Chapter 26 Properties of Light

Visible light originates with accelerated motion of electrons. It is an electromagnetic wave phenomenon.

1.ELECTROMAGNETIC WAVES l Accelerating charges give off EM waves. l Demo - Tesla Coil, Radio, and Fluorescent Lamp l Slide – EM Wave

Electromagnetic Wave Velocity l A certain speed is required to satisfy conservation of energy requirements. l That speed is c. l c = 300 million m/s = 186,000 mi/s186,000 mi/s l Maxwell discovered the nature of light.

THE ELECTROMAGNETIC SPECTRUM l Slide - Electromagnetic Spectrum l Different kinds of sources produce the different regions of the EM spectrum.

THE ELECTROMAGNETIC SPECTRUM EM Waves Sources Radio Waves Microwaves Infrared Visible Ultraviolet X-rays Gamma rays Vibrating charges Molecular rotations Molecular vibrations Atomic vibrations Nuclear vibrations

l We live in an “ocean” of waves. l For more information visit URL1.URL1 l Video - Microwave Gremlin l URL - X-ray of Hand URL l URL - X-ray of Lungs URL l Video - The Atomic Bomb

2. TRANSPARENT MATERIALS l Slide – Atom Spring Model l For glass the natural frequency is in the UV. l Energy of EM wave in UV stays with atoms longer causing vibrations with neighboring atoms with loss of energy. l Thus UV is absorbed.

l The atom holds the energy for less time, with less chance of collision with neighboring atoms, and less energy transformed to heat. l All visible frequencies get through clear glass. (Not all their energies do.) l l For visible light the electrons in the glass are forced into vibration, but at less amplitude.

llTllThere is an apparent slow down in the speed of light. l Slide - Light in transparent medium l Besides the UV, heat waves (IR) don't get through either.

TRANSLUCENT MATERIALS l Frosted glasses are good examples of translucent materials. l They scatter light on transmission. l You can’t read or identify objects from light passing through translucent materials. l Demo – Wax Paper

3.OPAQUE MATERIALS l Most things around us are opaque. l Metals are shiny and reflect. Because of free electrons - they don’t pass on energy to atoms. l Our atmosphere is transparent in visible and part of IR. l It's good that UV is absorbed in our atmosphere. UV causes sunburns.

Demo - Radiometer and Filters

Umbra Penumbra

SHADOWS l When light is blocked, shadows are created. l The kind of shadow (umbra or penumbra) depends on the source of light and the relative positions of the light source, object, and shadow of the object. l Extended light sources cause shadows of objects to be partial shadows (penumbras) that may or may not have umbras. l A point source of light will create a shadow that only has an umbra. l A penumbra will be small if the light source is far from the object and its shadow. l Demo – Shadows using Projector and Flashlight

Partial ShadowPenumbra Umbra Solar Eclipse Sun Moon Earth Full Shadow Looking at the sun while the moon passes in front of it.

Annular Eclipse Again looking at the sun while the moon passes in front of it.

Lunar Eclipse Looking at the moon while it passes through the earth’s shadow. Earth’s shadow New Moon Phase Full Moon Phase

l Cornea l Iris l Pupil - has the eye color and controls light intensity l Lens - the hole in the eye - does remainder of focusing l Retina- location of light sensors, has rods and cones l Blind spot l Fovea- center of vision, predominantly cones - optic nerve exit, no light sensors 4.SEEING LIGHT - THE EYE - does most of the focusing

l Primates and a species of ground squirrel are the only mammals that experience full color vision. l At the periphery of our vision we see Motions first as objects come from behind Colors second l Demo - Blind Spot

l Examples: stars and twilight l Demo - Turn off Room Lights l Rods see blue better. Blue will appear much brighter than red in dim light, though the red might be much brighter than the blue in bright light. l l Rods are more sensitive to low light levels than cones are.

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Lateral Inhibition We don't perceive the actual differences in brightness in fields of light and dark. We even out our visual field. This allows us to discern detail in very bright areas and in dark areas at the same time. We accentuate differences rather than similarities. Slide - Lateral Inhibition

Chapter 26 Review Questions

(a) you stand in the penumbra of the moon’s shadow (b) you stand in the umbra of the moon’s shadow (c) sunlight diffracts around the moon (d) sunlight reflects from the moon to the earth You will observe a total eclipse of the sun when (b) you stand in the umbra of the moon’s shadow

Which of the following will cast a shadow that has an umbra but no penumbra? (a) the sun (b) an incandescent lamp (c) a fluorescent lamp (d) a point source of light

The speed of light (a) has never been measured (b) is about the same as that of sound (c) is infinitely fast (d) is very fast, but not infinite

What is the purpose of the pupil in an eye? (a) to adjust the focal length of the lens (b) to adjust the amount of light passing through the lens (c) to focus the image (d) to change the width of the field of view (b) to adjust the amount of light passing through the lens

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