physics 2

Which have the longest wavelengths: light waves, X rays, or radio waves?

Radio waves have the longest wavelengths and the smallest frequencies.

Do radio waves travel at the same speed of sound, at the same speed of light, or at some speed in between?

Radio waves travel at the speed of light in air (3.00 X 108 m/s) which is much larger than the speed of sound waves in room temperature air (340 m/s)

What is the color of light that is transmitted through a piece of red glass?

Red light is transmitted through a piece of red glass.

How many vibrations per second are represented in a radio wave of 101.7 MHz. (1 MHz = 106 Hz)

101.7 x 106 = 101,700,000 vibrations per second. Frequency measures the number of vibrations per second.

What causes bending of light in refraction?

A change in the speed of the light as it enters the medium.

What is the difference between a compression and rarefaction of a sound wave?

A compression part of a sound wave is a region of high pressure and a rarefaction is a region of low pressure.

Is a converging lens or a diverging lens used to produce a real image? To produce a virtual image?

A converging lens can form both a real and virtual image depending on the location of the object relative to the lens. (see above) A diverging lens always forms a virtual image.

Why does a red apple appear to be the color red? Would the red apple appear to be the color red if blue light illuminated the apple?

A red apple appears red because when white light illuminates the red apple all of the colors in the white light are absorbed except the red light which is reflected. Since blue light one of the primary colors (and does not contain red light), when it illuminates a red apple the apple would appear black since there is no red light to be reflected back.

What causes the variety of colors seen in gasoline splotches on a wet street? In the same regard, what accounts for the variety of color in a soap bubble?

The colors we see in gasoline and soap bubbles are a result of interference of light when the light strikes a thin film. The light that passes through the thin film and reflects and the light that simply reflects off the surface of the thin film interfere with one another. Some frequencies of light cancel and we see the different colors.

What is meant by critical angle?

The critical angle is the angle at which total internal reflection occurs. This only occurs when light travels from a region of high index of refraction (light is traveling slower) into a region when of lower index of refraction (light is traveling faster).

What must be the minimum length of a plane mirror in order for you to see a full view of yourself? What effect does your distance from the plane mirror have on your answer?

The mirror must be 1⁄2 the height of the person viewing the mirror. Moving the mirror to various positions does not have any impact on the answer.

What are the primary colors of light? What color do you get if you mix blue and red? Green and red? Green and blue? What happens if you mix blue, red, and green light?

The primary colors are Red, Green, and Blue. Blue + Red = Magenta Red + Green = Yellow Green + Blue = Cyan If you mix the primary colors (red, blue, and green) you get white light.

Why is a secondary rainbow dimmer than a primary rainbow?

The primary rainbow comes from light that is incident on a raindrop. The light reflects off the back of the raindrop and then refracts at the front of the raindrop. The secondary raindrop comes from the light that is reflected inside the raindrop at the point of refraction and then it is reflected off the back of the drop again and refracted at the front. This light is dimmer because some of the light has already been refracted. The light that is left is dimmer.

For correct viewing, why are slides put into a slide projector upside down?

The projection of the slide is formed on a screen which means that it is a real image. All real images formed by converging lenses are inverted.

Does a single raindrop illuminated by sunlight disperse a spectrum of colors? Does a view see a spectrum from a single faraway drop?

A single raindrop illuminated by sunlight does disperse a spectrum of color but not all that color can reach a person's eyes. When a rainbow is formed, various raindrops illuminated by sunlight disperse the color spectrum. Since red light does not bend as much as violet light, a person sees the color red from raindrops that are higher in the sky. On the other hand, violet light bends the most and thus a person sees the color violet from raindrops that are lower in the sky. All the other colors of the spectrum appear in between. As described above a viewer does not see the entire spectrum from a single raindrop.

If a gas tap is turned on for a few seconds, someone a couple of meters away will hear the gas escaping long before he or she smells it. What does this indicate about the speed of sound and the motion of the molecules in the sound-carrying medium?

This indicates that speed of sound is greater than the speed of the gas molecules as they move through the air. The sound simply needs to vibrate the air molecules which is passed on. (The actual air does not move.) The gas molecules must actually move through the air from the source to the person. This takes a greater amount of time as the molecules cannot move as fast as the vibration of the sound wave.

In what direction are the vibrations in a transverse wave, relative to the direction of a wave travel? In a longitudinal wave?

Transverse waves have the vibrations perpendicular to the motion of the wave. Longitudinal waves have the vibrations parallel to the motion of the wave.

Why do troops "break step" when crossing a bridge? Consider the Tacoma Narrows bridge as you answer this question.

Troops break step in order to ensure that they do not match the natural frequency of vibration of the bridge. They do not want to cause resonance.

What physical phenomenon underlies beats?

Two sound waves with slightly different frequencies interfere with one another.

Which has the shorter wavelengths, ultraviolet or infrared? Which has the higher frequencies?

Ultraviolet has shorter wavelengths and higher frequencies. (This is the type of electromagnetic waves that cause sunburns.)

What is the fate of the energy in ultraviolet light incident on glass? Visible light?

Ultraviolet light is absorbed by glass while visible light passes through.

You can get sunburn on a cloudy day, but you can't get sunburn even on a sunny day if you are behind glass. Explain.

Ultraviolet light is the cause of sunburns and glass is an opaque material for ultraviolet light. In other words, ultraviolet light can not pass through the glass it is absorbed.

What does it mean to say that one wave is out of phase with another?

Waves are out of phase with one another if they meet one another crest to trough.

What does it mean to say that light is scattered? How does this explain a very blue sky?

When light is incident on the atmosphere which is made up of atoms, the light actually interacts with the atoms. The atoms behave as tiny optical tuning forks. When the light strikes the atoms, the atoms reemit light. The upper atmosphere is made up primarily of Nitrogen and Oxygen molecules. These particular atoms are "tuned" to higher frequency light (Blue). When white light in incident on the atoms, they reemit the higher frequency light more often and in all directions. This is termed scattering. The other colors are scatters in order of frequency with red being scattered the least. Since blue light is scattered the most, we see the sky as being very blue especially on a clear day. If there is a lot of water vapor in the air or dust particles, lower frequencies of light also undergo scattering. This causes the sky to appear less blue or even whitish.

What has the higher frequency, red light or blue light?

Blue light has a higher frequency.

What do radio waves and light have in common? What is different about them?

Both travel at the same speed in air. (3.00 X 108 m/s). They have different frequencies and wavelengths.

What is dispersion? Cite an example.

Dispersion is when white light is incident on a surface and the light is split up into the various colors. An example of this is when light is incident on a prism.

A cat can hear sound frequencies up to 70,000 Hz. Bats send and receive ultrahigh-frequency squeaks up to 120,000 Hz. Which hears sound of shorter wavelengths, cats or bats?

Higher frequency hears shorter wavelengths. (Bats)

Is interference restricted only to some types of waves or does it occur for all types of waves?

Interference can occur with any type of wave.

Does visible light make up a relatively large part or a relatively small part of the electromagnetic spectrum?

small part

what is the source of all waves

vibrating source

What is the relationship among frequency, wave-length, and wave speed?

Speed of a wave = frequency x wavelength. In a particular medium the speed of the wave will remain fixed so if the frequency increases the wavelength will decrease by the same amount.

Does the medium in which a wave travels move with the wave?

The medium itself does not move with the wave. A wave is a traveling disturbance. The disturbance moves through the medium.

A railroad locomotive is at rest with its whistle shrieking, and then its starts moving toward you. (a) Does the frequency that you hear increase, decrease, or stay the same? (b) How about the wavelength reaching your ear? (c) How about the speed of sound in the air between you and the locomotive?

(a) Whenasoundsourceismovingtowardanobserverthefrequencyofsoundheardbythe observer is larger compared to the actual sound frequency. (it increases) (b) Thewavelengthofthesoundwillappearshortersincethefrequencyincreased. (c) The speed of the sound will not change.

Distinguish between a virtual image and a real image.

A virtual image cannot be placed on a screen. Virtual images are always formed on the same side of the lens as the object. You must look through the lens to see the image. All virtual images are upright. They can be smaller or larger than the object depending on the lens that forms the image. A converging lens can form a virtual image that is larger (magnifying glass, farsightedness) if the image is placed close to the lens between the lens and the focal point. A diverging lens always forms a virtual image that is smaller (nearsightedness). A real image can be formed on a screen. Real images are always formed on the opposite side of the lens as the object. All real images are inverted. Only converging lenses can form real images.

What is the principal difference between a radio wave and light? Between light and an X ray?

All electromagnetic waves travel at the same speed in air (or a vacuum), 3.00 X 108 m/s. What makes the waves different is the fact that they have different frequencies and wavelengths. Radio waves have long wavelengths and low frequencies compared to visible light while X-rays have short wavelengths and high frequencies compared to visible light.

What kinds of waves exhibit interference?

All waves can exhibit inference.

How is the wavelength of light related to its frequency?

As with all waves, the wavelength and frequency are inversely proportional to one another. As one increases the other decreases.

If the frequency of sound in air is doubled, what change will occur in its speed? What change will occur in its wavelength?

Assuming that the properties of the air remain the same, the speed of the sound will not change. The wavelength will decrease by a factor of 1⁄2.

If, while standing on the bank of a stream, you wished to spear a fish swimming in the water out in front of you, would you aim above, below, or directly at the observed fish to make a direct hit? If you decided instead to zap the fish with a laser, would you aim above, below, or directly at the observed fish?

Due to the refraction of light, the fish appears to be higher up in the water then it actually is. Thus, you would want to aim below the image with a spear to actually hit the fish. If on the other hand you wanted to the hit the fish with a laser, you would aim at the image that you see because the laser will bend along the line shown when it enters the water and thus would hit the fish.

If a standing wave on a string has a fundamental frequency of f1 = 10 Hz, how many anti-nodes are present? How many nodes are present? What frequency would string need to vibrated in order to have the 2nd harmonic of standing waves, f2? How about the 3rd harmonic, f3? How many anti-nodes and nodes are present in each case?

For a fundamental frequency on a string there is always one antinode (in the middle) and two nodes (on the end) similar to a jump rope. The second harmonic always occurs at a frequency that is 2 times the fundamental frequency, thus for this case f2 = 2 f1 = 20 Hz. There would be 2 antinodes and 3 nodes in this case. The third harmonic always occurs at a frequency that is 3 times the fundament frequency, thus for this case f3 = 3f1 = 30 Hz. There would be 3 antinode and 4 nodes. For standing waves on a string there is always one more node than antinode.

How do frequency and period relate to each other?

Period and frequency are inverses of one another. As the frequency goes up the period goes down and vice versa.

If we double the frequency of a vibrating object, what happens to its period?

If you double the frequency of vibration, the period will decrease by a factor of 2 or will be reduced by 1⁄2.

Distinguish between a converging lens and a diverging lens.

In a converging lens light that strikes the lens along the principle axis will converge to a point on the opposite side of the lens (focal point). In a diverging lens light that strikes the lens along the principle axis will diverge on the opposite side of the lens. If the rays of light are followed back to the front of the lens they seem to come from a single point. (focal point)

The sound coming from one tuning fork can force another to vibrate. What is the analogous effect for light? (Think about light traveling through a transparent material and what the light does to the atoms of the material.)

In a transparent medium, light can cause the electrons in the atom that make up the transparent material to vibrate at the same frequency of the light. The electrons can then give off photons and that energy is transmitted to the next electron. Basically, the frequency of the light can cause the electrons in the material and sometimes the atoms themselves to vibrate at the same frequency.

Distinguish between constructive interference and destructive interference.

In constructive interference two waves with the same frequency (like sound waves from a speaker) are in phase with one another. This means that they meet in such a way that they add together. In destructive interference two waves with the same frequency are out of phase with one another. This means that they meet in such a way that they cancel each other out.

Distinguish between forced vibration and natural frequency? Explain.

In forced vibration and object is forced to vibrate at a particular frequency. This frequency can be changed and the object can be forced to vibrate at a different frequency. Natural frequency is the frequency that an object will naturally vibrate at when it is set into vibration. For example, a tuning fork has a natural frequency. When it is struck, it vibrates at that frequency. Other examples of natural frequency include, playing sound from wine.

What is required to make an object resonate?

In order to make an object resonate, you must cause it to vibrate at its natural frequency of vibration. This can cause large amplitudes in the vibration. (Tacoma Narrows Bride and breaking a glass with a particular frequency of sound wave.)

In the Doppler Effect, does frequency change? Does wavelength change? Does speed change? Does it depend on how the source of the sound is moving with respect to the observer?

In the Doppler Effect, the frequency changes. Depending on whether the source of the sound is moving or the observer is moving has an effect on the other answers. If the sound source is in motion and the observer is a rest, the speed of sound does not change but the wavelength of the sound does change. If the sound source is a rest but the observer is moving, the wavelength of the sound stays the same but the speed of sound changes.

Under what condition is light totally reflected? Will it be totally reflected when traveling from air into water, water into air, water into diamond, diamond into water?

Light is totally reflected (total internal reflection) when light travels from a region of high index of refraction (light is traveling slower) into a region when of lower index of refraction (light is traveling faster). It will not occur when light travels from air (nair = 1.00) into water (nwater = 1.33). It will occur when light travels from water (nwater = 1.33) into air (nair = 1.00). It will not occur when light travels from water (nwater = 1.33) into diamond (ndiamond = 2.42). It will occur when light travels from diamond (ndiamond = 2.42) into water (nwater = 1.33).

Rays of light in water that shine up to the water-air boundary at angles of more than 48° to the normal are totally reflected. No rays beyond 48° refract outside. How about the other way around? Is there an angle at which light rays in air meeting the air-water boundary will reflect totally? Or will some light be refracted at all angles?

Light traveling from air (nair = 1.00) into water (nwater = 1.33) will always be refracted. Total internal reflection can not occur when light travels from a region of low index of refraction (high speed of light) to a region of high index of refraction (lower speed of light).

Two observes standing apart from one another do not see the "same" rainbow. Explain.

Observers see different rainbows. The rainbow actually moves with the observer since it is related to how the sunlight strikes raindrops in the sky.

Why does the Sun look reddish at sunset and sunrise but not at noon?

Referring to the previous question, light scatters in the atmosphere. Blue light scatters the most and yellow, orange, and red the least. Light that isn't scattered is transmitted through the atmosphere. When the sun is low in the sky, the light must travel through more of the atmosphere. The thicker the atmosphere, the more scattering that can occur. The higher frequencies are scattered away and the red light is transmitted through the most since hardly any red light was scattered. At noon, when the sunlight travels through the least amount of atmosphere, and thus the yellow and orange light have not been scattered with the blue. The sun will appear to be a yellow or orange color as that color is transmitted through.

Distinguish between reflection and refraction. What is the law of reflection?

Reflection occurs when a wave (light) strikes a surface and bounces off. Refraction occurs when a wave (light) moves from one medium to another and bends due to a change in speed when it enters a new medium. The law of reflection states that the angle of incidence is equal to the angle of reflection (as measured from a line normal to the surface).

How does wave speed relate to the phenomenon of refraction (bending of waves)?

Refraction occurs when the speed of a wave changes from one location to another. This causes the wave front to bend or refract. In the case of sound, a different in temperature between the air at ground level and the air above the ground can cause a difference in the speed of sound and thus the sound wave bends. (see the PowerPoint slide from chapter 12 with the person blowing the horn and the dog.)

Suppose a sound wave and an electromagnetic wave have the same frequency. Which has the longer wavelength?

Since electromagnetic waves have a much larger speed in air (Speed of all electromagnetic waves in air is equal to the speed of light 3.00 X 108 m/s) they will have a larger wavelength.

Which travels more slowly in glass, red light or violet light?

Since violet light bends more than red light in glass this means that it travels more slowly. The bigger the difference in the speed of light between the mediums produces a greater the bending of the light. If we assume the light starts off in air and then enters glass (like a prism), the violet light bends the most which means that there is a bigger difference between it's speed of light in the glass as compared to red light.

Why is the Moon described as a "silent planet"?

Sound needs a medium to travel through. The moon does not have an atmosphere and thus there is no medium for the sound to travel.

Which requires a physical medium in which to travel, light, or sound? Or do both require a physical medium? Explain.

Sound requires a physical medium to travel through.

Does sound travel faster in warm air or in cold air?

Sound travels faster is warm air.

What causes a standing wave? What is a node? What is an antinode?

Standing waves are caused when a wave traveling forward interferes with the wave that is reflected back. When the frequency of vibration is just right for the medium, the interaction of the forward wave and the reflected wave results in regions where the waves cancel one another out completely and other regions where the amplitude of the wave is very large and there is a large vibration. The node is a region of zero vibration and the antinode is the region of the largest vibration.

What part of the electromagnetic spectrum is unable to penetrate the Earth's atmosphere? Why is this important?

The atmosphere is opaque (meaning that the electromagnetic waves can not be transmitted through) for high frequency ultraviolet light and other high frequency electromagnetic waves such as X-rays and Gamma rays. This is important because high frequency electromagnetic waves carry a great deal of energy. In fact, lower frequency ultraviolet light that makes it through the atmosphere is what causes sunburns. If other high frequency electromagnetic waves were able to penetrate the atmosphere, life on this planet would not be possible as we exist now.

How does the average speed of light in glass compare with its speed in a vacuum?

The average speed of light is glass is less than in a vacuum. Light always travels slower in a medium as compared to air (or a vacuum).

How does the frequency of an electromagnetic wave compare with the frequency of the vibrating electrons that produce it?

The frequencies are the same. All electromagnetic waves originate from a vibrating electron.

Why is it so quiet after a snowfall as compared to when there is not snow on the ground?

The fresh snow can absorb sound (similar to a carpeted room).

Short wavelengths of visible light interact more frequently with the atoms in glass than do longer wavelengths. Does this interaction time tend to speed up or slow down the average speed of light in glass?

The greater the number of interactions, the slower the speed of light in the material.

Relative to the distance of an object in front of a plane mirror, how far behind the mirror is the image?

The image is the same distance behind the mirror as the object is in front of the mirror.

Does the law of reflection hold for diffuse reflection?

The law of reflection also holds for diffuse reflection. In diffuse reflection, the surface does not appear smooth. The electromagnetic wave hit different bumps on the surface and reflect in all different directions due to the unevenness of the bumps; however, the law of reflection still holds.

How does the law of reflection hold for curved mirrors?

The law of reflection holds for all mirrors including curved/spherical mirrors.

How can a surface be smooth/polished for some waves and not for others? (This is an important question, refer to the homework on WebAssign and question regarding whether a surface is smooth/polished for some electromagnetic waves verses others)

The smoothness of a surface depends on the wavelength of the electromagnetic wave that strikes the surface. If the wavelength is large compared to the size of the imperfections in the surface (size of the bumps) then the surface will be considered smooth for that type of electromagnetic wave. Another type of electromagnetic wave with a much smaller wavelength may strike the same surface and the surface would not be considered smooth if the wavelength is small compared to the imperfections (size of the bumps) in the surface.

How does the speed of sound in water compare with the speed of sound in air? How does the speed of sound in steel compare with the speed of sound in air?

The speed of sound in water is greater than the speed of sound in air. The speed of sound in steel is greater than the speed of sound in water and thus is greater than the speed of sound in air. In general, sound travels fastest in solids, followed by liquids, and finally gases.

You dip your finger repeatedly into a puddle of water and make waves. What happens to the wavelength if you dip your finger more frequently?

The speed of the wave in the water will stay the same. Since speed = frequency X wavelength, as the frequency is increased, the wavelength will decrease.

Why does a struck tuning fork sound louder when it is attached to a sound box (demonstration)?

The tuning fork causes forced vibration in the attached sound box. This amplifies the sound. Another way to say this is that the tuning fork forces the box to vibrate at the same frequency as the tuning fork.

What is the main difference between the propagation of sound waves and the propagation of electromagnetic waves?

There are a few differences. (1) Sound needs a medium to travel through while electromagnetic waves do not. (2) Sound is a longitudinal wave and electromagnetic waves are transvers. (3) All electromagnet waves travel at the speed of light in air while sound waves travel at a much smaller speed.

Does sound tend to bend upward or downward when it is colder near the ground? Why does the sound bend?

When the temperature is colder at the ground, this means that the sound will travel slower in the air near the ground and faster in the air above the ground. Since the sound is traveling faster above the ground, the sound bends downward.

Distinguish between the white of this page and the black of this ink, in terms of what happens to the white light that falls on both? In other words, why does the page appear to be white and ink black?

When white light falls on this page, the white parts of the page reflect all the white light while the black ink absorbs all the white light. In fact, this particular page contains red ink. The red appears red because all but the red light is absorbed by the red lettering and only the red light is reflected bac.

How could you use the spotlight at a play to make the yellow clothes of the performers suddenly change to black? (same type of question: Under which light will a ripe banana appear black?) a. Red light b. Yellow light c. Green light d. Blue light

d. Blue light Since yellow light is a combination of both red and green light you would want to use a color of light that does not contain yellow. Blue is a primary color and does not contain yellow light.