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

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

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!

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.

2nd & 3th NUTS Workshop ( Jan 2010) Diffused beams 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”

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)

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

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

2nd & 3th NUTS Workshop ( Jan 2010) transparent object opaque object Transparent and Opaque Objects

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.

2nd & 3th NUTS Workshop ( Jan 2010) Shadows appear when beams are blocked Wall Wall unblocked bright penumbra A umbra Shadow B 2 point light sources penumbra Point light source Book Book bright unblocked Beams that are NOT blocked by the 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 thepenumbraeach get light from onlyoneof the two bulbs. Theumbra gets nolight from eitherof the two bulbs. The bright region gets light from bothof the bulbs.

2nd & 3th NUTS Workshop ( Jan 2010) Wall bright penumbra A umbra B 2 point light sources penumbra Book bright Umbra & 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 theumbraand penumbrato exist in space even without a wall or screen!

2nd & 3th NUTS Workshop ( Jan 2010) Shadows from Large Sources Two light sources produce two shadows. The region of overlap is theumbra. The penumbra is reached by beams from one of the light source. More shadows.

2nd & 3th NUTS Workshop ( Jan 2010) Beams from this part of the sunDO reach the upper penumbra Umbra Penumbra Penumbra Beams from this part of the sunDON'T reach the upper penumbrabecause they are blocked by Moon 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 A Sun Moon B

2nd & 3th NUTS Workshop ( Jan 2010) Solar Eclipse as Viewed from Space

2nd & 3th NUTS Workshop ( Jan 2010) Map 1 of Solar Eclipse, Jan 15, 2010

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.

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.

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 reflectsbeams in all directions (unless the object is a mirror) MANY reflected beams comefrom 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

2nd & 3th NUTS Workshop ( Jan 2010) 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 Your eye sees a brighter nose than with either light source alone Lightsource 2 Lightsource 1 Reflected beam from light 1 Reflected beam from light 2

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

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!

2nd & 3th NUTS Workshop ( Jan 2010) blocked rays 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 Image oflight bulb Light bulb • Notice that this image is upside down and left-right reversed. A pinhole camera works by blocking beams

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

2nd & 3th NUTS Workshop ( Jan 2010) Pinhole Size Larger is the pinhole, brighter and less resolved is the image ! … optimal pinhole diameter(Lord Rayleigh):

2nd & 3th NUTS Workshop ( Jan 2010) Pinhole Cameras in Gulu

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