light the facts of light – notes light intro shadows
TRANSCRIPT
Light
The Facts of Light – Notes
Light Intro
Shadows
Shadow
A book of height 20 cm is held 50 cm from the light and the wall is at a distance of 125 cm to the light. Find the size of the shadow.
ds =
What is Light?
• ELECTROMAGNETIC WAVES.
http://hawkins.pair.com/radiation/ewaves.gif
Electromagnetic Waves
Copywrited by Holt, Rinehart, & Winston
Light is
Produced when:• An electric charge is accelerated• Molecules, atoms or atomic nuclei
make transitions to lower energy states.
Cutnell & Johnson, Wiley Publishing, Physics 5th Ed.
Electromagnetic Spectrum
• What are other types of electromagnetic waves?
Cutnell & Johnson, Wiley Publishing, Physics 5th Ed.
Spectrum
Speed of Light
• Light travels at 3.00 x 108 m/s in a vacuum.
• Call this “c” – for constant speed.
What can cause the speed of light to change?
Passing through a transparent (clear) medium.
http://www.art.com/products/p12817857730/product.htm?RFID=217825&ProductTarget=40470995847&gclid=COnGmZyvkrkCFbE7MgodgAoAzg
ROY G. BIV
EM Spectrum l ( m)Visible Light 400-700 nm
• Red 674 nm• Orange 616 nm• Yellow 580 nm• Green556 nm• Blue 458 nm• Indigo 410 nm• Violet 400 nm
nanometer (nm) = 1.0 x 10-9 mhttp://www.ems.psu.edu/Courses/earth002/03012D.gif
Cutnell & Johnson, Wiley Publishing, Physics 5th Ed.
Transparent
• Transparent materials transmit light – allowing it to pass through undistorted.
• Notice that glass blocks both infrared and ultraviolet light, but is transparent to all the frequencies of visible light!
Visible Light
• Translucent materials transmit light – but distort it, which means objects can’t be seen clearly.
• Opaque materials do not transmit light. They either reflect or absorb the light.
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http://sprout.site.ne.jp/photo/archives/images/karasuma.jpg
Light & Materials
Principle of Least Time
• Light travels in straight lines.• Among all possible paths for light
to go from one point to another, light will take the path that requires the shortest time.
This idea is the basis for Laws of Reflection
http://jeremy.lwidof.net/downloads/beam-3.jpg
Light Behavior
What happens when light strikes a new surface (medium)?
1. Bounce off of the new medium - REFLECTION
2. Go through the new medium and bend as it changes speed - REFRACTION
3. Be ABSORBED and changed into heat
http://www.rpi.edu/dept/phys/ScIT/InformationTransfer/reflrefr/rr_content/images/refllaw.gif http://www.kfs.oeaw.ac.at/oeal/corr/10_Brechung2b.gif
http://www.glossary.oilfield.slb.com/files/OGL98151.gif
Light bounces off Homer
Light enters the eye of an observer
Light is produced by a luminous
object
How Can We See Homer?
If light bounces off Homer in only one direction…
…Homer can only be seen from one direction
This observer can see Homer
This observer receives no light from Homer so cannot see him
Why Can We See Homer in All Directions?
When light hits a rough surface, it is scattered in many directions, so…
… objects can be seen from many directions.
Light Scattering
Sealed box with non-reflective coating inside (preferably
black)Very
small hole(pin
hole!)
Translucent screen (e.g.
tracing paper)When the camera is pointed towards a bright object, an image of the object
appears on the screen
What is a Pinhole Camera?
How does a pinhole camera work?
Every point on Homer’s head will scatter light in many
directions, however…
… only one ray of light from each point on Homer’s head is travelling in
precisely the right direction to pass through the pinhole
Also…
… only one ray from each point on Homer’s foot will be
travelling in precisely the right direction to enter the pinhole
Light is scattered as it passes through the
translucent screen, so…
Pinhole Camera
Image is inverted
… we see an image on the screen
Light scattered by screen
Object
Pinhole Camera
Ray diagrams show the path of rays of light from an object.
They can be tricky to draw, so the best thing is to…
CHEAT!!!
Drawing Ray Diagrams for Pinhole Cameras
… so that the rays cross in the pinhole
Step 1: Draw your object
Step 3: Draw in rays from top and bottom of your object
Step 4: Draw in rest of pinhole camera …
Step 2: Draw your inverted image and your translucent screen
Pinhole Camera
4. Light always reflects off an object at the same point that it hits the object.
5. When using a diagram to explain how somebody sees something – show the ray of light entering the eye
2. Always use an arrow head to show which direction the rays of light are travelling in
1. Always use a ruler – light travels in straight lines
3. Rays of light should touch the objects that they are reflecting off.
Rules for Drawing Ray Diagrams
Pinhole Camera
Pinhole Camera
Pinhole Camera
Similar Triangles
Pinhole Camera
Using your pinhole camera, measure the height of an apartment building that is 320 m away.
Ray
• An arrow drawn on a diagram to show the direction of propagation of a set of waves
• A ray is always at 90° to the wavefront.
wavefront
ray
Refraction
• Change in direction of a wave• When waves travel across a boundary
between two different media (substances), their speed changes.
• The change in speed
can result in a change
in direction of
propagation of the
waves. Refraction ht
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Skateboarding on Grass
Think about a skateboard example:• If a skateboard is rolling from
pavement to this triangular plot of grass, show its path both into and out of the grass.
• If the grass plot were a piece of glass, and the skateboard path were a ray of light, the light would bend the same way due to refraction.
• Notice that the light bends toward the base of the triangle.
Ray Tracing
• Show how a ray of light will bend as it passes through each piece of glass below: (To simplify, the incident ray is drawn straight to the center of the glass. Bend the ray at that point and draw a straight line out of the glass.)
Lens
• If a piece of glass has just the right shape, it can bend parallel light rays so that they all cross – or appear to have crossed – at a single point – focal point
• Such a piece of glass is called a lens.
http://ebiomedia.com/gall/eyes/images/lensRefractBlack300.gif
Lens
• A lens is simply a combination of pieces of glass.
• The combination at the left is thicker in the middle and converges the light (convex lens).
• The arrangement at the right is thinner in the middle than at the edges; it diverges light (concave lens).
Convex Lens
• Convex lens – is a converging lens as it bends the light inward to a real focal point (F). – focal point can be seen– true if lens is properly shaped
Copywrited by Holt, Rinehart, & Winston
Concave Lens
• Concave lens – is a diverging lens as it spreads the light outward away from a virtual focal point (F’).– Focal point cannot be seen, but can be
found by projecting the rays back toward the light source (use dashed lines)
Copywrited by Holt, Rinehart, & Winston
Focal Length
• The focal length (f) is the distance from the center of the lens to the focal point (F).– measured to real focal point then the
focal length is positive– measured to virtual focal point then the
focal length is negative
Lens Power
• The greater the curvature of the lens the greater the power of the lens – more refraction or bending of light.
• Greater the power, the shorter the focal length.
Reflection
• occurs when the new medium is too different from the original medium for the light ray to pass through
• is the process of returning re-emitted light into the medium from which it came.
1st Law of Reflection:
• the angle of incidence is equal to the angle of reflection.
• Both of these angles are measured from a normal (which means perpendicular) to the surface at the point of incidence.
http://www.museumonline.at/1997/schulen/bg10/physik/refr14.gif