Color - Lecture Slides - Basic Concepts of Physics, Slides of Physics

Download Color - Lecture Slides - Basic Concepts of Physics and more Slides Physics in PDF only on Docsity!

Today:

Chapter 27 (Color)

Color: What gives an object color?

First, recall

simple model of atom and what light does to atoms

in

Ch. 26

There are natural frequencies associated with electronvibration - different for different atoms/molecules.Light of certain frequency impinging on material forcesvibrations of electrons at that frequencyIf resonant (i.e. = natural freq), material

absorbs

the light,

turning it into heat.If off-resonant, atoms re-emit the original frequency:

transparent materials

transmit

the light

opaque materials

reflect

the light

From last time

The color comes from those frequencies that are reflected, i.e. off-resonant in the visible spectrum: “

Selective reflection

Selective Reflection cont.

•^

Eg. incandescent vs fluorescent light bulb – both emit allfrequencies, but incandescent emits more at the lowerfrequencies (i.e red) and fluorescent emits more of thehigher freqs (i.e blue). So: Fluorescent light here – enhances blues, plays down reds, solipstick looks less red (pink) c.f. in sunlight!

Selective Transmission

•^

Similarly, color of transparent object determined by whatfrequencies are non-resonant

-^ Eg. Blue glass appears blue because all other frequenciesilluminating it coincide with natural frequencies of the atoms inthe

pigment

(dye), so get absorbed:

are fine particles that selectively absorbcertain freqs & transmit others

blue glass is transparent to blue,i.e. off-resonant in blue-frequency range

Clicker Question

When green light is shone on a red rose, what happens ?A)

The leaves become warmer than the petals

B)

The petals become warmer than the leaves

C)

Both petals and leaves warm up the same amount

Answer: B

Because green is absorbed by the red petals, turning the

light energy into heat. (The petals appear black). Theleaves, on the other hand, reflect green.

Mixing colored light

•^

White light from the sun is composed of all the visiblefrequencies - as demonstrated by a rainbow, or passing itthrough a prism.

-^

Intensity of light varies with frequency - “

Radiation curve

” of

sunlight is:

-^ Most intense in the yellow-green.• Our eyes have evolved tohave max sensitivity inyellow-green!• That’s why fire-engines &school-buses are nowpainted yellow-green, andwhy use yellow sodium-vapor street lamps at night.

docsity.com

Mixing colored light cont.

In fact,

any

color in the spectrum can be produced by various

amounts of red, green and blue: these are called the additive primary colors Eg. Project red, blue, green lamps on screen:Where two colors overlap, another isproduced. We say they the two “add” to eachother.(This is what’s going on in a color tv – tiny (<mm) spots,either R, B or G; gives a range of colors when viewed ata distance)

Adding colors…

Red + Green = Yellow

Blue + Green = Cyan

Red + Blue =Magenta

Since R + B = magenta, and R+B+G = white, thenmagenta + green = white.Magenta and green are called

complementary colors

: i.e.

Two colors that add together to make white.So, complementary pairs are:-

Magenta and Green- Yellow and Blue

(since = (R+G) + B)

• Cyan and Red

Addition of colors used in lighting stage performances – getinteresting colored shadows:Eg. R, G, B, colored lamps illuminating a golf ball:Ball appears white (= R+G+B).•^ This middle shadow is castby the green lamp. If the redand blue lamps were turnedoff, this shadow would beblack. But instead it isilluminated by the R and Blamps, which add tomagenta.

Similarly, can explain

• the cyan shadow cast by

the red lamp (lit by G and B lamps)

• the yellow shadow cast by the blue lamp (lit by R and G lamps)

If ball was bigger so that shadows from each lamp

overlapped, what would be the color of the shadowdirectly behind the ball? A) WhiteB) Green – the color of the middle lampC) Magenta – white minus greenD) BlackE) None of the above

Clicker Question

Answer: DBlack – i.e. no color, since that part of ball obstructs light fromall three lamps.

Mixing colored pigments cont.

Subtractive primaries Yellow pigment absorbs blue;cyan absorbs red. So bothabsorbed where overlap –which thus appears (white –blue – red) = green •Colored photos – composed of dots of magenta, cyan, yellow

• magenta, cyan, yellow

i.e. the colors obtained from subtracting the

additive primaries from white. (ie the complementarycolors to those).

Pigments:

Why the sky is blue

•^

Arises from selective

scattering

not

from color

addition/subtraction i.e. light striking atoms and molecules in a gas (ie far apartfrom each other) gets re-emitted in many directions.

-^ The

tinier

the particle, the

more

higher-frequency

light it

will re-emit( c.f.

sound vibrations – smaller bells have higher pitch thanlarger bells)• N

2

and O

in atmosphere are 2

like such tiny bells.

i.e.

scattered

So, what does this mean for the sky? Next slide…

Why the sky is blue cont.

•^

But violet is higher freq than blue – why don’t we see aviolet sky?

We would if our eyes were equally sensitive to violet

and blue, but the cones in our eyes sense blue much more,so we sense a blue sky.Other creatures with better violet vision like bumble bees,do see a violet sky!

-^

The lower freqs pass more or less in a straight line; e.g.red scatters a tenth as much as violet.

-^

DEMO

: shine flashlight through water with a few drops of

milk in it. View from the side, see bluish haze. If lookstraight through, see reddish-orange – as this doesn’t getscattered so much.

Remember this demo when we get to

sunsets!

Why the sky is blue cont.

•^

Why are some skies bluer than others?

Depends on what’s in the atmosphere, i.e. How

much water, dust, etc - these particles are larger than O

N

, so scatter lower freqs strongly as well, making sky less 2 blue and more whitish.

e.g. Drier atmospheres (eg in Greece and Italy) have

deeper blue skies than more humid ones

e.g. Polluted cities – even greyish skies, because of

exhaust gases absorbing rather than scattering…