- Slides: 39
Part 2: Light We get almost all of our information about “what’s up there” by looking at the light coming from those objects. We have landed on the Moon and have sent landers to Mars and have crashed satellites into some of the moons, but other than that, we use light to determine what’s up there. This section will look at the properties of light to see how we can use light to figure out what is up there.
Light What is it?
Light What is it? moving energy You can’t stop light and still have it be light. You can stop light but then the light changes into some other kind of energy, such as heat or electric energy (e. g. , in photocells). Does this light energy move as a Wave or as a particle?
Light What is it? moving energy Wave or particle? If a wave, what is waving? (waving even in a vacuum? )
Light What is it? moving energy Wave or particle? If a wave, what is waving? (waving even in a vacuum? ) Electric & Magnetic Fields Gravity also acts across the vacuum of space, and our theories say that gravity acts via gravitational fields in space. Electricity and Magnetism also act across the vacuum of space, and our theories say that both electricity and magnetism act via electric and magnetic fields. How do we decide between wave and particle?
Light What is it? moving energy wave or particle? If a wave, what is waving? (waving even in a vacuum? ) Electric & Magnetic Fields In the particle theory of light, a particle of light is called a photon. How do we decide between wave and particle? Look at properties of light and see which theory explains the properties the best.
Properties of Light • • speed of light colors reflection shadows refraction (bending) energy theory absorption of light emission of light
Property 1: Speed of Light • particle (photons) ? • wave (oscillations of E&M fields that move through space) no prediction in vacuum, v = c; in material, v < c (Here c stands for the speed of light in vacuum, which is 300, 000 meters/second, or about 670 million miles per hour. For comparison, the speed of sound in air is about 700 miles per hour, or about a million times slower than light!) From experiment, we find that the wave prediction works!
Property 2: Color • Experiment ? White light is a combination of all the colors. Below is an image taken with a camera of white light broken into its colors. We’ll have a demonstration in class where you can see this for yourself. • Particle (photon) explanation? • Wave (E&M) explanation?
Property 2: Color Experiment: visible order: • red • orange • yellow • green • blue • Violet This is the same order of colors as in the rainbow. Are there “colors” of light beyond red and violet that are not visible to our eyes?
Property 2: Color Are there “colors” of light beyond red and violet that are not visible to our eyes? Yes! Experiment: There are forms of light that are invisible as well as visible. The technology used to “see” or “make” the light determines how we group them. Total spectrum order: • radio (there are different “colors” of radio that correspond to the different radio stations) • microwave (there are different “colors” of microwaves that correspond to our different cell phone numbers) • • IR (infrared) visible UV (ultraviolet) x-ray and gamma ray
Property 2: Color Particle (photon) explanation? Amount of energy per photon determines “color”
Property 2: Color Particle (photon) ? amount of energy among the different types: x-ray - most energy; radio - least in visible portion: violet - most energy; red - least If a photon has enough energy to ionize an atom, that photon cause biological damage. X-rays have the most energy per photon so they are the most dangerous; radio photons have the least energy so they (along with microwave, IR, and visible) don’t cause biological damage unless there are so many of them that they end up heating the object as in a microwave oven.
Property 2: Color • Particle (photon) explanation ? Amount of energy • Wave (E&M) explanation?
Property 2: Color • Particle (photon) explanation ? Amount of energy • Wave (E&M) explanation? Frequency among different types of “light”: high frequency is x-ray & gamma ray low frequency is radio (AM is 500 -1500 KHz) Different radio stations have different frequencies. in visible spectrum: violet is highest frequency (just below UV) red is lowest frequency (just above IR)
Wavelength and Frequency “Nice” sine waves have a simple relation for wavelength and frequency: λ*f = v where λ is the wavelength (distance from one crest to the next one), where f is the frequency (how many times one location goes up and down a second), and where v is the speed of the wave (how fast the crest of the wave moves). λ v
Light For light in vacuum, the speed of the light wave is 300, 000 meters/sec, or about 670 million miles/hour. We use the symbol “c” to denote this value. Therefore for light in vacuum, we have: λ*f =c. Example: for a radio wave of frequency 100 MHz, the wavelength is: λ * (100 * 1, 000 Hz) = 300, 000 m/s, or λ = 300, 000 m/s / 100, 000 Hz = 3 meters.
Nanometers The wavelength of visible light is in the range of 0. 000000400 meters to. 000000700 meters. This is an awkward way to write these numbers. In Scientific Notation, this becomes 4 x 10 -7 m to 7 x 10 -7 m. This is still somewhat awkward, so we often use the unit of nanometers (nm) which is 10 -9 meters; this gives the range for the wavelengths of visible light to be 400 nm to 700 nm. In some cases it is easier to determine the frequency of the light. In other cases, it is easier to determine the wavelength. Hence both frequency and wavelength are used to measure exact “colors” of light.
Colors: frequencies & wavelengths (in vacuum) AM radio 1 MHz 100’s of m FM radio 100 MHz m’s microwave 10 GHz cm - mm Infrared (IR)1012 - 4 x 1014 Hz mm - 700 nm visible 4 x 1014 - 7. 5 x 1014 Hz 700 nm - 400 nm Ultraviolet (UV) 7. 5 x 1014 - 1017 Hz 400 nm - 1 nm x-ray & gamma ray > 1017 Hz < 1 nm You are responsible for the order of the types of light. Based on their wavelengths, AM radio the longest, ray the shortest. Based on frequency the order is opposite: AM radio have the smallest frequencies, rays have the highest.
Colors and Wavelengths in the visible Red Orange Yellow Green Blue Violet 700 – 650 nm 650 – 600 nm 600 – 550 nm 550 – 500 nm 500 – 450 nm 450 – 400 nm All of these values are rough. You are responsible for these ranges. 1 micrometer = 1, 000 nm = (1/1, 000)mm
Property 3: Reflection • Particle (photon) explanation? • Wave (E&M) explanation? mirror
Property 3: Reflection • Particle (photon) ? • Wave (E&M) ? bounces “nicely” bounces nicely means angle incident = angle reflected Does light bounce nicely off of a mirror? Does light bounce nicely off of a white sheet of paper, or off the screen? What is the difference? Does light bounce off of a black sheet of paper? In this case, little light bounces off since most of the light is absorbed instead of reflected. Black is the absence of light, so it is not really a “color”.
Reflection Does light bounce nicely off of a mirror? Does light bounce nicely off of a white sheet of paper, or of the screen? What is the difference? The difference is in the flatness of the reflecting surface. The surface needs to be flat on the order of the wavelength of the light – in the case of visible light it must be smooth on the order of micrometers. A laminated sheet of paper is more mirror-like and harder to write on – the ink smears out. Another example is water – smooth water is mirror-like, but rough water is not.
Property 4: Light and Shadows Consider what we would expect from particle (photon) theory: sharp shadows dark light dark
Light and Shadows Consider what we would expect from wave theory: shadows NOT sharp crest Waves spread out behind the barrier dark dim light dim dark
Light and Shadows laser What DOES happen? Look at a very bright laser beam going through a vertical slit. (A laser has one frequency unlike white light. ) Narrow slit pattern screen
Diffraction: single slit How can we explain the pattern from light going through a single slit? screen w x L
Diffraction: single slit The wave theory can explain this. We break the beam up into 2 n pieces since pieces will cancel (when a crest of one wave from one piece meets the trough of in pairs. This leads to: (w/2 n) sin( n) = /2 , screen or w sin( n) = n for MINIMUM. another wave from anther piece) x w L
Diffraction: circular opening If instead of a single SLIT, we have a CIRCULAR opening, the change in geometry makes the single slit pattern into a series of rings; and the formula to be: 1. 22 n = D sin( n) The smaller the diameter of the opening, the bigger the angle the dot spreads out. A bigger diameter for the opening means a smaller spreading out and so a clearer image on the screen. The smaller the wavelength, the smaller the angle spreads out and the smaller the fuzzy dot, and so the clearer the image. This means that viewing in the IR (with the help of technology) is less clear than in the visible since IR has a longer wavelength than visible light.
Diffraction: circular opening Since the light seems to act like a wave and spreads out behind a circular opening, and since the eye (and a camera and a telescope and a microscope, etc. ) has a circular opening, the light from two closely spaced objects will tend to overlap. This will hamper our ability to resolve the light (that is, it will hamper our ability to see clearly – why we can’t read small print if the print is too far away).
Rayleigh Criterion: a picture The lens will focus the light to a fuzzy DOT rather than a true point. Side view (We’ll talk about lenses in the next slide set. ) D lens picture of the pattern on the screen
Rayleigh Criterion: a picture If a second point of light makes an angle of limit with the first point, then it can just be resolved. Side view screen D lens x x’ s s’
Limits on Resolution: • Imperfections in the eye (correctable with glasses) • Rayleigh Criterion due to wavelength of visible light [A point of light going through an opening can only be focused to a finite fuzzy dot rather than a point. The wider the opening, the smaller the fuzzy dot and so the better the ability to “resolve” the image. ] • Size (graininess) of retinal cells [Each retinal cell can only tell how much light is hitting it – it can’t tell if more light is hitting one part of it and less light hitting another part or if the light is hitting it evenly – in other words an individual cell can’t read. ]
Pixels The bigger the size of the dots (pixels), the less we can resolve. e e. As you can see, with this first size of dots (pixels) and this size of the letter, we couldn’t tell that the letter was an “e”. With the second, smaller, size dots, we can start to tell that the letter is an “e”. The smaller the pixel size, the more pixels we need for the same space.
Pixels on TV’s Mp = megapixels Mp 480 i/p 720 x 480 0. 35 720 p 1, 280 x 720 0. 92 768 p 1, 366 x 768 1. 05 1080 i/p 1, 920 x 1, 080 2. 07 4 K 3, 840 x 2, 160 8. 29 4 K cinema 4, 096 x 2, 160 8. 85 8 K 7, 680 x 4, 320 33. 18 For each resolution, the number of pixels is the same regardless of the size of the TV. For cameras, the higher the pixel count, the finer the detail in the image, and the more you can blow the image up and still see the small details. Higher pixels also means more memory is required to store the images. https: //www. lifewire. com/what-is-a-pixel-1846929
Limits on Resolution: further examples • hawk eyes and owl eyes • cameras: – lenses (focal lengths, diameters) – films (speed and graininess) – shutter speeds and f-stops • Amt of light D 2 t • f-stop = f/D – f-stops & resolution: resolution depends on D
Limits on Resolution: further examples • surface must be smooth on order of • other types of light – x-ray diffraction (use atoms as slits) – IR – radio & microwave (satellite dishes don’t have to be mirror smooth)
Review • Light is a form of energy that is moving. • There are two ways of thinking about light: photons and E&M waves. property photon prediction wave prediction speed of light color of light reflection of light shadows no prediction correct and correct but explains danger doesn’t explain danger correct wrong correct In the next ppt set, we’ll continue to look at more properties of light and see what the two theories predict, and how all the properties allow us, or limit us, to “see” the universe.
Review Important Results for Astronomy • Light has a finite speed so it allows us to look back in time when we look out into space. • We can bend the path of light by reflection. • Because light acts as waves when it is moving, and waves spread out behind obstacles, we have limits on our ability to see clearly (resolve). The bigger the opening and the smaller the wavelength of the incoming light, the better we can see it (resolve it).