8/7/2019 6F04pp_L31

1/22

L 31 Light and Optics [1] Measurements of the speed of light

The bending of light refraction Dispersion

Total internal reflection

Dispersion Rainbows

Atmospheric scattering Blue sky and red sunsets

8/7/2019 6F04pp_L31

2/22

Measurement of the speed of light

Speed of light in vacuum = c =

300,000,000 m/s = 186,000 miles/s

Galileo was the first to consider whetherthe speed of light was finite or infinite

He attempted to measure it by stationing

himself on one mountain and an assistanton a nearby mountain

8/7/2019 6F04pp_L31

3/22

Galileo and the speed of light

Galileo turns on his flashlight and starts his clock When his assistant sees Galileos light, he turnson his flashlight

When Galileo sees the light from his assistants

flashlight, he notes the time

D

Galileo Galileosassistant

8/7/2019 6F04pp_L31

4/22

Galileos result

FAST

Suppose D = 2 miles, then the time delay

would be t = D/c = 5 millionths of a sec. It is not surprising that Galileo was not

able to measure this!

Modern measurement of the speed of lightusing fiber optics we can do this!

8/7/2019 6F04pp_L31

5/22

The speed of light inside matter

The speed of light c = 300,000,000 m/s invacuum

In any other medium such as water orglass, light travels at a lower speed.

The speed of light in a medium can befound by using v = c/n, where c is the

speed in vacuum (300,000,000 m/s) and nis a number (greater than 1) called theindex of refraction.

8/7/2019 6F04pp_L31

6/22

MEDIUM

INDEX OF

REFRACTION

(n)

SPEEDOF

LIGHT (m/s)

(v)

Vacuum

air

Exactly 1

1.000293

300,000,000

water 1.33 225,564,000

glass 1.52 197,368,000

diamond 2.42 123,967,000

v = c / n

8/7/2019 6F04pp_L31

7/22

VISIBLE LIGHT

Color WAVELENGTH OR FREQUENCY

WAVELENGTH X FREQUEN

CY=C

8/7/2019 6F04pp_L31

8/22

COLOR Any color can be made by combining primary

colors Red, Green and Blue

A color TV uses mixtures of the primary colors to

produce full color images

Perceived color is a physiological effect

depends on how our eyes work

8/7/2019 6F04pp_L31

9/22

Refraction the bending of light

One consequence of the fact that light

travels more slowly in say water compared

to air is that a light ray must bend when it

enters water this is called refraction

the amount of refraction (bending) that

occurs depends on how large the index of

refraction (n) is, the bigger n is, the morebending that takes place

8/7/2019 6F04pp_L31

10/22

Reflection and refraction

at a surfaceIncidentLight ray reflected

Light ray

refractedLight ray

Normal line

8/7/2019 6F04pp_L31

11/22

Refraction of light

Water n= 1.33 Glass (n=1.5)

Incidentray

refractedray

The refracted ray is bent more in the glass

8/7/2019 6F04pp_L31

12/22

Normal incidence

If the ray hits the interface at a right angle

(we call this normal incidence) there is no

refraction even though the speed is lower

The wavelength is shorter, however

8/7/2019 6F04pp_L31

13/22

Refraction from air into water

water

n = 1.33

n = 1.0

normalWhen a light raygoes from airinto water, the

refracted ray is bentaway toward thenormal.

8/7/2019 6F04pp_L31

14/22

Refraction works both ways:

water into air

water

n = 1.33

n = 1.0

normal

When a light raygoes from water

into air, the refractedray is bent away

from the normal.

8/7/2019 6F04pp_L31

15/22

Some interesting effects

due to refraction

Underwater objects appear to be closer to

the surface than they actually are Total internal reflection fiber optics

Seeing through a window

Dispersion rainbows

8/7/2019 6F04pp_L31

16/22

Where is the fish? Closer

than you think!

fish

Apparent locationOf the fish

8/7/2019 6F04pp_L31

17/22

Total internal reflection

n1 > n2

n2

When the incidentangle is too big, the

refracted ray disappearsand the incident ray is

totally reflected back.

8/7/2019 6F04pp_L31

18/22

Fiber optics

A fiber optic cable is a bunch (thousandths) ofvery fine (less than the diameter of a hair) glass

strands clad together.

The light is guided through the cable bysuccessive internal reflections.

8/7/2019 6F04pp_L31

19/22

fiber optic communications

can carry more info with less distortion

over long distances

not affected by atmospheric conditions orlightning and does not corrode

copper can carry 32 telephone calls, fiber

optics can carry 32,000 calls

takes 300 lbs of copper to carry same info

as 1 lb of fiber optics

downside expensive

8/7/2019 6F04pp_L31

20/22

Seeing through the window

A light ray is offset slightlyWhen it passes thru a pane

Of glass. The thinner theGlass, the smaller the offset.

8/7/2019 6F04pp_L31

21/22

Seeing thru a window

When the angle of incidenceIs small, most of the incidentLight passes thru the glass,

Only a small amount isreflected

8/7/2019 6F04pp_L31

22/22

Windows behaving as mirrors

transmitted

When the angleOf incidence isLarge (grazing

Incidence) moreLight is reflected,The window isLike a mirror