How LightBehaves

•Optics Study of light

Illumination

• Luminous – when a body gives of light

• Incandescent – when a body that gives

of light through when heated

A body is illuminated when you can see it

because it reflects light towards your eyes.

• Radiant flux – the amount of energy radiated in a unit of

time by an electromagnetic wave source. Part of this

radiation that enables us to see is called the luminous

flux, F. Measured in lumen (lm).

• Illuminance, E, is measured in lm/m2 or lux.

• If you don’t have constant\ illumination in all directions,

you can specify the luminous intensity of a source in a

particular directio, I, using the unit candela (cd)

• 1 cd = 1lm/Ω

• The steradian is the unit of solid angle given by A/R2 .

A – Area of sphere

R – Radius of the sphere

Reflection

• When a ray of light reaches between two types of materials, several

things can happen. Some may reflect from the surface, while some

may pass through the surface.

• Refract – The light that enters from the second material.

Some light may be absorbed by molecules on the surface or within

the

second material.

• Transparent – lets light rays pass through it without mixing them up.

• Translucent – allows rays to pass through it but mixes them up so

that you cannot see them clearly through the material.

• Opaque – when it blocks all light.

Imagine a line perpendicular to te reflecting surface. Such

line is called normal . The angle between the path of an

incoming ray and the normal is called the angle of incidence.

The reflected ray makes the same angle to the normal as the

incoming ray, but the other side of the normal. Reflection works

this way even when it involves rough surfaces. Whenever a ray

reflects from a surface, it has an equal angle to the normal at

that spot it had before the reflection.

• When a light reflects from a smooth surface, all of its rays reflect in

the same directions. When light reflects from a rough surface, the

rays reflect in many directions because the normals at all spots on

the surface points many ways.

• Waves behave according to the Law of Reflection.

• If you have a nice, clean, fat, shiny mirror, the light will still be

recognizable as the beam of the flashlight. It is called specular

reflection. But if you shine a flashlight at foil that has been

crumpled and then straightened out, the reflecting surface is no

longer flat and shiny; the reflected light goes in many different

directions and is much more spread out. This is diffuse reflection.

Refraction

• Waves that do not reflect back from a boundary

but travel into the new medium instead are said

to refract.

• In one direction, the wave just moves straight into

the new material, adjusting only its speed. In two

dimensions, the angle formed by the refracted ray

and the boundary will change from the incident

angle, because the wave moves at a different

speed in the new medium.

Snell’s Law• Gives a relationship between indices of refraction, the

angles of incidence and refraction and speeds in this problem:

We have a relation sin i/sin r = v1/v2

To observe refraction, place a pencil in a glass of water and then look at the pencil from the top and one side. The pencil appears bent at the water surface. The light from the top part of the pencil comes directly to you eyes. The rays from the bottom part pass through the surface between the water and air. There the rays refract, and so they seem to have come from a pencil bottom bent from the pencil top.

Index of Refraction

• The ratio between the speed of light c and it

speed v in a particular medium is called the index

of refraction of the medium. The greater the index

of refraction, the greater the extent to which a

light beam is deflected on entering or leaving the

medium. The symbol for the index of refraction is

n, so that

n = c/v

n1 sin i=n2 sin r

Substance n

Air 1.0003

Benzene 1.5

Carbon Disulfate 1.63

Diamond 2.42

Ethyl Alcohol 1.36

Glass, crown 1.52

Glass, flint 1.63

Ice 1.31

Lucite and plexiglas 1.52

Quartz 1.46

Water 1.33

Zircon 1.92

Absorption• Opaque materials absorb certain colors of light.

Transparent materials also absorb certain colors if they contain dyes or pigments.

• Scattering describes what happens when light rays stike atoms, molecules, or other individual, tiny particles. These particles send the rays of light off in new directions – that is, they cause the rays to scatter. Most of a clear sky appears blye because air molecules scatter more blue rays toward us than they do te other colors in sunlight. When the sun is near the horizon, it looks orange or red because the light reaching us has lost so muc of its other colors by scattering.

Interference• When two lights waves cross through the same

spot, they interfere with each other – that is, they add to or subtract from each other.

• Constructive Interference, gives brighter light than either wave would have separately.

• Suppose instead that wheneve a crest of one wave crosses through the spot, a trough of the other wave also does. The trough reduces the height of the crest, leaving the spot dim or even dark. This process is called destructive interference.

• The wavelength of the light can be calculated from the

distance between the center and the first bright band of the

screen.

• In the thickness of various thin films, different colors of light

are able to constructively interfere when path difference

between light reflected from the top surface and from the

bottom surface matches with the light’s wavelength, causing

the changing stripes of color we see in soap bubbles. As you

view the reflection t larger angles, the path difference

becomes larger and a longer wavelength of light will be able

to constructively interfere as well, and more bands of color

will appear.

• Hologams, are specialized interference patterns

recorded on a thin film emulsion on glass or

plastic, enabling you eyes to see exactly the

pattern of light waves that was reflected by a

three-dimensional object.

Diffraction

• In young’s experiment, the light passing through

each slit spread. It results from the fact that light

behaves as a wave.

• However, diffraction serves a purpose when a

device called a diffraction grating is used to

study the colors in a light beam. A grating used

with a telescope can separate the colors in the

light from a star, enabling the scientists to learn

what materials make up the star.

Dispersion

• Is the spreading of light into its colors.

• One way of to disperse a light beam is to send it

though a prism. Red light is refracted the least,

and purple light is refracted the most. This is

because the speed of the various wavelengths in

glass is different, slowest for violet light and

fastest for red light.

•

Polarization• It involves the oscillations (regular variations in

strength) of the electric fields that make up a light wave. These directions may be represented by arrows.

• In most of lights we see, the arrows point in many directions perpendicular to the ray’s path. Such light is unpolarized.

• If these arrows all point in one direction or just opposite it, the light is polarized.