Light

Physics 100

Chapt 17

Galileo’s attempt to measure the speed of light

Galilean moons

Ole Roemer

Eclipses of Jupiter’s moons

Sun

Earth

Jupiter

Io

Light from eclipse #1 travels this far

Eclipses of Jupiter’s moons

Sun

JupiterIo

Light from the next eclipse hasto travel this much farther

Earth

Measured time between eclipses is a little longer than actual time

6 months later

Sun

Earth

JupiterIo

Light from eclipse #1 travels this far

6 months + later

Sun

JupiterIo

Light from the next eclipse travelsa distance that is this much shorter

Earth

Measured time between eclipses is a little shorter than actual time

Reflections

Reflection from a flat surface

angle of incidence = angle of reflection

Image

Wall mirror

left handright hand

Speed of lightis slower

Refraction

Speed of lightis faster

air

h2o

Refraction

where should you aim?

At the fish?

Above it?

Below it?

answer

“Broken stick”

Broken pencil

Dispersion

Dispersion in a water droplet

rainbowSun

Wave nature of light

Interference from two wave sources

Light thru a single narrow slit

central b

right

stripe wider

than shadow

of the slit!

Light thru a narrower single slit

central b

right

stripe gets wider!

Single slit “diffraction pattern”

“Intensity pattern”

Light thru two narrow closely spaced slits

Diffraction patte

rn

with dark strip

es

Two slit interference pattern

“Intensity pattern”

Bright stripes

Dark stripes

Two slits

Single slit diffraction

Single-slit interference

Any aperture produces interference

Two adjacent holes

Move holes closer together

Photo using IR light

Human vision

Visible light wavelengths 0.75x10-6m 0.4x10-6m

Human hair: Thickness = ~100 x 10-6 m (100 m)

Red blood cell: diameter = ~5 x 10-6 m (5 m)

hemoglobin molecule: ~0.01 x 10-6 m (0.01 m)No

way!

(albeit with a microscope)

OK

!

OK

!

Rule of thumb

To view an object using a wave, the wavelength of the wave must be shorterthan the dimensions of the features that

you want to examine.

“proof” that light is a wave

With 2 slits openno light goes here

Block off one slit

Now lightcan go here

Somehow the light going through

one slit knows whether or not the other slit is open or

closed

This is possible for a wave, which is extended in space, but not for a localized particle

Doppler effect applies to light also

Red-shift --- Blue shift