Color

Why is the Sky Blue?

Atmosphere scatters blue light because of the shorter λ.

http://www.youtube.com/watch?v=u4hw-aXiQmc

Violet light refracts more than red light.

Dispersion - separation of light into colors

Primary Rainbows

Inside the raindrop..1. Refraction2. Reflection3. Dispersion

colors change from red on the outside to violet on the inside

Secondary rainbow - appears outside of a primary rainbow and develops when light undergoes two internal reflections instead of one

Primary Rainbows

What is color?

• Different wavelengths and frequencies of light being perceived as different colors.

• White light contains equal amounts of these colors. (ROY G. BIV)

• Black objects absorb light.

• White objects reflect light.

Practice Problem

• If an object out in the sun absorbs all frequencies of white light except for 4.2 x 1014 Hz, what color does the object appear to be? (v = 3.0 x 108 m/s)

7.14 x 10-7 m = 714 nm = red

Two ways of color mixing!!

Color Addition - mixing lightex. Pixels on TV's and monitors are red, green, and blue.

Color Subtractive – mixing paintex. Printers and copiers use yellow, magenta, and cyan.

Red, Green, and Blue are the primary colors of light.

Yellow, Magenta, and Cyan are the secondary colors.

magenta + green = white

yellow + blue = white

cyan + red = white

Complementary Colors - any two colors that add together to produce white

Retinal Fatigue or “after images” - after staring at a picture for a while, the human eye will see complimentary color.

Airplane shadow demo

airplanes appear to switch directions

Why do the planes switch directions? The record player doesn’t change directions, what is going on?

No depth perception so you can’t tell which planes are in front and which are going which way.

Incandescent vs Fluorescent

Who cares?

Incandescent bulbs enhance redFluorescent bulbs enhance blue

To save energy costs, find the bulbs with the light output you need, then choose the lowest watts.

Brightness is based on LUMENS not WATTS.

Why don’t people buy energy efficient bulbs?

Luminous – emits light

Illuminated – reflects light

Luminous Flux (P) – how much light is emitted from a source, measured in lumens (lm)

Illuminance or Illumination (E) - how bright a surface is due to light reflecting off the surface, measured in lux (lx)

P E

d

How bright a paper is (illumination E) depends on…

1) brightness of bulb (P)2) distance from bulb (d)

E depends on P and d

The bulb emits an amount of light = PThe paper has a certain brightness = E

P E

PE

Inverse

1~PE

Direct

~

If P increases, how does E change?

If distance increases, how does E change?

P E

dE

Inverse

1~dE

Direct

~

d

P E

d

PE ~

d

Our equation so far…

Inverse square law – quantity varies inversely as another quantity squared(Illumination varies inversely with the distance squared)

2~d

PE

24 d

PE

24 d

PE

Illumination (lx)

Luminous Flux (lm)

distance (m)

2d

IE

Luminous Intensity (cd)

Luminous Intensity (I) – another measure of how much light is emitted from a source, measured in candela (cd)

Summary:

Symbol Units

Luminous flux P lm

Illumination or Illuminance E lx

Luminous Intensity I cd

2d

IE 24 d

PE

Summary

Example 1) What is the illumination of a book that is 4 m below a 405 lm lamp? • Write down the information given in the problem

E = ? d = 4 m P = 405 lm

• Write down the equation you need to use…

• Substitute the numbers

• Solve

24 d

PE

244

405

E

lxE 2

Ex 2) What is the illumination on a surface 3.0 m below a 150 W

incandescent lamp that emits a luminous flux of 2275 lm? • Write down the information given in the problem

E = ? 150 Watts d = 3 m P = 2275 lm

• Write down the equation you need to use…

• Substitute the numbers

• Solve

24 d

PE

234

2275

E

lxE 20

Lens – focuses light onto retinaIris – colored part that controls the amount of light that entersPupil – opening that appears blackCornea – transparent covering, helps focus the lightRetina – tissue on the back of the eye

Rods - brightness receptorsCones - color receptors

Optic nerve – carries info to your brain, cause the blind spot

Myopia is also called _______________. A ___________ lens corrects the problem. Images are formed _______________ the retina because the eye is too ______________ or the refractive powers of the cornea and lens are too ______________. ________ objects appear blurry.

nearsightedness

in front of

stronglong Distant

Hyperopia is also called __________________. A ___________________ lens corrects the problem. Images are formed _______________ the retina because the eye is too ______________ or the refractive powers of the cornea and lens are too __________. _________ objects appear blurry. weak

farsightedness

short

Nearby

behind

Astigmatism occurs when light focuses ____________________. Both nearby and distant objects are ____________ because there is an ________ ________ of the cornea or lens.

in multiple places distorte

dirregular curvature

Optical Illusions