Electromagnetic Radiation … aka Light

The EM spectrum•Not just the “light” we can see…•Ranges from low frequency radio waves to high frequency x-rays•Small window of frequencies called “visible light”

Light Properties: Review

• Electromagnetic radiation travels Electromagnetic radiation travels

________________in a vacuum ________________in a vacuum

– or about 186,000miles/secondor about 186,000miles/second

• This velocity has a symbol of?This velocity has a symbol of?

• Travels in rays… straight line from an origin.Travels in rays… straight line from an origin.

3.00 3.00 E8E8 m/s m/s

cc

Visible Light • A small slice of the EM spectrum can be A small slice of the EM spectrum can be

detected by human eyes.detected by human eyes. • ColorColor is the brains code for the frequency of is the brains code for the frequency of

perceived light.perceived light.• Roy G. Biv – acronym Roy G. Biv – acronym • On the left are low frequencies and long On the left are low frequencies and long

wavelengths. On the right are high frequencies wavelengths. On the right are high frequencies and short wavelengthsand short wavelengths

R O Y G B I V

Color # 1- Rules for Color

• White is ALL colors of the VS (visible spectrum)

Color # 1- Rules for Color• Black is the absence of all colors.• Black appears black because nothing is

reflected from it… therefore the light energy is turned to heat!

Color # 1- Rules for ColorColor # 1- Rules for Color

• Objects of a particular color absorb all other Objects of a particular color absorb all other colors except for the color they “normally” colors except for the color they “normally” appear.appear.

• Ex. Green plants absorb all colors (roybiv) Ex. Green plants absorb all colors (roybiv) and use these energy frequencies to and use these energy frequencies to produce sugar during photosynthesis…, produce sugar during photosynthesis…, except green… they reflect green.except green… they reflect green.

Color # 2- Rules for Color

• An object’s color depends SOLELY on the color of light that is reflected off of it.

White light (roygbiv)

Green

onl

y is re

flect

ed

Object looks green

Color # 3- Rules for Color

• An object’s color depends SOLELY on the color of light that is reflected off of it.

No light is reflected, no light reaches the eye, so object appears black!

The turtle absorbed red light

Filtered red light

Velocity of light through other Velocity of light through other materials – Snell’s Lawmaterials – Snell’s Law

• When light travels through substances, its velocity is effected.

• The amount the speed is reduced is the optical density (n)

• n = c / velocity in the substance

Find the v of each of these.

Material nVacuum 1.0000

Air 1.0003

Ice 1.31

Water 1.333

Ethyl Alcohol 1.36

Crown Glass 1.52

Zircon 1.923

Diamond 2.417

n = c/v

3.00 E8 m/s

3.00 E8 m/s

2.29 E8 m/s

2.25 E8 m/s

2.21 E8 m/s

1.97 E8 m/s

1.56 E8 m/s

1.24 E8 m/s

Law of Reflection

• Establish “normal” – an imaginary perpendicular line to the surface of a reflective object.

mirror

normalnormal

ΘAngle of

incidence

i ΘrAngle of reflection

=

Inverted imagesInverted images• Mirrors produce images which have an

“apparent distance” equal to that of the real object is from the mirror.

• Images are also “inverted”• Mirrors need not be the same size as the object

to produce a full sized image.

d d

Mirrors and Reflection

• Light rays travel in parallel rays from a source to an object that reflects them.

• A reflection can be “diffuse”, “perfect”, or “focused”

• Mirrors create either perfect (specular) or focused reflections depending on their shape.

Diffuse Reflection

• The rays are scattered in different directions. This is the way we see most objects in everyday life.

Perfect (specular) reflection

• Plane mirrors create perfect parallel reflection of rays.

mirror

Focused reflection• Concave and Convex Mirrors create

focused reflections. They still obey the Law of Reflection.

• ConvexConcave Focal

point

Mirrors and Reflection #4enlarged

Multiple normals, resulting in slightly different reflected

angles

RefractionRefraction• High frequencies are more affected by this

phenomenon.

• Snell’s Law also predicts how much it will be bent in the equation…

ni sinΘi =nrsinΘr

• n = optical density or Index of refraction.

Refraction#1

When light changes the material in which it travels, changes in velocity cause a change in

direction. The θincidence is different than the θrefracted.

Refraction#2

Optical density of the material determines amount of “bending”

Optical density low

Optical density high

Optical density low

High O.D. = small Θ

Low O.D. = larger Θ

parallel

water

air

Actual fishVirtual fish

LensesLenses• Lenses obey Snell’s Law by refracting light to a

certain degree.• Lens shape has the biggest effect on light

behavior.• Two types :

Concave Convex

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