Physics Light Unit

Property of Wave

1. Reflect 2. Refract 3. Lamda x frequencies = speed

Property of Particles

1. Travels through a vacuum. 2. Carries energy. 3. Travels in a straight line.

How long does it take for light to travel across the diameter of Earth's orbit around the sun?

1000s

What is the speed of light?

2.99 x 10^8

How long is the diameter of Earth?

300,000,000km

Shadow

A feature that is formed where light rays cannot reach.

Penumbra

A partial shadow.

When does penumbra appear?

A penumbra appears where some of the light is blocked, but where other light fills in. This can happen where light from one source is blocked and light from another source fills in or where light from a broad source is only partially blocked.

Polarization

A process that produces light traveling in only one plane

Vibrating Electrons v. Direction of Polarized Light

A single vibrating electron emits an electromagnetic wave that is polarized. A vertically vibrating electron emits light that is vertically polarized, while a horizontally vibrating electron emits light that is horizontally polarized.

Ray

A thin beam of light.

Umbra

A total shadow.

Electromagnetic Wave

A wave that is partly electric and partly magnetic.

Spring Strength

All materials that are springy (elastic) respond more to vibrations at some frequencies than others. Different materials have different electric spring strengths.

Opaque

An object will not transmit light. It will either reflect or absorb.

Transparent

An object will transmit light

Is it correct to say that a radio wave is a low-frequency light wave? Is a radio wave also a sound wave?

Both a radio wave and a light wave are electromagnetic wave originated from the vibrations of electrons. Radio waves have much lower frequencies of vibration than light waves, so a radio wave may be considered to be a low-frequency light wave. A sound wave, on the hand, is a mechanical vibration of matter and is not electromagnetic wave. Thus, a radio was is not a sound wave.

Speed of Light in Water, Glass, and Diamond

Diamond (.67c) < Glass (.67c) < Water (.75c)

Ultraviolet

Electromagnetic waves of frequencies higher than those of violet of visible light.

Infrared

Electromagnetic waves of frequencies lower than the red of visible light.

Electrons and Ultraviolet Light

Electrons in glass have natural vibration frequency in the ultraviolet range. When ultraviolet light shines on glass, resonance occurs as the wave builds and maintains a large vibration between the electron and the atomic nucleus. If ultraviolet light interacts with an atom that had the same natural frequency, the vibration amplitude of its electrons becomes large. The atom typically holds on to this energy for quite a long time. During this time the atom makes many collisions with other atoms and gives up its energy in the form of heat.

Light v. Gamma

Gamma has higher frequencies but light had longer wavelengths.

Ultraviolet v. Gamma

Gamma has higher frequencies but ultraviolet has longer wavelengths.

Transparent Materials

Glass and Water

Why are metals shiny in appearance?

In metals, the outer electrons of atoms are not bound to any particular atom. They are free to waned with very little restraint throughout the material. When light shines on metal and sets these free electrons into vibration, their energy does not transfer from atom to atom in the material, but is reemitted as visible light. This reemitted light is seen as a reflection.

Electrons and Infrared Light

Infrared waves, which have frequencies lower than visible light, vibrate not only the electrons, but also the entire structure of the glass. This vibration of the structure increase the internal energy of the glass and makes it warmer.

Albert Michelson's Experiment

Light from an intense source was directed by a lens to an octagonal mirror initially at rest. The mirror was carefully adjusted so that a beam of light was reflected to a stationary mirror located on a mountain 35km away, and then reflected back to the octagonal mirror and into the eye of an observer. If the rotating mirror made exactly one-eighth rotation in the time the light made the trip to the distant mountain and back, the mirror would be in a position to reflect light into the eyepiece. When he accomplished this by adjusting the speed of the octagonal mirror at a high rate and he knew that the time for the light to make the round-trip and the time for the octagonal mirror to make one-eighth of a rotation was the same. He divided the 70-km round-trip distance by this time.

Radio v. Light

Light has higher frequencies but radio has longer wavelengths.

Light and Electromagnetic Spectrum

Light is a small portion of the broad family of electromagnetic waves that includes such familiar forms as radio waves, microwaves, and X-rays.

Light

Light is both a wave and a particle. Light is a transverse wave.

Axis of Polaroids

Light is transmitted by a pair of polaroids when their axes are aligned together and and light is blocked when their axes are crossed at right angles. Transmitted light is polarized in a direction the same as the polarization axis of the filter.

Light incident upon a pane of glass slows down in passing through the glass. Does it emerge at a slower speed or at its initial speed?

Light reemerges from glass at its original speed. The delays caused by absorption and reemission of light in the glass are not present once light leaves the glass.

How long does it take to travel from the sun to Earth? From the star Alpha Centauri to Earth?

Light takes 8 minutes to travel from the sun to Earth, and 4 years from the next nearest star, Alpha Centauri. It could also make 7.5 trips around the Earth in one second.

Reflected Light's Vibration Direction

Light that reflects at glancing angles from nonmetallic surfaces, such as glass, water, or roads, vibrates mainly in the plane of the reflecting surfaces. Therefore, glare from a horizontal surface is horizontally polarized.

Time Delay of Reemitted Light

Light travels at different average speeds throughout different materials.

Warm Water v. Cold Water

Light travels at slightly different speed in warm and in cold water. The difference in the speed bends the light.

Photons

Massless bundles of concentrated electromagnetic energy particles

Which type of eclipse is dangerous to your eyes if viewed directly?

People are cautioned not to look at the sun at the time of a solar eclipse because brightness and ultraviolet radiation of direct sunlight is damaging to the eyes. Staring at the sun when it is high in the sky is harmful whether or not an eclipse occurs. In fact, staring at the bare sun is more harmful than when part of the moon blocks it. The reason for special caution at the time of an eclipse is simply that more people are interested in looking at the sun during an eclipse.

ROYGBV

Red, Orange, Yellow, Green, Blue, and Violet.

Why do reflection and transmission occur?

Reflection and transmission of light waves occur because the frequencies of the light waves do not match the natural frequencies of vibration of the objects. When light waves of these frequencies strike an object, the electrons in the atoms of the object begin vibrating. But instead of vibrating in resonance at a large amplitude, the electrons vibrate for brief periods of time with small amplitudes of vibration; then the energy is reemitted as a light wave. If the object is transparent, then the vibrations of the electrons are passed on to neighboring atoms through the bulk of the material and reemitted on the opposite side of the object (transmitted). If the object is opaque, then the vibrations of the electrons are not passed from atom to atom through the bulk of the material. Rather the electrons of atoms on the material's surface vibrate for short periods of time and then reemit the energy as a reflected light wave (reflected).

Three sets of polaroids, one atop the other, are shown above. In each set the polarization axes of two polaroids are at 90 degrees to each other, and a third is at 45 degrees to the two. Which set will pass light where the three overlap?

Set B will pass light where the three overlap. In set A, all light is blocked by the back pair of polaroids because they are at 90 degrees to each other. Likewise with the front pair of polaroids in set C. In set B, the vertical component gets through the second polaroid because it is at 45 degrees.

Shapes of Shadow

Shapes of shadow depend on how the light is bent.

Solar System Map

Sun - Mercury - Venus - Earth - Mars - Jupiter - Saturn - Uranus - Neptune - Pluto

Albert Michelson

The American physicist who measured the most exact speed of light in 1880 and received the 1907 Nobel prize in physics for this experiment.

Olaus Roemer

The Danish astronomer who did the first demonstration that light travels with finite speed in 1675. He carefully measured the periods of Jupiter's moon and found out the irregularity in the measurement. He discovered when Earth was moving towards Jupiter, the measured periods were shorter than average and when Earth was moving away from Jupiter, the measured periods of the moon were longer than average.

Christian Huygens

The Dutch scientist who argued that light was a wave and interpreted the change in the measured periods of Jupiter's rotation due to the change in the location in a right way that when Earth was further away from Jupiter, it was the light that was late, not the moon.

What is one factor that affect the average speed of electromagnetic waves?

The actual speed of an electromagnetic wave through a material medium is dependent upon the optical density of that medium. Different materials have their atoms more closely packed and thus the amount of distance between atoms is less. As a result, the speed of an electromagnetic wave is dependent upon the material through which it is traveling.

Polaroid Axis 30 degrees v. 40 degrees

The amount of light that gets through the polaroids at 30 degrees, compared to the amount that gets through the 45 degrees polaroids is more.

Polarization Vectors

The amplitude of a light wave has magnitude and direction, and can be represented by a vector. Polarized light vibrates in a single direction and is represented by a single vector. To the left the single vector represents vertically polarized light. The vibrations of non-polarized light are equal in all directions. There are as many vertical components as horizontal components. The pair of perpendicular vectors to the right represents non-polarized light.

Speed of Light in Glass

The average speed of light that goes through a piece of glass is decreased due to the time delays that accompany the absorption and re-emissio of light from atom to atom.

Light Year

The distance light travels in one year is called a light year.

Polarizing Filter

The filter is said to have a polarization axis that is in the direction of the vibrations of the polarized light wave.

Frequency of reemitted light

The frequency of the reemitted light passed from atom to atom is identical to that of the light that produced the vibration to begin with. The main difference is a slight time delay between absorption and reemission.

Why is glass transparent to visible light but opaque to ultraviolet and infrared?

The natural frequency of vibration for electrons in glass matches the frequency of ultraviolet light, so resonance in the glass occurs when ultraviolet waves shine on it. These energetic vibrations of electrons generate heat instead of wave reemission, so the glass is opaque to ultraviolet. In the range of visible light, the forced vibrations of electrons in the glass are more subtle, and reemission of light rather than the generation of heat occurs, so that the glass is transparent. Lower-frequency infrared waves cause entire atomic structure to resonate so heat is generated and the glass is opaque to infrared.

Electromagnetic Spectrum

The range of electromagnetic wave. The color spectrum is only a small portion of the the range of electromagnetic wave.

Polarized Wave

The vibrations that are back and forth in only one direction.

Why are lunar eclipses more commonly seen than solar eclipse?

There are usually two of each year. However, the shadow of the moon on Earth is very small compared with the shadow of the larger Earth on the smaller moon. Only a relatively few people are in the shadow of the moon (solar eclipse), while everybody who views the nighttime sky can see the shadow of Earth on the moon (lunar eclipse).

Ultraviolet v. Infrared

Ultraviolet has higher frequencies but infrared has longer wavelengths.

Source of Electromagnetic Waves

Vibrating electrons emit electromagnetic wave that carrie energy called light.

Wagon Wheel Illusion

Wagon wheels in movies appear still or moving backward as the wagon moves forward. This illusion occurs because humans eyes can't detect the gaps between still frames. If the wheel with six identical spokes turns four times each seconds, that is 24 spokes each second, which is exactly the number of frames. So a picture of the spoke is always at the same place on the screen. When the speed/frequency of the wheel doesn't match with the number of frames each second, being a little further behind, the wheels appear to be moving backward.

Lunar Eclipse

When the Earth casts a shadow, the shadow extend into space and the moon passes into it, the moon is not visible on certain parts of the Earth. Whereas a solar eclipse can be observed only in a small region of Earth, a lunar eclipse can be seen by all observers on the nighttime half of Earth.

Electrons and Visible Light

When the electromagnetic wave has a lower frequency than ultraviolet, the electrons are forced into vibration with smaller amplitudes. The atom holds the energy for less time, with less chance of collision with neighboring atoms, and less energy is transferred as heat. The energy of the vibrating electrons is reemitted as transmitted light.

Solar Eclipse

When the moon passes between Earth and the sun, because of the large size of the sun, the rays taper to provide an umbra and a surrounding penumbra. The moon's shadow. In this case, the moon's shadow barely reaches the Earth and the sun is not visible on small area of Earth.

Is it correct to say that in every case, without exception, any radio wave travels faster than any sound wave?

Yes, it is correct to say that any radian wave travels faster than any sound wave. A radio wave is an electromagnetic wave so any radio wave is simply a low frequency light wave. Therefore, any radio wave travels at the speed of light. On the other hand, the speed of sound in air is 34om/s. It is true that sound travels faster in other media but in no case at the speed of light. Therefore, no sound wave can travel as fast as any radio wave.

Light entered the eyepiece when Michelson's octagonal mirror made exactly one-eighth of a rotation during the time light reflected to the distant mountain and back. Would light enter the eyepiece if the mirror turned one-quarter of a oration in this time?

Yes, light would enter the eyepiece whenever the octagonal mirror turned in multiplies of one eighth rotation in the time the light made its round trip. What is required is that any of the eight faces be in place when the reflected flash returns from the mountain. Michelson did not spin the mirror fast nougat for these other possibilities to occur.