Light

The most famous and accurate

1880

C = 186,282.3960 miles per second, plus or minus 3.6 feet per sec.

C = 299,792.4562 kilometers per second, plus or minus 1.1 meters per second

Michelson’s results

The Speed of LightThe speed of light in a vacuum is

a universal constant

Accepted values 3.00 x 108 m/s or 186,000 mi/s

A beam of light could travel around the earth, it would make 7.5 trips in one second.

The distance light travels in one year is called a light-year.

Electromagnetic WavesLight is energy that is emitted by

accelerating electric charges—often electrons in atoms.

This energy travels in a wave that is partly electric and partly magnetic.

Such a wave is an electromagnetic wave.

Light is a small portion of the broad family of electromagnetic waves

Electromagnetic WavesThis list includes in order of increasing (f) :

radio waves, microwaves, infrared waves, visible light waves, ultraviolet waves, X rays, and gamma waves

Electromagnetic WavesPOP QUIZ

QUESTION:

Is it correct to say that a radio wave is a low-frequency light wave? Is a radio wave also a sound wave?

ANSWER:

Both a radio wave and light wave are electromagnetic waves originating from the vibrations of electrons. Radio waves have lower frequencies of vibration than a light wave, so a radio wave may be considered to be a low-frequency light wave. A sound wave is a mechanical vibration of matter and is not electromagnetic. A radio wave is not a sound wave.

Electromagnetic WavesThe range of electromagnetic waves, or the electromagnetic

spectrum

The lowest frequency of light we see in our eyes appears red

The highest frequency of light we see in our eyes appears violet

Electromagnetic waves of frequencies lower than the red of visible light are called infrared

Electromagnetic waves of frequencies higher than those of violet are called ultraviolet

Light and Transparent MaterialsLight is energy carried in an

electromagnetic wave

Light incident upon matter, forces electrons to vibrate

How a receiving material responds when light is incident upon it depends on the frequency of light and the natural frequency of electrons in the material

Propagation

The natural vibration frequencies of an electron depend on how strongly it is attached to a nearby nucleusDifferent materials have different electric “spring strengths.”

Light and Transparent Materials

The energy of vibrating electrons is reemitted as transmitted light.

The (f) of the reemitted light = (f) incident light

A slight time delay occurs between absorption and reemission.

There is lower average speed of light through a transparent material.

Light and Transparent Materials

Light travels at different average speeds through different materials.

Light travels slightly more slowly in water than in the atmosphere, but its speed there is usually rounded off to c.

When light emerges from these materials into the air, it travels at its original speed, c.

Glass is transparent to visible light, but not to ultraviolet and infrared light.

The Secret of the Archer Fish

Light is not changed because of the perpendicular

Opaque Materials

Most materials absorb light without reemission and thus allow no light through them; they are opaque.

In opaque materials, any coordinated vibrations given by light to the atoms and molecules are turned into random kinetic energy—that is, heat

Why are metals Shiny?

Metals are also opaque.

When light shines on metal and sets these free electrons into vibration, their energy does not “spring” from atom to atom in the material, but is reemitted as visible light.

This reemitted light is seen as a reflection and that is why metals are shiny.

Opaque MaterialsOur atmosphere is

transparent to visible light and some infrared, but almost opaque to high-frequency ultraviolet waves.

The small amount of ultraviolet that does get through is responsible for sunburns.

PolarizationUnpolarized light

A light wave which is vibrating in more than one plane

EX: Light emitted by the sun, by a lamp in the classroom, or by a candle flame

Polarized light

are light waves in which the vibrations occur in a single plane

PolarizationPolarization explains why light

waves are transverse and not longitudinal

A single vibrating electron emits an electromagnetic wave that is polarized.

A vertically vibrating electron emits light that is vertically polarized,

A horizontally vibrating electron emits light that is horizontally polarized.

Polarization

More Polarization

Polarization Quiz

What is the result of shining light through two polarizing filters whose transmission axes are parallel to each other The first filter will polarize the light, blocking half of its vibrations. The second filter will have no effect.

Which of the three pairs of sunglasses to the right would be best for blocking road glare?

A. The light coming from the road will have a slight vertical polarization

.

Reflection &

Refraction

Lumination

Luminous objects

are objects which generate their own light

IlluminationIlluminated objects

are objects which are capable of reflecting light to our eyes

None of us are light-generating objects

It is only by reflection that we, see

ReflectionReflection: when a wave reaches a boundary,

some or all of it bounces back into the first medium

Totally Reflected waves are reflected back rather than transmitted into another object

Partially Reflected some energy is transmitted into a new medium

ReflectionLuminous objects emit light in a variety

of directions

your eye only sees the very small portion of rays coming towards it

Ray = a narrow beam of light

The Law of Reflectionangle of incidence = The angle made by the

incident ray and the normal

angle of reflection = The angle made by the reflected ray and the normal

Normal = imaginary line drawn perpendicular to the surface

The Law of Reflectionangle of incidence = angle of reflection

The Law of ReflectionReflected waves travel back to the

medium from whence they came

Incident rays and reflected rays make equal angles with the normal

The Law of Reflection

Do Now Problems

.

1. Differentiate between the angle of incidence and the angle of reflection2. What is meant by the normal to the surface?3. What is the law of reflection

Pop Quiz 1.  Consider the diagram at the right. Which one of the angles (A, B, C, or D) is the angle of incidence? Which one of the angles is the angle of reflection?

2. A ray of light is incident towards a plane mirror at an angle of 30-degrees with the mirror surface. What will be the angle of reflection? 60 degrees. Note the

angle of incidence is not 30 degrees!

Mirrorslight reflects off surfaces in a very predictable

manner - law of reflection

Rays of light are reflected from mirrors in all directions

                                                     

The Law of Reflection is Always Observed (regardless of the orientation of the surface)

MirrorsVirtual images are images which are formed in

locations where light does not actually reach

Virtual Image are formed through reflection that can be seen by an observer but can not be projected on a screen

light from the object does not actually come to a focus

MirrorsYour eyes cannot differentiate between a

real image and a virtual one

Your image in a plane mirror is?

ALWAYS

VIRTUAL !!!

Refraction of Light• The bending of the path of light • Refraction occurs as light passes across the

boundary between two medium

• A synonym for refraction is "bending”

RefractionThe change in speed that occurs at an interface

or boundary between two different media

Refraction of Light

• The speed of a light wave is dependent upon the optical density of the material through which it moves

RefractionThe optical density of a material relates to

the tendency of the atoms of a material to maintain the absorbed energy of an electromagnetic wave

One indicator of the optical density of a material is the

index of refraction value

Refraction of Light

When light rays enter a medium in which their speed increases (less dense) the rays bend away from the normal.

SFA = Slow to Fast, Away From Normal

If a ray of light passes across the boundary from a material in which it travels slowly into a

material in which travels faster, then the light ray will bend away from the normal line

Refraction of Light

When light rays enter a medium in which their speed decreases (more dense) the rays bend toward the normal

FST = Fast to Slow, Towards Normal

If a ray of light passes across the boundary from a material in which it travels fast into a

material in which travels slower, then the light ray will bend towards the normal

Refraction of LightHow does refraction causes the bottom of a clear

lake or pool to appear?

CLOSER !!!!

Refraction of LightWhich medium is more dense?

Refraction of Light

This is caused by the change in speed of light as it passes from one medium to another

How many times is the light refracted in the picture?

Refraction of LightThe wave speed is always greatest in

the least dense medium

The wavelength is always greatest in the least dense medium

Refraction of LightThe frequency of a wave is not altered

by crossing a boundary

However, both the wave speed and the wavelength are changed

RefractionINDEX OF REFRACTION (n)Defined as the ratio of the speed of light in

vacuum to the speed of light in the medium

index of refraction : n = c / v The speed of light in a given material is

related to this quantity

Every substance has its own specific index of refraction. The next slide has a few examples

Material Index of Refraction  

Vacuum 1.0000 <--lowest optical density

Air 1.0003  

Ice 1.31  

Water 1.333  

Ethyl Alcohol 1.36  

Plexiglas 1.51  

Crown Glass 1.52  

Light Flint Glass 1.58  

Dense Flint Glass 1.66  

Zircon 1.923  

Diamond 2.417  

Rutile 2.907  

Gallium phosphide 3.50 <--highest optical density 

Index of refraction values (represented by the

symbol (n)

It is the number of times slower that a light wave would be in that material

A vacuum is given an (n) value of 1.0000

v = c / n

n = c / v

Snell’s LawSnell's Law

The relationship between the angles of incidence and refraction and the indices of refraction of two media

ni sin θi = nr sin θr

Snell’s Law

light crosses a boundary into a medium with a higher index of refraction, the light bends towards the normal

Light traveling across a boundary from higher (n) to lower (n) will bend away from the normal

Snell’s Law

Index of refraction ( n ) = c / v material

Describes the extent to which the speed of light in a material medium differs from that in a vacuum

ni sin i = nr sin r

ni = index of refraction of incidence material

i = angle of incidence

nr = index of refraction of refractive material

r = angle of refraction

Find the angle of refraction

ni sin i = nr sin r

ni sin i = nr sin r

r = 32o r = 35o