2 nd & 3 th N.U.T.S. Workshops Gulu University Naples FEDERICO II University

2nd & 3th N.U.T.S. Workshops

Gulu University

Naples FEDERICO II University

1 - Light Beams, Rays & Shadows

1- Light Beams, Rays & Shadows

2nd & 3th NUTS Workshop ( Jan 2010)

2

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!



3

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.



4

Non-luminous Objects

(… illuminated objects)

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”

Diffused beams



5

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)



6

Extended Light Source

We can imagine an extended light source as a collection of point light sources

Two point light sources look like this !



7

Point and Extended Light Sources

a point light source

an extended light source

1 cm

5 m

1 m

5 m



8

Transparent and Opaque Objects

transparent object

opaque object



9

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 rayVery often Light Ray is used for Light Beam.



10

Point light source

Wall

Beams that ARE blocked by the book

Shadows appear when beams are blocked

Beams that are NOT blocked by the book

What happens to the shadow if we move the book back from the wall ?

unblocked

unblocked

The two parts of the penumbra each get light from only one of the two bulbs. The umbraumbra gets no light from either of the two bulbs. The bright region gets light from both of the bulbs.

Sha

dow

Book

Wall

2 point light sources

AB

umbra

Bookbrightbright

penumbrapenumbra

penumbrapenumbra

brightbright

blocked



11

UmbraUmbra & Penumbra

2 point light sources

AB

umbra

Bookbrightbright

penumbrapenumbra

penumbrapenumbra

brightbright

Wall

The light from A doesn't reach this penumbra

The light from B doesn't reach this penumbra

We can extend the definition of the umbraumbra and penumbrapenumbra to exist in space even without a wall or screen!



12

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.



13

UmbraSun

• 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

A

B

Penumbra

Penumbra

Beams from this part of the sunDO reach the upper penumbra

Beams from this part of the sunDON'T reach the upper penumbrabecause they are blocked by Moon

Solar Eclipse

Moon



14

Solar Eclipse as Viewed from Space



15

Map 1 of Solar Eclipse, Jan 15, 2010



16




17




18

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.



19

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.



20

Extend Source and ObjectAn "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)

Incident beams from a frosted light bulb

MANY reflected beams comefrom each point on Alex.This is diffuse reflection

Many beams from different points on a large frosted lightbulb hit Alex’s nose



21

More Light Beams Reach the Nose, More Brighter It Is

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

LightLightsource 2source 2

LightLightsource 1source 1

Reflected beamReflected beamfrom light 1from light 1

Reflected beamReflected beamfrom light 2from light 2

Your eye sees a brighter nose than with either light source alone



22

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.)

Op

tical

Devic

e

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



23

Virtual Image?

Op

tical

Devic

e

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!

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.



24

Pinhole Camera (using ray tracing)

Pinhole Camera

Light bulb

Image oflight bulb

blocked rays

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

• Notice that this image is upside down and left-right reversed.

A pinhole camera works by blocking beams



25

Ray Tracing

Is the image of Alex smaller or larger than the real Alex?a) Smallerb) Largerc) Same size

Is the image of Alex smaller or larger than the real Alex?a) Smallerb) Largerc) Same size

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!



26

Pinhole Size

Larger is the pinhole, brighter and less resolved is the image ! … optimal

pinhole diameter (Lord Rayleigh):



27

Pinhole Cameras in Gulu

2 nd & 3 th N.U.T.S. Workshops Gulu University Naples FEDERICO II University

Documents

Transcript of 2 nd & 3 th N.U.T.S. Workshops Gulu University Naples FEDERICO II University