Light

Light – Wave or particle?• For many years scientists argued over the nature of light, "Is light

a wave or a stream of particles?" • In some experiments light exhibits wave like properties, the

Doppler effect, interference, refraction, diffraction• and in other experiments, like the photo electric effect, it exhibits

particle like properties • The fact is that light exhibits behaviors which are characteristic of

both waves and particles.

Early Concepts of Light• Early Greek philosophers: tiny

particles entering the eye to create vision

• Socrates & Plato: streamers/filaments from the eye contacting the object.“How else can you explain why you don’t see an object until you look at it?”

• Newton: light made of particles

The Speed of Light

• Does light take time to travel, or does it arrive at the same instant it departed?

• Ancients: speed of light is infinite• 1880: Michelson measured speed to be about

300,000 km/s

1840 1860 1880 1900 1920 1940 1960 1980 2000297,000,000

297,500,000

298,000,000

298,500,000

299,000,000

299,500,000

300,000,000

300,500,000

Year of Measurement

Mea

sure

d sp

eed

of li

ght,

m/s

Electromagnetic Waves• Visible light is a small part of the

electromagnetic spectrum• EM waves with higher frequency than visible

light are ultraviolet• EM waves with lower frequency than visible

light are infrared

Light and Transparent Materials

LIGHT

3 of many atoms

LIGHT

GULP!

GULP!

GULP!

BURP! BURP!

BURP!

GLASS

a b

c d e

f g

Chain of absorptions/re-emissions

Opaque Materials• Reflect light instead of transmitting• Opacity in one part of the EM spectrum,

transparent in another part– EX: People are opaque in visible light, more

transparent in X-raysAlso called M31, this is

closest neighboring galaxy in the wider universe and is

just about visible from a good site.

Radio, microwave, infrared, visible, ultraviolet

and x-ray.

Opaque Materials• Can only reflect a color if that color is present

in the incident light

http://phet.colorado.edu/simulations/sims.php?sim=Color_VisionShadows: http://www.schulphysik.de/ntnujava/shadow/shadow.html

RGB Color Mixing

Polarization

BUT . . . There’s a controversy!

• “DST” – Dark Sucker Theory• Electric bulbs = “Dark Suckers”• Candles are primitive DSs• Portable DSs need a Dark Storage Unit• Dark has mass• Dark is heavier than light• Dark is faster than light

BUT . . . There’s a controversy!

• Finally, we must prove that dark is faster than light. If you were to stand in a lit room in front of a closed, dark closet, and slowly opened the closet door, you would see the light slowly enter the closet. But since dark is so fast, you would not be able to see the dark leave the closet. Next time you see an electric bulb, remember that it is a Dark Sucker.

The Color Spectrum

• Homework for next time:• Read Chapter 27, Hewitt: Do RQ 1-23, T&E1,4-

10• Chapter 28 – Will pull extra credit questions

from here!