K.7 SOL The student will investigate and understand that shadows occur when light is blocked by an object. Key concepts include *shadows occur in nature when sunlight is blocked by an object; and * shadows can be produced by blocking artificial light sources.
5.3 SOL The student will investigate and understand basic characteristics of white light. Key concepts include * the visible spectrum, light waves, reflection, refraction, diffraction, opaque, transparent, translucent; *optical tools (eyeglasses), lenses, flashlight, camera, kaleidoscope, binoculars, microscope, light boxes, telescope, prism, spectroscope, mirrors); and * historical contributions in understanding light.
Reflection returns n light returns after striking an object n the angle of reflection equals the angle of incidence
Angles of Reflection Normal, at 90 o from surface
Angles of reflection n Angle of incidence = Angle of reflection n Normal n Normal is an imaginary line perpendicular to the reflecting surface
Activity: Reflection n Bouncing light off mirrors u using a flashlight and mirror in a dark room. Place a mirror on the floor and shine the light on the mirror at an angle. Hold a book in the path of the reflected light. Using a protractor can you measure the angle? (Ticotsky, p 30) n The Case of the Vanishing Reflection u Carefully cut 8 - 10 inches of foil and look at your reflection. Crinkle the foil and flatten it again. Can you still see your reflection? (Churchill, p 95)
Refraction bent different density. n Light is bent when it traveled from one medium to another of a different density. n Snell(1621) The law of refraction u When the light passes through a denser medium, the light is bent toward the normal, because the light slows down (velocity decreases). u When light passes through a less dense medium the light is bent away from the normal.
AIR WATER GLASS AIR Refraction occurs at the interface between two mediums Water more dense than air--> Air less dense than glass -->
Activity: Refraction n You can see this effect when you place a coin under a clear glass. Look straight down, what do you see? Now look from an angle. What happens? n Turn this around. Place a coin in a bowl and move so that you can not see it. Have your partner slowly add water to the bowl, what happens
Diffraction edge bends n light that passes very close to an edge bends Theoretical Diffracted light Obstacle
Transmission - Absorption transmitted n Light that passes through an object is transmitted absorbed n Light neither transmitted nor reflected is absorbed n Transmitted light strikes the retina of the eye to stimulate vision
Absorbed light causes the heating effect n Prove that dark colors absorb light and light colors reflect light. u Put two cups containing water and a thermometer behind a white sheet of paper and a black sheet of paper. Place a lamp very close to the cups, allow the lamp to shine only on the paper not the thermometer or water. What happens? n What colors do we wear in the summer? Why?
What Path Does Light Follow? u Set up 3 or 4 index cards standing up. Punch a hole in each card. With a flashlight on one side, move the cards until the light passes through each. Look through the last back at the light. What will you see? Move the cards, what happens ?
Shadows n Shadows are a consequence of light moving in a straight line. absorbed n If an object passes in front of the light source, the light is absorbed(blocked) and a shadow forms. n Shadows are always darkest in the middle and lighter on the edges.
n The more light rays that are blocked by the object, the larger the shadow. n The closer the object to the light source the BIGGER the shadow
Activity: Shadow n Place a projector across the room from a black wall, turn the projector on and the lights off. Slowly walk toward the projector. What happens to the size of the shadow formed?
Why are shadows darker in the middle, lighter at the edges? Diffraction
Optical tools n Lenses n Eyeglasses n Microscope n Camera n Kaleidoscope n Binoculars n Flashlight n Light boxes n Telescope n Prism n Spectroscope n Mirror
Lens n Lenses alter the angle of incidence, and thus the angle of refraction of light rays n Lenses determine how much light rays are bent n Lenses determine in which direction light rays are bent n Lenses bring light rays together (converge) n Lenses spread light rays apart (diverge)
Convex Lens A convex lens curves outward; “is rounded”. Light passing through a convex lens is bent inward, or made to converge.. Convex Lens
myopic, A concave lens is curved inward (caves in). Light passing through a concave lens bends outward, or diverges. Concave lenses are generally prescribed for myopic, or near-sighted, people. Concave Lens
Tools that use lenses / mirrors n Eyeglasses - to correct poor vision n Telescopes- to see things that are far away n Microscope - to see very small things n Binoculars - to enlarge an image n Camera - focus image on film
Kaleidoscope n Sir David Brewster, 1817 n internal mirrors repeat the pattern n http://kaleidoscope heaven.org/info.ht ml
Lasers n Light Amplification by the Stimulation Emission of Radiation
History n Mesopotamia n Ptolemy n Classical Greeks n Pythagoreans n Newton n Young & Helmholtz
Mesopotamia (1500 BC) n polished metal used as mirror n piece of glass used as burning lens
Ptolemy (127-135 AD) n Alexandria n rules of additive light mixtures n spinning disks* n light bends when passing through glass (refraction) n Wrote Optica on optical phenomena
Classical Greeks n Sought underlying principles n Observation without experimentation n Light travels in straight lines n Reflected light: the angle of incidence = the angle of reflection
Plato: n believed that light was emitted by the eye. n This was believed as late as 1644 when Descartes published a book elaborating a similar theory.
Aristotle (350 BC) n all colors are derived from mixture of black and white, White light is the purest color, others are contaminated n “These juxtapositions involving simple ratios may the most pleasing colors such as purple or crimson, like the concords in music. Irrational ratios may produce impure colors” n Sense and the Sensible
Isaac Newton (1600’s) n Beginning of modern scientific understanding /testing n Prisms n white light consists of light of may different colors with different “refrangibility” and power n Perception of color is due to physical properties of light
Make a prism n Make a prism by placing a small mirror in about an inch of water in a baking pan. Lean the mirror against one edge near bright sun light and direct the reflection to a white surface.
Young (1802) & Helmholtz n trichromatic (three color receptor) theory of color vision n demonstrated that the wide range of colors can be reproduced by superimposing various proportions of red, green and blue light n there are three color receptors in the eye
Today n the three cone pigments genes have been located on specific chromosomes
Human eye structures n Cornea - most anterior n Aqueous humor - watery solution behind cornea n Lens - fairly rigid structure n Vitreous Humor- jelly like colloid n Retina -delicate light receiving layer n Sclera - tough protective white outer layer
Where does most refraction occur? n At the air/cornea interface. n This is the site of astigmatism n A normal cornea is like a baseball - evenly curved in all directions n An astigmatic cornea is like a football, more curved in one direction than the other.
Human Lens F The lens in the human eye is convex, but unlike a glass lens, it is elastic so that it can change shape to focus on objects at varying distances. The lens becomes short and fat when viewing close objects and elongated and thin when viewing distant objects.
Hyperopia (far sighted) hyperopics n Sometimes eye muscles are unable to focus light on the retina, the screen at the back of the eyeball. If the image forms behind the retina for nearby objects, farsightedness (results. Convex lenses are prescribed for hyperopics to assist the eye in making light converge on the retina for nearby objects
Presbyopia - old eyes n As the eye ages, the lens becomes less transparent and absorbs more blue light. What is the effect on vision ?
Cataracts n A loss of transparency - usually of the cornea n May be congenital or aquired (uv light) What is the effect on vision ?
Vision occurs as n Light energy collected by the eye n Is transduced in the retina to neural energy n Travels via nerves n and is processed by the brain
Color vision The eye has three types of light receiving units, red, green and blue cones
Differential stimulation of cones YELLOW n If you stimulate only red and green cones, not blue, you see YELLOW n Try this with different colors of cellophane on an overhead projector or tied over flash lights.
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