Electromagnetic Spectrumand Light

Chapter 16

Electromagnetic (EM) Waves – transverse waves consisting of changing electric fields and changing magnetic fields.

They differ from mechanical waves in the way they are produced and how they travel. They are produced by constantly changing fields.

Electric Field – exerts electric forces on charged particles.

Magnetic Field – produces magnetic forces produced by magnets or by changing electric fields and vibrating charges.

Electromagnetic waves are produced when an electric chargevibrates or accelerates.

The figure below shows that the electric and magnetic fields are at right angles to each other. This is a transverse wave because the fields are also at right angles to the direction in which the wave travels.

Unlike mechanical waves, electromagnetic waves do NOT need a medium. Electromagnetic waves can travel through a vacuum,or empty space, as well as through matter.

The transfer of energy by EM Waves traveling through matter is called electromagnetic radiation.

Why can’t the electric field waveexist without the magnetic fieldwave?

They both produce each other!

Mirror

Mirror

Semi-silveredMirror

Michelson’s Experiment- 1926 – American physicist Albert Michelson measured the speed of light more accurately than ever.He reflected and refracted light off a mountain series of mirrorsand lenses and by knowing the differences, and by timing the light,he concluded the light’s speed.

He earned theNobel Prize inphysics makinghim the firstAmerican everto get this award.

Light and all electromagnetic waves travel at the same speed ina vacuum. The speed of light in a vacuum is 3.00 X 108 m/s.Actually, it is 299,792,458 m/s.

Even though all EM waves travel at the same speed in a vacuum,they are not all the same. EM Waves vary in wavelength and frequency. As we already know, the speed is a product of itswavelength and frequency.

Wave Speed = Wavelength X Frequency

A radio station broadcasts a radio wave with a wavelength of3.0 meters. What is the frequency of the wave?

Frequency = Wave Speed / Wavelength

Frequency = 3.00 X 108 m/s = 1.0 X 108 Hz 3.0m

A global positioning satellite (GPS) transmits a radio wave with awavelength of 19cm. What is the frequency of the radio wave?

Hint: Wavelength will need to be converted to meters.

Wavespeed = Wavelength X Frequency

Frequency = Wave Speed / Wavelength(3.00 X 108 m/s) / (0.19m)

1.6 X 109 Hz

The radio waves of an AM radio station vibrate 680,000 timesper second. What is the wavelength?

Wave Speed = Wavelength X Frequency

Wave speed / Frequency = Wavelength

(3.00 X 108 m/s) / (680,000 Hz) = Wavelength

(300,000,000 m/s) / (680,000/s) = 440m

Radio waves that vibrate 160,000,000 times per second are usedon some train lines for communications. If radio waves that vibratehalf as many times were used instead, how would the wavelengthchange?

At 160MHz, WL = WS / F

(3.00 X 108 m/s) / (160,000,000Hz) = 1.9m

At 80MHz, WL = WS/F

(3.00 X 108 m/s) / (80,000,000 Hz) = 3.8m

3.8m – 1.9m = 1.9m Therefore, The wavelength would be 1.9m longer at 80MHz than at 160MHz.

Wave or Particle????The fact that light casts a shadow has been used as evidence forboth the wave model of light and the particle model of light.

Evidence of the Wave Model – English physicist Thomas Young in1801, showed that light behaves like a wave.

Evidence for the Particle Model

The emission of electrons from a metal caused by light strikingthe metal is called the photoelectric effect. When dim blue lighthits a metal such as cesium, an electron is emitted. When a brighterblue light is emitted, more electrons are emitted. But red light, nomatter how bright, does not cause the emission of electrons in thisparticular metal. WHY???

In 1905, Albert Einstein proposed that light consists of packets of energy that he named photons. Each photon’s energy is proportionalto the frequency of the light. The greater the frequency, the moreenergy each of its photons has.

Blue light has a higher frequency than red light so the photonsof blue light have more energy than those of red light. Blue lightphotons have enough energy to cause electrons to be emittedfrom a cesium surface.

Intensity – The rate at which a wave’s energy flows througha given unit of area. The intensity of light decreases as photonstravel farther from the source.

As the light rays move farther from the source, the lit area becomeslarger, but less intense.

The waves of a spectrum – How do you investigate something that is invisible??? This was the problem of astronomerWilliam Herschel in 1800. He used a prism to separate thewavelengths present in sunlight and placed thermometers atvarious places along the color bands. He discovered that thetemperature was lower at the blue end, higher at the red end.

Herschel wondered if the temperature increased beyond the redend. He concluded that the area just beyond the red are recordedan even higher temperature showing that there must be invisibleradiation beyond the red color band. This is now called infraredradiation.

The electromagnetic spectrum consists of radio waves, infrared rays, visible light, ultraviolet rays, X-rays, and gamma rays. This diagram shows the electromagnetic spectrum.

Radio waves – used in radio and television technologies, as wellas in microwave ovens and radar.

AM – Amplitude modulation – Theamplitude of the wave is varied butthe frequency remains the same.AM stations use 535 KHz – 1605 KHz.

FM – Frequency modulation – Thefrequency of the wave is varied butthe amplitude remains the same.FM stations use 88MHz – 108MHz.

The shortest wavelength radio waves are called microwaves. Thesehave a wavelength from about 1 meter to about 1 millimeter. Thesecan cook food for us. When the water or fat molecules in the foodabsorb microwaves, the thermal energy of these molecules increase.Microwaves also carry cell phone conversations, data transfer, and high distant tv and radio transmissions.

Infrared rays – wavelengths vary from about 1 millimeter to about750 nanometers or millionth of a millimeter (billionth of a meter).

Infrared rays are used as a source of heat and to discover areasof heat differences. Thermographs use infrared sensors to createthermograms which are color-coded pictures that show variations in temperatures. These are used to find places where a buildinglooses heat and problems in the path of electric current. Searchand rescue teams use infrared cameras to locate victims.

Visible Light – Each wavelength in the visible spectrum correspondsto a specific frequency and has a particular color. People usevisible light to see, to help keep safe, and to communicate with one another.

Ultraviolet Rays – Has higher frequency than violet light and hasapplications in health, medicine, and agriculture.

In moderation, UV rays help your skin produce Vitamin D which helpsthe body absorb calcium from foods which produce healthy bonesand teeth. Excessive exposure can cause skin cancer, sunburn, and wrinkles. It can also permanently damage your eyes.

UV rays are used to kill microorganisms as well as help plants to grow.

X-Rays are used in medicine, industry, and transportation to makepictures of the inside of solid objects. They have higher frequenciesthan ultraviolet rays. X-Rays have high energy and can penetrate objects that light cannot.

Gamma Rays – Have highest frequencies and the most energy andthe greatest penetrating ability of all electromagnetic waves. Overexposure can be deadly! Gamma rays are used in medicalfield to kill cancer cells and make pictures of the brain. They areused in industry as an inspection tool.

Light and MaterialsLight can be:

1. Transparent – transmits light which allows most of the light topass through it.

2. Translucent – scatters light such as frosted glass. You can seethrough it but the objects are fuzzy or lack detail.

3. Opaque – absorbs or reflects all of the light that strikes it. It does not allow any light to pass through making it so youcan’t see through it.

When light strikes a new medium, the light can be:

Reflected

Absorbed

TransmittedWhen light is transmitted, it can be:

Refracted

Polarized

Scattered

Regular reflection – occurs when parallel light waves strike a surfaceand reflect all in the same direction. EX. a calm lake acting as a mirror.

Diffuse reflection – occurs when parallel light waves strike a roughuneven surface and reflect in many different directions. EX. a dog.

Image – a copy of an object formed by reflected or refractedwaves of light.

Mirage – a false or distorted image. mirages occur because lighttravels faster in hot air than in cooler dense air. On a sunny day,air tends to be hotter just above the surface of a road than higherup. Light is gradually refracted as it moves into the layers of hotterair. This causes some of the light to curve rather than being on adirect path to the ground.

Polarized light – light with waves that vibrate in only one plane.Polarized filters transmit light waves that vibrate this way.

Scattering – light is redirected as it passes through a medium. Ascattering of light reddens the sun at sunset and sunrise. The tinymolecules in the Earth’s atmosphere can scatter sunlight causing this.

The small particles in the atmosphere scatter shorter wavelengthblue light more than light of longer wavelength. By the time thesunlight reaches your eyes, most of the blue and even some of thegreen light has been scattered by the small particles. Most of whatremains are the reds and oranges.

When the sun is high in the sky, the light travels a shorter distancethrough the atmosphere. It scatters blue light in all directions whichexplains why the sky appears blue on a sunny day even though theair itself has no color.

As white light passes through a prism, shorter wavelengths refractmore than longer wavelengths, and the colors separate.

The process in which white light separates into colors is dispersion.

The color of any object depends on what the object is made ofand on the color of light that strikes the object. Sunlight containsall of the visible colors but when you look at a yellow car in thesunlight, the yellow paint reflects mostly yellow light. Most of theother colors in white light are absorbed at the surface.

Mixing ColorsPrimary Colors – three basic colors (red, green, blue) that can becombined to produce white light. These colors can combine invarying amounts to produce all possible colors.

Secondary Colors – A combination of two primary colors makingcyan, yellow, and magenta. If you add a primary color to the propersecondary color, you get white light. Any two colors of light thatcombine to form white light are complementary colors of light. Thisis a secondary and primary color that combines to form white.

Blue and yellow = whiteRed and cyan = whiteGreen and magenta = white

A pigment is a material that absorbs some colors of light andreflects other colors. Paints, inks, photographs, and dyes get their colors from pigments.

The primary colors of pigments are cyan, yellow, and magenta.Color printers use these colors plus black to create almost anycolor.

The secondary colors of pigments are red, green and blue. Anytwo colors of pigments that combine to make black pigmentare called complementary colors of pigments.

Sources of LightObjects that give off their own source of light are luminous. EX. Sun, lamps, headlights, flashlights, fire, etc.

Common light sources include:

incandescentflourescentlaserneontungsten-halogensodium-vapor bulbs

Incandescent Light-The light produced when an object gets hot enough to glow.

When electrons flow through the filament of an incandescentbulb, the filament gets hot and emits light.

Filament of regular light bulbs are made oftungsten. The bulbs are filled with a mixtureof nitrogen and argon gas at a very low pressure. These gases do not react with the filament as oxygen does so the filament doesn’t burn out as fast. Incandescent bulbsgive off most of their energy as heat……..not light!

Fluorescent LightDuring fluorescence, a material absorbs light at one wavelengthand emits light at a longer wavelength. A phosphor is a solidmaterial that can emit light by fluorescence. Fluorescent lightbulbs emit light by causing a phosphor to steadily emit photons.

A fluorescent bulb is a glass tube thatcontains mercury vapor and a glass coated with phosphors. When currentflows through the bulb, small electrodesheat up and emit electrons. Theelectrons hit the atoms of mercury andemit UV rays. The UV rays strike thephosphor coating and emit visible light.They emit most of their energy as light.One 18w fluorescent = one 75 w incand.

Laser LightA laser is a device that generates a beam of coherent light. The word laser stands for light amplification by stimulated emissionof radiation. Laser light is emitted when excited atoms of a solid, liquid, or gas emit photons.

Light in which waves have the same wavelength, and the crestsand troughs are lined up is called coherent light. A beam ofcoherent light does not spread out from its source.

Neon LightNeon lights emit light when electrons move through a gas or a mixture of gases inside glass tubing. Many contain gases otherthan neon such as helium, argon, and krypton for the differentcolors they produce.

SODIUM –VAPOR LIGHTSSodium-vapor lights contain small amounts of solid sodiumplus a mixture of neon and argon gases. As electric currentpasses through a sodium-vapor bulb, it ionizes the gas mixture.This mixture warms up and the heat causes the sodium to change from a solid into a gas. The sodium atoms emit light. These are very energy efficient and give off a very bright light but are slow to come on. They can also alter the color of the objects below.

Tungsten-Halogen LightsWorks much like an incandescent but it has a small amount ofhalogen gas such as iodine, bromine, or fluorine. These bulbslast much longer than incandescent because the halogen gasreduces wear on the filament. The light is made of quartz because the heat generated can melt glass. These make colors“pop” and are used in accent lighting, recessed lights, studios,and concert arenas.