1 light light mike maloney physics, shs © 2003 mike maloney2 objectives what is light? where does...

1

LightLightMike MaloneyPhysics, SHS

© 2003 Mike Maloney 2

Objectives

• What is LIGHT?• WHERE DOES IT COME FROM?


Light

• What is LIGHT?• WHERE DOES IT COME FROM?


What is Light?

• Light is a wave, or rather acts like a wave.

• How do we know since we cannot see it?

• We can detect how it acts through …– Reflection– Refraction– Dispersion– Diffraction– Interference– Polarization


What is Light• Light is a special type of wave• What we know as VISIBLE

LIGHT is actually a type of something called ELECTROMAGNETIC RADIATION.

• So, what is electromagnetic radiation and what are electromagnetic waves?Do you know any other types?


Electromagnetic Waves

• Electromagnetic waves are everywhere.

• Light is only a small part of them

– Radios– TVs– Microwaves– Light

(Visible/UV/InfraRed)

– Radiation– Lasers– Tanning

Huts– X-Rays


Electromagnetic Waves• Where do these waves come

from?• When something releases

energy it also emits radiation. Depending on the amount of energy, the object will emit different types of electromagnetic radiation.

• But what is creating the disturbance? What is emitting this energy?

• For visible light it is ELECTRONS, oscillating electrons! link



• Electrons in materials are vibrated and emit energy in the form of photons, which propagate across the universe.– http://www.colorado.edu/physics/2000

/waves_particles/wavpart4.html• Photons have no mass, but are

pure quantized packets of energy.• Electromagnetic Waves are waves

that are made up of these “photons”.

• When these photons come in contact with boundaries, E-M waves interact like other waves would.

http://www.colorado.edu/physics/2000/waves_particles/wavpart4.html

http://www.colorado.edu/physics/2000/waves_particles/wavpart4.html



• When we studied mechanical waves, they were all transferred through a medium. What medium is light transferred through?

• LIGHT DOES NOT NEED ONE!• Electromagnetic waves are special

in the fact that they do not need a medium to propagate through.

• Then what is oscillating?– SPACE ITSELF!– EM Wave


Electromagnetic Spectrum


What creates the different types?

• gamma rays are produced by radioactive decay and nuclear processes. They also can be produced in black holes and other stellar events.

• X-rays are produced by decelerating electrons.• ultraviolet (UV), visible, and infrared (IR)

waves are produced by electron transitions and by vibrating atoms and molecules.

• Microwaves, radio and TV waves are produced by charges accelerated in antennas.

• waves with even lower energy are known as long waves, because of their very long wavelength


How do they affect us?Electromagnetic waves can have a number of

different interactions with the human body:• Gamma rays and X-rays are known as ionizing

radiation. They can cause chemical changes as well as mutations of DNA. (remember the vest they put on you when you get an XRAY)

• Ultraviolet light is associated with sunburns, cataracts, skin cancer and can damage cells.

• Rods and cones in our eyes are sensitive to visible light, which is why we can see.

• Heat sensors in our skin can detect infrared waves which is why we can tell if stuff is hot.

• We're not particularly sensitive to EM waves with longer wavelengths than infrared.


Speed of E/M Waves

• From last chapter, we found that – V = f *

• We also said that the speed of a wave in a certain medium is always constant.

• It has been found that the speed of E-M waves and light in a vacuum is– 3 x 108 or 300,000,000 m/s– 671,000,000 mph– 186,000 miles per second– We call this value “c”

– For comparison vsound ≈ 780 mph


c = f *

• C is constant throughout the universe, as long as light is in a vacuum.

• When it is in other materials, vlight can decrease, but can never be larger than its value in a vacuum [c].

• Since “c” is constant, all of E-M waves will have a corresponding frequency to go along with their wavelength.

• Lets find the corresponding frequency ranges for a few of the groups of E-M waves.


Energy in E-M Waves• Which waves do you think have more energy,

Radio waves or gamma waves?• Because waves don’t really have a mass, we can’t

really talk about their energy like mechanical waves.

• The greater the frequency of an E-M wave, the more crests pass a point in a certain amount of time, therefore the more photons pass that point.

• Which means More energy moves past that point in a certain amount of time or the wave is more energetic.

• We can even find out how much energy each of those wiggling photons has E = h * f, where h is Plank’s constant .. but don’t worry you won’t need to know this until next year, now just knowing that higher frequency light has more energy is good enough.

• HANG ON A SECOND?!



Let’s Talk about some of these again…



• Electromagnetic waves are everywhere.

• Light is only a small part of them

– Radios– TVs– Microwaves– Light

(Visible/UV/InfraRed)

– Radiation– Lasers– Tanning

Huts– X-Rays

What makes some of these dangerous?


Back to Light

• There seem to me an incredible about of light in the universe that we don’t even notice.

• So, why can we only see a small portion of these E-M waves?


Our Eyes


Visible Light

• We now know what we see is part of the electromagnetic spectrum. We know that the light waves enter our eye, and stimulate parts of it that cause a electrical impulse to be sent to the brain which creates this visual image.

• But everything does not emit EM radiation. How do we see my shirt? And why can we barely see a window?


Seeing things• We know that when waves run into a

boundary they are partially transmitted and partially reflected.

• Light behaves as a wave, so it too is reflected.

• Therefore, an object does not need to emit photons itself to be seen, it just has to reflect light back to our eyes where we can detect it.

• Objects that do not allow light to pass through them are called opaque.

• Objects that allow light to pass through them are considered transparent.

• Objects in between are called translucent.


Color• Different objects may emit

different wavelengths of E-M radiation, so we would see that light as different colors.

• But why do we see colors in objects that reflect light? If you shine a white light on my clothes, and it gets reflected why doesn’t all of my clothes appear white?

• When I shine white light through a colored piece of plastic, why does it change color?


Color

• The light we see is know as visible or white light – although it is not that simple.

• The light is not really white, the white we see is a combination of all the colors of the rainbow.

• Remember R-O-Y G. B-I-V from art class.

• When all of these light waves are combined we see white light.


Color Reflection

• So if we see something as WHITE, that means …– It reflected back all the

wavelengths of light to our eyes

• If we see something as RED or BLUE– It reflected only the RED or only the

BLUE wavelengths– The others were absorbed.

• And if we see something as black ?– It did not reflect back any of the

light.


Color Transmission

• Filters work in a similar way. They control what wavelengths of light leave a material.– RED filters only let RED light thru.– BLUE let only BLUE light thru.

• What do you think that UV sticker means on your sunglasses?

• Why do they sell those orange glasses that are supposed to reduce glare?


Some Sweet Color Tricks• Combining color lights • Combining colors in art class• How does color printing work?• What color is your shirt really?• Why is the sky blue?• Why are sunsets red?• Why is water greenish-blue?• How does a greenhouse work?• InfraRed, that’s HOT• How does 3-D work?• Polarization• How does Phosphorescence (blacklight)

work?• How can you help people who are color

blind? --http://colorfilter.wickline.org/

• Doppler Effect / Red shift of stars• Lasers


Flux

• We now know how light behaves, but we must measure how strong it is.

• The rate at which a source emits light is called the LUMINOUS FLUX (P).(greek or latin for light flow)

• What do you think this is measured in? What are light bulbs measured in.

• LUMINOUS FLUX (P) is actually measured in something called a lumen (lm).

• A typical 100-W bulb emits about 1500 lm.


Illuminance

• Flux is the total of all the light that is emitted from a source.

• This is not very useful, often we would like to know how much of that light is hitting a surface at some point.

• The illumination of a surface is called illuminance, E. It is measured in lumens per square meter, lm/m2


Illuminance

• How do you think illuminance is affected when the object moves away from the source?– Right the illuminance decreases

• So what would you expect an equation to look like for E in terms of P and the distance away d?– Close for an expanding sphere of

light, it isE = P4d2

Where did the 4 d2 come from?



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Visible Light Spectrum

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Short wavelength

High Frequency

Long wavelength

Low Frequency


Excited Electrons

1. The atom is normal. 2. The electron absorbs a packet of energy. 3. Because of the excess energy, the electron must move to a higher

energy level. 4. The atom is now "excited". 5. The electron immediately drops back to its normal energy level. 6. To do this, the electron releases the excess energy in the form of a

photon. 7. Light is produced.

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Combining Colored Lights• Also known as color addition.

• When you combine light colors

from different sources, you add that color to the light that gets reflected to our eyes.

• If you add two complimentary colors, such as blue and yellow, or green and magenta, or red and cyan, you end up with white light.

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Combining Colored Pigments• Also known as color subtraction.

• When you combine pigment colors, as

you do when mixing paints, you end up subtracting colors that get reflected to our eyes.

• If you add two complimentary color pigments, such as blue and orange, or green and magenta, you end up with black, because all colors end up absorbed.

• The same is true if you filter colors from a source. If you have a white source, and you first filter with blue then with orange, almost all the light will be blocked.

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Why is the Sky Blue?• As light moves through the atmosphere, most of the

longer wavelengths pass straight through. Little of the red, orange and yellow light is affected by the air.

• However, much of the shorter wavelength light is absorbed by the gas molecules. The absorbed blue light is then radiated in different directions. It gets scattered all around the sky. Whichever direction you look, some of this scattered blue light reaches you. Since you see the blue light from everywhere overhead, the sky looks blue.

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Why is the Sunset Red?• As the sun begins to set, the light must travel farther through the

atmosphere before it gets to you. More of the light is reflected and scattered. As less reaches you directly, the sun appears less bright. The color of the sun itself appears to change, first to orange and then to red. This is because even more of the short wavelength blues and greens are now scattered. Only the longer wavelengths are left in the direct beam that reaches your eyes.

• The sky around the setting sun may take on many colors. The most spectacular shows occur when the air contains many small particles of dust or water. These particles reflect light in all directions. Then, as some of the light heads towards you, different amounts of the shorter wavelength colors are scattered out. You see the longer wavelengths, and the sky appears red, pink or orange.

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What color is your shirt really?

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• You are wearing a white shirt. • What color is it really? Is it always white?• The color of something depends on what type of light it is

reflecting. • A “white” shirt in blue light looks blue not white.• What would a pure RED shirt look like in pure BLUE light?

– BLACK


Why is Water Blue?• It absorbs the reds, yellows and

greens allowing only the blues and purples to pass thru to reach our eyes.

• It IS NOT just a reflection of the sky.

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3-D

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Light Amplification by the Stimulated Emission of Radiation• Laser light is very different from normal

light. • The light released is monochromatic. It

contains one specific wavelength of light (one specific color). The wavelength of light is determined by the amount of energy released when the electron drops to a lower orbit.

• The light released is coherent. It is “organized” -- each photon moves in step with the others. This means that all of the photons have wave fronts that launch in unison.

• The light is very directional. A laser light has a very tight beam and is very strong and concentrated. A flashlight, on the other hand, releases light in many directions, and the light is very weak and diffuse. BACK


Light Amplification by the Stimulated Emission of Radiation

• Lets see more detail of a ruby laser

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• Lets see more detail of a ruby laser


Doppler Effect and Color Shifts

• The radiation emitted by an object moving toward an observer is squeezed – its pulses are pushed closer together; its frequency appears to increase and is therefore said to be blueshifted because it moves towards the blue end of the light spectrum. In contrast, the radiation emitted by an object moving away is stretched or redshifted.

• Blueshifts and redshifts exhibited by stars, galaxies and gas clouds also indicate their motions with respect to the observer.

See the animation

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Greenhouses• Greenhouses work on the

same principle that the Mythbusters used to beat the IR camera.

• Think glass allows visible light to pass through, but does not allow IR light to pass through.

• The energy in the sunlight in the form of visible light can get in to feed the plants.

• The plants then radiate IR energy in the form of heat, like all living things.

• This IR radiation gets reflected when it tries to pass through the glass.

• The energy is trapped, and the greenhouse stays warm.

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Polarization

• Another reason we know light is a wave it because it can be polarized.

• Polarization is a phenomenon of light that is used in sun-glasses and 3-D movies.

• Play with the two polarizing filters for a few minutes and note what is happening and see if you can think of any reasons for it.

• Think about the following:– What happens to the strength of the

light when it goes through one polarizer.– What about two or three?BACK


Polarization Hint• Light vibrates in all directions.• A polarizing filter acts like a

picket fence. It only lets certain direction vibrations pass through it.

• Therefore, if you pass light through two of them you can completely block the light from passing through.

• HOW?BACK


Polarization

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1 light light mike maloney physics, shs © 2003 mike maloney2 objectives what is light? where does...

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