Download Ch 18 Notes

Ch. 18 Notes – Electromagnetic Spectrum and Light
You use and are surrounded by electromagnetic waves (like radio, television, visible light, microwaves) all
the time, most of which are invisible. While these waves exhibit some of the behaviors of mechanical
waves, they also have some unique characteristics.
Electromagnetic Waves
Like mechanical waves, electromagnetic waves carry energy from place to place. But how they travel and
how they are produced is very different.
electromagnetic waves – transverse waves consisting of changing
electric and magnetic fields
electric field – region of space exerting electric forces on
charged particles
magnetic field – region of space producing magnetic forces
• electromagnetic waves are produced when an electric charge vibrates or accelerates
• electromagnetic waves can travel through a vacuum (empty space) as well as through matter
The transfer of energy by electromagnetic waves traveling through matter or across space is called
electromagnetic radiation
Electromagnetic waves (light) travel VERY fast. So fast that for many years, people thought it was infinite.
Albert Michelson is famous for accurately measuring the speed of light in a famous experiment.
speed of light (c) – 3.00 x 108 m/s (300,000,000 m/s or 300,000 km/s = 186,000 miles per second)
Although all EM waves travel the same speed in a vacuum, EM waves vary in wavelength and frequency
Wave or Particle?
EM radiation travels in a wave exhibiting properties of wave behavior, but it also behaves like a particle
Wave Model – one of the best demonstrations on the wave behavior of light was shown by a famous experiment by Thomas
Young. He used diffraction to create interference patterns of alternating dark and light bands of light occurring
where the waves overlapped (alternating destructive interference and constructive interference patterns).
Particle Model – Another famous experiment that demonstrated the particle nature of light was the photoelectric effect.
When dim blue light was shown on the surface of certain metals, the metal emitted electrons. A brighter blue
light caused more electrons to be emitted, but red light, no matter how bright, did not cause electrons to be
emitted. Albert Einstein proposed that EM waves consists of packets of energy called photons. Each
photon’s energy is proportional to the frequency of the light: the greater the frequency of an EM wave, the more
energy it has. Since blue light has a greater frequency than red light, photons of blue light have enough energy
to cause the emission of electrons, whereas photons of red light do not have enough energy to cause the metal to
emit electrons.
Intensity – the rate at which a wave’s energy flows through a given unit of area.
We all know that the closer you are to a source of light, the brighter the light appears. As photons travel
outward from a source of light, the spread out over a larger area. This decreases the light’s intensity
(brightness). The intensity of light decreases as photons travel farther from the source. This also
demonstrates the wave nature of light. As waves travel away from the course, they are covering a larger and
larger area. Since the total energy does not change, this decreases the wave’s intensity.
The Electromagnetic Spectrum
Beyond the electromagnetic waves we can see (visible light) scientists have discovered many other
frequencies of EM waves. The different frequencies of these waves gives them particular uses. The full
range of electromagnetic waves is what we call the electromagnetic spectrum.
Radio Waves – the longest wavelengths (lowest frequencies)
in the EM spectrum – ranging from 1 mm to thousands
of kilometers.
• Radio waves are used in radio and television
technologies, in radar and microwave ovens
Radio – music and voices are changed into electronic signals and
coded onto radio waves either by varying the amplitude
of the wave (amplitude modulation - AM) or by varying
the frequency of the wave (frequency modulation – FM).
Your radio receives the signal, decodes it, and converts the signal back into sound waves.
Television – similar to radio waves, but the signals also carry information for pictures as well as sound. To avoid weather
and location affecting the signal sent by the antenna, TV broadcasts are often transmitted by satellite.
Microwaves – the shortest wavelength radio waves (1 m to 1 mm). Used to carry cell-phone conversations as well as
increase the thermal energy of molecules in foods and cook them.
Radar – short bursts of radio waves are reflected off objects they encounter. The returning waves are interpreted by a radio
receiver to determine the location of the objecet. The Doppler effect (the apparent change in frequency if the
object is moving) also allows detection of the speed and direction of the object.
Infrared Rays – EM waves with a higher frequency (shorter wavelength) that radio waves, but lower
frequency than red light – ranging from 1 mm to 750 nm.
• Infrared waves are used as a source of heat and to discover heat differences – warmer objects give
off more infrared radiation than cooler objects
• We cannot see infrared waves (like radio waves) but our skin detects them as warmth
• Thermograms are color-coded pictures that show variations in temperature.
Visible Light – wavelength ranges from 750 nm to 400 nm
• The part of the EM spectrum that the human eye can see.
• Visible light consists of a range of wavelengths corresponding
to a specific frequency that has a particular color. This range is
what we call the color spectrum (rainbow – ROYGBIV)
Ultraviolet Rays – wavelength ranges from about 400 nm to 1 nm
• frequencies just beyond violet light
• UV rays have applications in health, medicine, and agriculture
• UV rays are used to kill microorganisms, disinfecting air and medical devices.
• UV rays also used in heating and cooling systems of large buildings and to help plants grow
• UV light helps your kin produce vitamin D, but excessive exposure causes sunburns and cancer
X-Rays – EM waves with very high frequencies and short wavelengths, ranging from 12 nm to 5 m.
• because of their high energy, X-rays can penetrate matter that other light cannot.
• X-rays are used in medicine, industry and transportation to view the inside of solid objects – rather
than passing through them, dense objects like bones and many metals absorb x-rays allowing us to
see them inside other objects.
• Too much exposure to x-rays can damage or kill living tissue
Gamma Rays – EM waves with the highest frequency and shortest wavelength – less that 5 m.
• they have the highest frequency, and thus the most energy and penetrating ability of all EM waves
• Gamma rays are used in the medical field to kill cancer cells and to make pictures of the brain and
in industry as an inspection tool
• Overexposure to gamma rays can be deadly
Behavior of Light
How light behaves when it strikes an object depends on many factors, including the material the object is
made of. Objects are either transparent, translucent, or opaque.
transparent – a material that transmits light – it allows most of the light that hits it to pass through
ex. air, water, glass
translucent – a material that scatters light – some light passes through, but not most.
ex. frosted glass, some plastics
opaque – a material that either absorbs or reflects all of the light that strikes it – you cannot see through it
ex. wood, metals, etc.
Objects that may be opaque to visible light, may also be transparent to other EM waves (like X-rays)
When light strikes a new medium, the light is either reflected, absorbed, or transmitted. When light
is transmitted, it can be refracted, polarized, or scattered.
reflection – the interaction of light that occurs when a wave bounces off a surface that it can’t pass through
regular reflection – when parallel rays of light strike a smooth, polished surface and
reflect all in the same direction.
Ex. visible light on a mirror
diffuse reflection – when parallel rays of light strike a rough, uneven surface and
reflect in many different directions.
Ex. visible light on a piece of white paper
• Whether or not a surface is “smooth” or “rough” depends on how the wavelength of the wave
compares to the irregularities of the surface of the material.
refraction – when light passes from one medium to another at an angle, it will change direction or bend
• refraction can cause object to appear closer or larger than they really are (objects underwater), or
distort them – like a mirage (the “wet” spot ahead on the road on a hot, sunny day)
polarized light - Light with waves that vibrate in only one direction (one plane)
• Unpolarized light vibrates in all directions.
• Polarizing filters transmit light that vibrates in only one direction,
so vertical polarizing filters only allow light vibrating vertically
to be transmitted and block all horizontal light.
• When sunlight reflects from a horizontal surface, that light is
polarized horizontally – this reflected light is glare. Polarized
sunglasses have vertically polarized filters which block the glare.
scattering – a process of redirecting light as it passes through a medium
• although our atmosphere is transparent, the tiny particles that make it up can redirect the light that passes through it. Since
not all frequencies of visible light are scattered equally, this is why the sky is blue – the smaller the particle the higher
frequency of light that is scattered. More of the high frequency blue light is scattered by the atmosphere so the sky looks
blue. More of the reds and greens reach your eyes, which your brain combines to see yellow – which is why the sun looks
yellow at noon. At sunrise and sunset, most of the greens have also been scattered by the atmosphere (since the light has
traveled through more atmosphere) leaving the reds/oranges to reach your eyes – which is why the sun looks red or orange at
sunrise and sunset.
Color
Dispersion – as white light (all colors of visible light combined) passes through a prism, shorter
wavelengths refract more than longer wavelengths, separating the colors into the visible spectrum
• Isaac Newton is famous for doing this. He also found that once separated, the individual colors could not be further separated.
• Dispersion (and internal reflection) is what causes a rainbow to form
The color of any object depends on what colors of light are absorbed and reflected by the object, and
on the color of light that strikes the object. – If white light strikes a red shirt, most of the red light is
reflected back to your eyes, while the other colors of the visible spectrum are absorbed. If green light is
shone on the shirt, since it absorbs the green light, the shirt would look black.
Mixing Colors of Light
Primary colors – three specific colors that can be combined in varying amounts to produce all possible colors
Colors of Light
The primary colors of light are red, green and blue
Secondary colors – a combination of two primary colors
• If red light and blue light both shine on a white shirt, the shirt
will look like the combination of red and blue, which is magenta
The secondary colors of light are cyan, magenta and yellow
Complementary colors of light – any two colors of light that
combine to form white light
• Red and Cyan light (its complementary color) combine to form white light
Ex. Red & Cyan , Green and Magenta , Blue and Yellow
Since adding different colors of light adds to the frequencies that are present to be reflected, mixing colored
light is often referred to as additive color mixing.
Mixing Pigments
Colors of Pigment
Pigment – a material that absorbs certain colors of light and reflects other colors
The primary colors of pigment are cyan, magenta and yellow
The secondary colors of light are red, green and blue
• Mixing Cyan and Magenta pigments produces Blue
Complementary colors of pigment – any two colors of pigment
that combine to form black
• Green and Magenta pigments combine to form black
Ex. Red & Cyan , Green and Magenta , Blue and Yellow
Since adding different colors of pigment subtracts from the frequencies that are reflected, mixing colored
pigment is often referred to as subtractive color mixing.
Sources of Light
Some things are able to give off their own light. We call objects that give off their own light luminous.
Incandescent Light – produced when an object gets hot enough to glow.
• when electrons flow through the filament of an incandescent light, the filament gets hot and emits light
• incandescent lights give off most of their energy as heat, not light, and are not very efficient
Fluorescent Light – when electric current flows through a fluorescent bulb, electrodes emit electrons which hit atoms of
mercury vapor. The mercury emits UV rays that strike the phosphor coating inside the bulb. The phosphors emit visible light.
• much more of the energy is emitted as light, instead of heat, which is why the bulbs don’t get as hot
Laser Light –atoms of a solid, liquid or gas are given energy and “excited”, emitting photons of light.
• Waves of laser light all have the same wavelength and are in phase (crests and troughs line up) so it is called coherent light.
• coherent light does not spread out significantly so it has a constant intensity, and the energy can be focused on a small area.
Neon Light – many gases (not all neon) emit photons of energies and therefore emit different colors when excited
• The different photons emitted combine to give each gas a distinctive color
Sodium-Vapor Light – electric current passes through a mixture of solid sodium, neon gas and argon gas.
The gas mixture is ionized, which warms up and causes the sodium to turn from a solid to a gas. As
the electrons in the sodium jump up to higher energy levels, and then move back down they emit light.
Tungsten-Halogen Light – work the same as incandescent lights, but the halogen gas inside the bulb reduces wear on the
tungsten filament so the bulb last longer.