· From the Fenway Park bleachers, why do you see Jacoby Elsbury hit the baseball before you hear it?

LIGHT

What is Light? Early theories of light varied. One was that your eyes emitted beams that would illuminate things for you. That was why you could only see when your eyes were pointed at something(~500 BC). Eventually people came to believe that light was either a wave or a bunch of moving particles. Einstein coined the term “photon” which represents the tiny massless particle that light may exist as.

What is light? The fact of the matter is that light behaves in certain ways so that currently it can only be described as both a wave and a particle. Therefore, it is appropriate to discuss light as if it were a wave. Electromagnetic radiation (electromagnetic waves):

Measuring the speed of light Can you think of a way that a person might measure the speed of light? During early attempts to measure the speed of light, scientists would up just measuring their own reaction time.

Measuring the speed of light Using the relative motion of Earth toward and away from Io (the innermost moon of Jupiter), Roemer was able to calculate the speed of light to an amazingly accurate approximation (1675).

Measuring the speed of light Accurately measured the speed of light on earth in 1880 first American to win the Nobel prize in 1907 reflected light from a mirror 35 km away spinning octagonal mirror allowed him to measure the time it took 299,920 km/s 300,000 km/s Michelson

/UNIT_4/Animation/michelson.html Measuring the speed of light

Just how fast is light? Speed of light in a vacuum is constant in universe 7.5 round trips around the earth in one second 8 minutes from the sun to the earth 4 years from the nearest star, Alpha Centauri 100,000 years to cross our galaxy some galaxies are 10 billion light years away

Measuring the speed of light A “light year” is a measurement of distance based on how far light can travel in a single Earth year. How far is a light year? Distance = velocity x time Light year = 300,000 km/s x 1 year = 300,000 km/s x 1 yr x 365 day/yr x 24 hr/day x 3600 s/hr Light year = 9.5 x km

Electromagnetic Spectrum Light is energy that is emitted by vibrating electric charges called an electromagnetic wave radio waves, microwaves, infrared, ultraviolet, x-rays, gamma rays are also electromagnetic waves lowest frequency we see is red highest frequency we see is violet (more energetic)

R ed O range Y ellow G reen B lue I ndigo V iolet ROY-G-BIV

The Doppler Effect and Light What is the Doppler Effect? How do we perceive the Doppler Effect for light waves? Red Shift Blue Shift In what ways can scientists use this knowledge?

When we see When light waves hit a material a number of things can happen depending on the resonant frequency of the particles of the material and the type of light that hits it. Light has a very high frequency –100 trillion times/second –10 14 hz Light hitting an object causes its electrons to vibrate

Transparent to Light Transparent: lets light pass through in a straight line Glass and water are transparent to light Spring and ball model of light transmission –light causes springs and balls to vibrate –vibration transfers within the material –energy is released on the other side

Transparent to Light Glass Example: Glass has a natural frequency similar to that of UV rays. When UV rays hit glass it resonates and the amplitude of vibration increases. BUT! Glass doesn’t transmit UV rays (in other words you are safe from sunburn due to UV radiation if you sit by a closed window). Glass likes to hold onto UV radiation. But what happens to that energy then? Results in heat

Transparent to Light Particles in glass hold onto frequencies lower than UV rays for less time than UV rays. As the light gets transmitted from atom to atom through the glass, the relative speed of light reduces to lower than c. So, glass does transmit visible light but doesn’t transmit UV rays and IR rays.

The speed of light (c) Speed of light = c = 300,000 km/s speed of light in glass = 0.69 c speed of light in water = 0.75 c speed of light in diamond = 0.40 c Just like all other waves, the speed of light in different materials has different speeds, so as light goes from on material to another, the speed changes!

Opaque Materials An opaque material absorbs certain frequencies of visible light but reflects other frequencies (it does not transmit light). We perceive the color of something based on the colors that get reflected off an opaque material.

Rays A thin beam of light is a ray Any beam of light can be considered a bundle of rays Rays travel in straight lines Example: Lasers are like “ray” guns

Shadows A shadow is formed when a light ray cannot reach a surface sharp shadows –produced by small source close by –large source far away total shadow: umbra partial shadow: penumbra –light from another source fills in –large source only partially blocked

solar eclipse and lunar eclipses are examples of shadows on a large scale

Polarization Light is a transverse wave Light from most sources vibrates in all planes Each light ray can be considered to have horizontal and vertical components Separating vertical and horizontal components is called polarization

Polarization Polarizing filters are like sewer gratings that look like slits. Light waves vibrating in the plane of the slit can make it through Light waves that vibrate perpendicular to the grates cannot make it through

Polarization A single polarizing filter will let about one half of the light through Two polarizing filters aligned in the same direction will still let about one half of the light through Two polarizing filters aligned perpendicular to one another will let almost no light through

Applications of Polarizing Filters Sun Glasses –reduce glare –block out half of the light 3-D movies

Why is the sky blue? Where do rainbows come from?

Diffraction When waves bend around an object The smaller the object compared to the wavelength, the more the bending. Wavelength and velocity stay the same.

Diffraction of Light Radio waves can be broadcast through cities or over far distances due to diffraction. What type of wave gets diffracted more?

· Why is it hard to pick a penny off of the bottom of a swimming pool?

Why is it difficult to catch a fish with a spear? (think of why it your hand looks all squishified when you look at it in a sink full of water?)

Refraction Refraction is the bending of the wave when it goes into a different medium. sol.sci.uop.edu/~jfalward/refraction/coneofdar knessforfish.jpg&imgrefurl= du/~jfalward/refraction/refraction.html&usg=__ wdp8rn5LcygwwDT8yIuH997IrCc=&h=291&w =363&sz=24&hl=en&start=10&sig2=d2h337U NTOgdgdu53dvuDA&um=1&tbnid=MVDsEpD IBBtIkM:&tbnh=97&tbnw=121&prev=/images %3Fq%3Drefraction%2Bof%2Blight%26ndsp %3D21%26hl%3Den%26sa%3DG%26um%3 D1&ei=T90IStP2HaLNlQf54sznCw

Light can be bent as it passes past barriers. This is how a mirage occurs. It is the bending of light as it passes through different temperatures of air. And it is why some roads look wet on a hot day

Reflection - how a mirror works Smooth surface angle of incidence of ray equals angle of reflection - no matter the shape of the mirror (law of reflection) All light is reflected, none is absorbed

Plane mirror

Bill looks at Emily’s reflection in the mirror. Draw the position of Emily’s image. Draw the path of the light rays that travel from Emily to Bill. Bill Emily

Concave Mirrors: (like a cave!) Satellite dishes are like concave mirrors shaped like a parabola Image from a concave mirror depends on the placement of the object in relation to the mirror

As you begin to learn to drive you notice your mirrors say “objects in mirror are closer than they appear.” What the heck?

Both of these types of mirrors still obey the law of reflection Convex Mirrors A convex mirror is shaped like the outside bottom of a spoon. Convex mirrors are used as rearview mirrors in cars and in stores to observe shoppers.