu
2nd & 3th NUTS Workshop ( Jan 2010)
Pinhole Cameras in Gulu
1- Light Beams, Rays & Shadows