Ph Ch 13 Define electromagnetic wave
an energy carrying wave emitted by vibrating electrical charges (often electrons) and composed of oscillating electric and magnetic fields that regenerate one another
Ph Ch 13 Define electromagnetic spectrum
the range of electromagnetic waves taht extends in frequency from radio waves to gamma rays
Ph Ch 13 Define transparent
the term applied to materials through which light can pass w/out absorption, usually in straight lines
Ph Ch 13 Define additive primary colors
the three colors--red, blue, and green--that, when added in certain proportions, will produce any color in the spectrum
Ph Ch 13 Define subtractive primary colors
the three colors of absorbing pigments--magenta, yellow, and cyan--that, when mixed in certain proportions, will reflect any color in the spectrum
Ph Ch 13 Define diffraction
the bending of light that passes around an obstacle or through a narrow opening, causing the light to spread and to produce light and dark fringes
Ph Ch 13 Define interference
the result of superposing different waves of the same wavelength. Constructive interference results from crest to crest reinforcement; destructive interference results from crest to trough cancellation. The interference of selected wavelengths of light produces colors known as interference colors
Ph Check Yourself 13.1 Is it correct to say that a radio wave is a low frequency light wave? Is a radio wave also a sound wave?
Yes, both radio waves and light waves are electromagnetic waves that originate in the vibrations of electrons. Radio waves have lower frequencies than light waves, so radio wave might be considered to be a low frequency light wave (and a light wave might be considered to be a high frequency radio wave). A radio wave is definitely not a sound wave. A sound wave is a mechanical vibration of matter, not an electromagnetic vibration. (Dont confuse a radio wave w/ the sound that a loudspeaker emits)
Ph Check Yourself 13.2 Why is glass transparent to visible light but opaque to ultraviolet and infrared?
The natural frequency of vibration for electrons in glass is the same as the frequency of ultraviolet light, so resonance in the glass occurs when ultraviolet waves shine on it. The absorbed energy is transferred to other atoms as heat, not reemitted as light, so the glass is opaque at ultraviolet frequencies. In the range of visible light, the forced vibrations of electrons in the glass are at smaller amplitudes--vibrations are more subtle, reemission of light (rather than the generation of heat) occurs, and the glass is transparent. Lower frequency infrared causes whole molecules, rather than electrons, to resonate; again, heat is generated and the glass is opaque.
Ph Check Yourself 13.2 Pretend that, while you are at a social gathering, you make several momentary stops across the room to greet people who are "on your wavelength." How is this compared to light traveling through glass? In what way is it not compared?
~Your average speed across the room would be less because of the time delays associated w/ your momentary stops. Likewise, the speed of light in glass is less because of the time delays in interactions w/ atoms along its path.
~In the case of walking across the room, it is you who begin the walk and you who complete the walk. This is not comparable to the similar case of light, for (according to our model for light passing through a transparent material) the light that is absorbed by an electron that has been made to vibrate is not the same light that is reemitted--even though the two, like identical twins, are indistinguishable
Ph Check Yourself 13.2 What are two common fates for light shining on a material that isn't absorbed?
Transmission and/or reflection. Most light incident on a pane of glass, for example, is transmitted through the pane. But some reflects from its surface. How much transmits and how much reflects varies w/ different conditions
Ph Check Yourself 13.3 Why do the leaves of a red rose become warmer than its petals when illuminated with red light?
The leaves absorb rather than reflect red light, so the leaves become warmer
Ph Check Yourself 13.3 When illuminated w/ green light, why do the petals of a red rose appear black?
The petals absorb rather than reflect the green light. Since green is the only color illuminating the rose, and green contains no red to be reflected, the rose reflects no color at all and appears black
Ph Check Yourself 13.3 If you hold a match, a candle flame, or any small source of white light between you and a piece of red glass, you'll see two reflections from the glass: one from the front surface of the glass and one from the back surface. What color reflections will you see?
You see white reflected from the top surface. You see red reflected from the back surface because only red reaches the back surface and reflects from there
Ph Check Yourself 13.3 white-red=
Cyan. Interestingly enough, the cyan color of the sea is the result of the removal of red light from white sunlight. The natural frequency of water molecules coincides w/ the frequency of infrared light, so infrared is strongly absorbed by water. To a lesser extent, red light is also absorbed by water- enough so that it appears a greenish-blue or cyan color
Ph Check Yourself 13.4 If molecules in the sky were to scatter low frequency light more than high frequency light, what color would the sky be? What color would the sunsets be?
If light of low frequencies were scattered, the noontime sky would appear reddish orange. At sunset, more reds would be scattered by the longer distance traveled by the sunight, and the sunlight would be predominantly blue and violet. So sunsets would appear blue
Ph Check Yourself 13.4 distant dark mountains are bluish in color. What is the source of this blueness?
If we look at distant dark mountains, very little light from then reaches us, and the blueness of the atmosphere between us and the mountains predominates. The blueness is the low altitude "sky" between us and the mountains. Thats why distant mountains appear blue
Ph Check Yourself 13.4 Distant snow covered mountains reflect a lot of light and are bright. But they appear somewhat yellowish, depending on how far away they are. Why do they look yellowish?
The reason that bright, snow covered mountains appear yellow is that the blue in the white light from the snowy mountains is scattered on its way to us. So by the time the light reaches us, it is weak in the high frequencies and strong in the low frequencies-hence, it is yellowish. From much greater distances, from farther away than mountains are usually seen, they would appear orange for the same reason a sunset appears orange
Ph Check Yourself 13.5 Why does a microscopist use blue light rather than white light to illuminate objects being viewed?
there is less diffraction w/ blue light. This allows the microscopist to see more detail (just as a dolphin beautifully investigates fine detail in its environment by means of the echoes of ultra short wavelengths of sound)
Ph Check Yourself 13.6 If the double slits were illuminated w/ monochromatic (single frequency) red light, would the fringes be more widely or more closely spaced than if they were illuminated w/ monochromatic blue light? Why is it important that monochromatic light be used?
They would be more widely spaced. If light of various wavelengths were diffracted by the slits, dark fringes for one wavelength would be filled in w/ bright fringes for another, resulting in no distinct fringe pattern.
Ph Ch 13 Hmwk 4 In electromagnetic waves, what happens?
electric and magnetic fields are perpendicular to each other, and both are perpendicular to the direction the wave is traveling
Ph Ch 13 Hmwk 4 A basket ball appears orange to humans. Why does it appear to be this color?
the ball reflects only the orange frequency of light from its surface, and absorbs all of the other colors
Ph Ch 13 Hmwk 4 As you shine white light on the basketball, what would you expect to happen?
it gets hotter, since the ball absorbs most frequencies of light
Ph Ch 13 Hmwk 4 Why do we see the sunlight as whitish instead of yellow green?
we see not only yellow green, but also red and blue. All together, they mix to produce the white light we see
Ph Ch 13 Hmwk 4 What color do you get when your remove red light from white light and then remove green light from that light?
Ph Ch 13 Hmwk 4 Overly optimistic people are sometimes accused of looking at the world through "rose colored glasses." If you really did have a pair of glasses w/ red lenses, what would be true about them?
they would transmit only light w/ red frequency, and absorb all other frequencies
Ph Ch 13 Hmwk 4 You look at your eye at point P to view a reflection in the mirror. Which of th ecards do you see reflected in the mirror?
Ph Ch 13 Hmwk 4 You have a water table. If you wanted to maximize the amount of diffraction you observe as the plane waves in the table crash into the barrier, what could you do?
make the slit in the barrier more narrow
Ph Ch 13 Hmwk 4 Which frequency of visible light travels slowest through glass, and will thus be refracted the most?
Ph Ch 13 Hmwk 4 For a given object, the critical angle for total internal reflection is 45 degrees. What happens if you shine a light at that object at an angle of 50 degrees?
the light will undergo total internal reflection and not emerge from the other side of the object because the angel of incidence was greter than the critical angle
Ph Ch 13 Hmwk 4 List 3 things true about lenses
~when you hold an object near converging lens (inside its focal point), the lens produces a virtual image that is right side up
~a magnifying glass is an example of a converging lens
~When an object is far from a converging lens (outside its focal point), the lens produces a real image that is upside down
Ph Ch 13 Hmwk 4 What percentage of light is transmitted by two ideal polarizing squares, one on top of the other w/ their polarization axes aligned?
Ph Ch 13 Hmwk 4 What percentage of light is transmitted by these two polarizing squares w/ their axes at right angles to each other?
Ph Ch 13 Electro. PP What is light? What is its properties?
electromagnetic phenomenon; its own kind of wave
~brightness/intensity and color
Ph Ch 13 Electro. PP What does a light wave look like?
light waves consist of perpendicular electric and magnetic fields and both of these fields are also perpendicular to the motiono f the wave
Ph Ch 13 Electro. PP What is the electromagnetic spectrum?
A continuous spectrum of EM waves at different frequencies and wavelengths
~radio waves have the lowest frequencies and the biggest wavelength
~gamma waves have the highest frequency and the smallest wavelength
~visible light is just a small section of the EM spectrum, somewhere in the middle (closer to gamma rays than to radio waves)
Ph Ch 13 Electro. PP How much of the EM spectrum can we see?
EM waves w/ a frequency of visible light account for less than one millionth of one percent of the wave frequencies of the EM spectrum
Ph Ch 13 Electro. PP Transparent materials allow light to...
pass through, temporarily absorbing it, and then reemitting it. Light wave comes in, is absorbed by an atom in the material, and is then passed on (reemitted) to the next atom until it has reached the other side of the material.
~transparent materials are only transparent to visible light--they wont let other frequencies of waves pass through
Ph Ch 13 Electro. PP What are opaque materials?
they absorb light w/out reemission; some do reflect light such as metals
Ph Ch 13 Electro. PP an object that appears red has a ____ frequency. an object that appears violet has a ____ frequency.
Ph Ch 13 Electro. PP What are the 3 additive primary colors of light?
red, green and blue---these colors combined make white light
Ph Ch 13 Electro. PP A colors complement is
the color you would need to add to it to get white light
Ph Ch 13 Electro. PP Why is the sky blue?
Selective scattering. Nitrogen and oxygen make up the majority of our air, and the molecules ring like tiny bells when they are hit w/ sunlight, which is then reemitted in all directions. The molecules reemit all frequencies of light, but molecules of nitrogen and oxygen scatter higher frequencies light more effectively than lower frequencies of light. since violet and blue light have the highest frequencies, they are scattered the most. We dont perceive the sky as violet since our eyes are not as sensitive to that color as blue.
Ph Ch 13 Electro. PP What is diffraction
when a plane of light waves tries to go through a hole, it continues through but w/ some distortion; these distortions are called fringes. Fringes depend upon the wavelength of the wave compared to the size of the hole. the smaller the hole, the more fringing that occurs. The larger the wavelength, the more fringing that occurs
Ph Ch 13 Electro. PP What is the law of reflection?
the angle of reflection equals the angle of incidence; the frequency of light does not change when it is reflected in a mirror
Ph Ch 13 Electro. PP Concave mirrors have surfaces which curve _________________ . If you view an object from inside the focal point the image produced is _____________ and is __________ and ______________ away from the mirror than the object. If you view the object from outside the focal point of the mirror, the image is ___________ and ___________.
~ virtual; farther away
~ real; upside down
Ph Ch 13 Electro. PP Convex mirrors have surfaces which curve _____________. The virtual image is _____________ and ______________ to the mirror than the object
Ph Ch 13 Electro. PP When light is reflected from a highly polished surface what happens?
the law of reflection holds, and the angle of the incident light is equal to the angle of reflected light for all light rays. If the surface is rough, the reflection is diffused. The Law of Reflection holds for each individual ray, but each ray hits the rough surface at a different angle
Ph Ch 13 Electro. PP Light slows down by about ______ when it travels through water compared to a vacuum
Ph Ch 13 Electro. PP When light travels through one medium and then passes through another where its speed is different, we call it _______________
refraction. the light wave bends since its speed in one medium is different than its speed in the other medium
Ph Ch 13 Electro. PP What are the effects of refraction?
~because of refraction, objects under water appear to be nearer to the surface than they actually are (they are magnified).
~sunsets "linger" in the sky for a few minutes after the sun has sunk below the horizon since the atmosphere is thin at the top and dense at the bottom;
~air over hot pavement makes the ground appear wet
Ph Ch 13 Electro. PP Each material that light can travel through has its own ______________
index of refraction
~the index of refraction tells us how much the speed of light changes as it travels through that material
~ n = c/v
~ n is the index of refraction; c is the speed of light in air (constant) and v is the speed of light in the material
~if light slows down more as it goes through a material, its index of refraction will increase
Ph Ch 13 Electro. PP What is snells law?
When light travels from a region where it is traveling faster (small index of refraction) to a region where it is traveling slower (higher index of refraction) the light bends toward the normal line.
~n1 sin (l) = n2 sin (R)
n1 is the first material; l is the angle of incidence; n2 is the second material; and R is the angle of refraction
Ph Ch 13 Electro. PP What is dispersion?
different frequencies of light travel at different speeds through materials. Violet light travels about 1% slower than red light through glass. So, different colors of light refract by different amounts, and light can be separated into different colors "dispersion" refers to the separation of light by frequency (like a prism)
Ph Ch 13 Electro. PP Explain rainbows
As light goes through the drop of water and reaches the lower surface some of the light is refracted, and exits the drop bent at varying angles and some of it is reflected back into the drop, to bounce back through the drop surface and be refracted again. This twice refracted light then exits the drop separated by color.
Ph Ch 13 Electro. PP What is total internal reflection?
When you shine a light through a material at a certain angle (called the critical angle) it will not emerge from the other side, but instead will be reflected by each surface.
Ph Ch 13 Electro. PP Two main types of lenses are
converging lens-brings light rays together
diverging lens-spreads light rays apart
Ph Ch 13 Electro. PP What is the anatomy of a converging lens?
~principle axis- the line that connects both convex faces of the lens
~focal point- the point where light converges on either side of the lens
~focal length- the distance between the center of the lens and either focal point
Ph Ch 13 Electro. PP how do eye glasses help to correct our vision?
by adjusting the focal point of light for us
Ph Ch 13 Electro. PP how do our eyes work w/ light/images?
~light enters your eye through the cornea (which does about 70% of the bending you need to see normally), and passes through your pupil
~light then passes through the lens of your eye, which provides the rest of the bending necessary to focus images of nearby objects, which are spread over your retina
~there is a very sensitive part of your retina (called the fovea) in which your vision is most acute and focused
~but eyes do have a blind spot
Ph Ch 13 Electro. PP What happens if a magnifying glass is held outside of a focal point?
a real image is formed, but it is upside down
Ph Ch 13 Electro. PP Explain ray diagrams for converging lenses
~if an object is outside of the focal length, a real image appears upside down on the other side
~if the object is inside the focal length, then a virtual image appears on the same side of the lens, but it is larger
Ph Ch 13 Electro. PP Explain ray diagrams for diverging lenses
~ virtual image is smaller, and on the same side of the lense as the object (always true for concave lenses)
Ph Ch 13 Electro. PP What is an aberration? a spherical aberration? a chromatic aberration?
~any distortion in an image
~ light passing through the edge of a lens focuses in a slightly different place than light passing though the middle
~ different colors of light have different frequencies, and thus pass through a lens at a slightly different speed due to refraction
Ph Ch 13 Electro. PP What is polarization?
~ the polarization of light shows us that it is a transverse wave
~ each transverse wave is "plane polarized" (ie the plane of the vibration is the same as the plane of the wave)
~ common light sources emit unpolarized light (because electrons that emit light vibrate in random directions)
~ all transparent crystals have the ability to polarize light, dividing light into two internal beams perpendicular to one another