Nature of Light

Read Pages 62-68

Measuring the speed of Light• c celeritas Latin for speed• Galileo 1600’s

– Two lanterns on hills far away– Very fast

• Romer late 1600’s – Appearance of the moons of Jupiter– Finite speed

• Michelson – early 1900’s– First great American scientist (excepting Franklin)

The Speed of Light• In empty space, light travels at a speed of

about 300,000 km/s. • Light travels so fast that light emitted from

the Sun travels 150 million km to Earth in only about eight and a half minutes.

• When light travels in matter, it interacts with the atoms and molecules in the material and slows down.

• As a result, light travels fastest in empty space, and travels slowest in solids.

Light Year

• 299,792.458 km/s• speed of light = 300,000 km/s.• 1 year = 365.25 days• 1 day = 24 hours• 1 hour = 3600 s

• Light year = (3x 105 km/s)(365.25d/y)(24h/d)(3600 s/h)

White to Color to White

The Prism isn’t producing the Color

Fundamental Property

• Color is a fundamental property of light.• The prism doesn’t produce the color.• The prism separates the light into different

colored portions.• The prism can put the colors back to gether

again.• The prism does not do anything to a single

color.

Newton’s Particle Theory

• Based on his study of color, Newton concluded that light is made up of different types of particles and that these particles produce different colors.

• The particles would be shot like BB’s and travel in straight lines, spreading out like buckshot from a shotgun.

Wave Theory

• Other scientist of the time believed that light was a wave phenomenon.

• Particle versus Wave Debate

• Particle won because Newton championed it.

• Young’s Experiment

Superposition Principle

½ λ

Constructive Interference

Destructive Interference

Interference

• A wave is a disturbance that moves through matter or space.

• Waves carry energy from one place to another.

• In water waves, the energy is transferred by water molecules.

Waves Carry Energy, not Matter

• When waves travel through solids, liquids, and gases, matter is not carried along with the waves.

• Waves are produced by something moving back and forth, or vibrating.

Types of Waves

• It is the energy of the vibrating object that waves carry outward.

• Some waves, known as mechanical waves, can travel only through matter. Sound

• Waves called electromagnetic waves can travel either through matter or through empty space. Light

• Light, radio waves, and X rays are examples of electromagnetic waves.

Electromagnetic Waves

• Electromagnetic waves are transverse waves.

• They contain electric and magnetic parts that vibrate up and down perpendicular to the direction the wave travels.

Properties of Light Waves• An electromagnetic wave contains an electric

part and a magnetic part.• Both parts are called fields and vibrate at

right angles to the wave motion.

• The frequency of a wave is the number of wavelengths that pass by a point each second.

Frequency

• If you were watching a transverse wave on a rope, the frequency of the wave would be the number of crests or troughs that pass you each second.

Period and Frequency• Frequency (f) of a wave is the number of

wavelengths that pass a fixed point each second.

• SI Unit is hertz (Hz) 1 Hz = 1 wavelengths/sec• Period (T) of a wave is the amount of time it

takes one wavelength to pass a point• SI Unit is sec/wavelength• f= 1/T and T= 1/f

• The speed of a wave depends on the medium in which the wave travels.

Wave Speed

• You can calculate the speed of a wave if you know its wavelength and frequency using this equation.

• Wave Speed Equationwave speed (m/s) = wavelength (m) X frequency (Hz)

v= λf

• Waves don’t always travel in the same direction.

Waves Can Change Direction or Amplitude (p 304-306)

• Waves can change direction when they travel from one material to another or when they strike some media.

• The waves can reflect (bounce off a surface), refract (change direction), or diffract (bend around an obstacle).

• Waves can also change amplitude when they pass through another wave, this is referred to as interference.

The Law of Reflection• According to law of reflection, the angle

that the incoming wave makes with the normal equals the angle that the outgoing wave makes with the normal.

θrθi θi= θr

• . When a light wave moves from air to water, it slows down

Refraction

• This change in speed causes the light wave to bend.

• Refraction is the change in direction of a wave when it changes speed traveling from one material to another.

• Waves can change direction by diffraction, which is the bending of waves around an object.

Diffraction

• The amount diffraction or bending of the wave depends on the size of the obstacle the wave encounters.

• If the size of the obstacle is much larger than the wavelength, very little diffraction occurs.

Observing Assignment

• Observe Venus and Jupiter• Evening Just after sunset

– East for Venus– After sunset

• Morning before 7:30– Overhead for Jupiter

• Record: Date/Time/Object near Venus

Intensity of Sound or Light• The amount of energy that a wave carries

past a certain area each second is the intensity of the sound.

This figure shows how the intensity of sound from the cymbals decreases with distance.

• The intensity of sound waves is related to the amplitude.

Doppler EffectSounds moving toward a listener rise in pitch while sounds

moving away from a listener lower in pitch.

λ decreases

f increases

V=f λ

λ increases

f decreases

Doppler Effect• Easiest experienced with sound• Also applies to light• Blue Shift• Light source approaching the detector has the

light wave lengths shortened- all the colors shift toward the blue end of the spectrum.

• Red Shift• Light source moving away from the detector has

the light wave lengths lengthened- all colors shift toward the red end of the spectrum.

Wavelength and Frequency of Light

• Wavelengths of light are usually expressed in units of nanometers (nm), or Angstroms.

• One nanometer is equal to one billionth of a meter. 1nm = 10-9m , 1 A0 = 10-10m

• Green light has a wavelength of about 500 nm, or 500 billionths of a meter. A light wave with this wavelength has a frequency of 600 trillion Hz.

Speed, Wavelength, Frequency

• Speed = frequency x wavelength

• V= f λ• frequency increases- wavelength decreases• frequency decreases- wavelength increases

The Electromagnetic Spectrum• At one end of the spectrum the waves have low

frequency, long wavelength, and low energy. At the other end the waves have high frequency, high energy and short wavelengths.

Long Wavelength Short Wavelength

Low FrequencyLow Energy

High FrequencyHigh Energy

Radio Waves and Microwaves• The wavelengths of radio waves are greater

than about 0.3 meters.

• Some are even thousands of meters long.

• The shortest radio waves are called microwaves.

• These waves have a wavelength between about 0.3 meters and 0.001 meters.

A new kind of energy

Infrared WavesInfrared waves have wavelengths between

0.001 meters and 700 billionths of a meter. • All warm bodies emit infrared waves. • Law enforcement officials and military

personnel sometimes use special night goggles that are sensitive to infrared waves. These goggles can be used to help locate people in the dark.

Visible Light and Color• The range of electromagnetic waves between

700 and 400 billionths of a meter is the range of wavelengths people can see.

Ultraviolet Waves

• Electromagnetic waves with wavelengths between about 400 billionths and 10 billionths of a meter are ultraviolet waves.

• Ultraviolet waves carry more energy than visible light waves.

• Sunlight that reaches Earth’s surface contains a small fraction of ultraviolet waves.

X Rays and Gamma Rays• The electromagnetic

waves with the highest energy, highest frequency, and shortest wavelengths are X rays and gamma rays.

• X rays pass through soft tissues, but are blocked by denser body parts, such as bones.

X-Ray of Roentgen’s Wife’s Hand

X Rays and Gamma Rays

• Gamma rays are even more energetic than X rays.

• One use of gamma rays is in the food industry to kill bacteria that might increase the rate of spoilage of food.

Light Discoveries• 1800 William Herschel - Infrared

• 1865 James Clerk Maxwell developed equations for electromagnetic waves.

• 1888 Hertz produced radio waves

• 1895 Roentgen discovered X-rays

Page 67Figure 3-7

Passage of Electromagnetic Waves Through the Atmosphere

Reflected Light

• Angle of reflection = angle of incidence• Regular Reflection – mirrors – reflect light in

single direction and form sharp images.• Diffuse Reflection- irregular surfaces like brick

walls reflect light in many different directions and do not form an image.

• We see objects because they reflect light.

Refracted Light

• Light rays are bent as they pass from one material into another.

• In prisms and rain drops, the different colors (wavelengths) are bent different amounts.

• The bending separates the colors into the spectrum (red, orange, yellow, green, blue, indigo, violet)

Color

• When white light (mix of all colors) falls on an object, some colors are absorbed, some are reflected.

• We see objects in the color that is reflected by the object.

• The other colors are absorbed.• The colors that are absorbed or reflected

depend on the characteristics of the material.

Page 67Figure 3-7

More Information from Light

Page 89-101

Electromagnetic Waves from the Sun

• Most of the energy emitted by the Sun is in the form of ultraviolet, visible, and infrared waves.

• Only a tiny fraction of this energy reaches Earth.

Blackbody Radiation

• A “black body” is an object that absorbs all of the energy that hits it and emits energy resulting from its own temperature.

• The moon is not a black body since it reflects light from the sun.

• The sun and stars are approximately black bodies. They may reflect a very small fraction of the light that hits them.

Color and Temperature

• Hot - Red• Hotter - Orange• Still Hotter- Yellow• Hottest - White• Most Hottest - Blue

Light from a heated object• If we put the light from a heated object

through a prism, we find that all wavelengths are present but one is brightest.

• 1. As an object heats up, it gets brighter, emitting more electromagnetic radiation at all wavelengths.

• 2. The brightest color (most intense wavelength) of the emitted radiation changes with temperature.

Temperature from Color

• If we know the wavelength of the most intense light emitted by an object we can find the temperature of the object no matter how far away.

• Wien’s Law• λmax = .0029 m / T (Kelvin)

Thermometers

Fahrenheit Celsius or centigrade Kelvin

Freezing temp of water

Boiling temp of water212o

100o 373o

32o 0o 273o

-273o 0o

Stefan-Boltzman Law

• An object emits energy per unit area at a rate proportional to the fourth power of its temperature in Kelvin.

• Hotter objects are brighter

• E/A = σ T4

• 2T 16 E/A

Powerful Concepts• Wein’s Law allows us to determine the temperature of a

star by observing the color of the most intense wavelength.

• Stefan-Boltzman Law allows us to determine the amount of energy per unit area emitted by the object.

• We can measure the energy per unit area that we receive on earth.

• Knowing the inverse square relationship between energy emitted and energy received we can determine the distance to the star.

Section 1 Characteristics of LightChapter 13

Electromagnetic Waves, continued• Illuminance decreases as the square of the distance

from the source.

• The rate at which light is emitted from a source is called the luminous flux and is measured in lumens (lm).

Photoelectric Effect

Photoelectric Effect

Photoelectric Effect

• Refers to the emission of electrons from a metal when light shines on the metal

• For a given metal, no electrons were emitted if the light’s frequency was below a certain minimum (and wavelength above a corresponding level), regardless of the light’s intensity.

• Einstein won the Nobel prize for physics for connecting this to the particle nature of light and to Planck’s equation E= h*frequency.

Quantum Physics

• Light energy is transferred in “packets” called quanta.

• If the packet does not have sufficient energy it will not dislodge the electron from the metal.

• If it has more than enough energy, some of the energy is used to dislodge the electron and the rest is transferred to the electron to make it move faster.

Model of Atom

Photon Emission and AbsorptionHydrogen- the simplest atom

Planck’s Law

• E= h ν• c= λ ν• ν = c/λ• So E= hc/λ• The energy carried by a photon of light

is inversely proportional to the wavelength. Directly proportional to the frequency.

Hydrogen’s Line-Emission Spectrum

Section 1 The Development of a New Atomic ModelChapter 4