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----------------- 1810 - Photons are Everywhere.
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- A photon is the smallest “bit” of light. It can only move at one speed, 186,000 miles per second. A photon has no mass and all massless objects have to move that fast. Even at that speed a photon takes 8.3 minutes to reach us after it leaves the Sun.
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- Photons are massless. If an object has mass it must slow down. Mass and energy are the same thing but mass is energy that has slowed down enough for us to see it.
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- Photons have no mass but they do have momentum (p).
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------------------------------- p = h * f / c
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------------------------------ “h” is Planck’s Constant = 6.6 *10^-34
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- That is a very small number.
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----------------------------- “f” is frequency, or, cycles per second
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----------------------------- “c” = the speed of light, 186,000 miles per second
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------------------------------ c = 3 * 10^8 meters / second
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------------------------------ h / c = 2.2 *10^-42
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------------------------------- p = 2.2*10^-42 * f
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----------------------------Momentum is directly proportional to frequency
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- The higher the frequency, the higher the energy of the photons. Radio waves have low frequency, and low energy. Kilohertz frequency and the radio needs a lot of amplification in order to listen to them. Microwaves have high frequency, high energy. These photons just brought my coffee to a boil.
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------------------------ Energy = Planck’s Constant * frequency
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---------------------------- E = h * f
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--------------------------- E = 6.6*10^-34 * f
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- A photon has momentum (p) because it carries energy (E). Kinetic Energy is equal to momentum times velocity (v) squared.
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------------------------------ E = p * v^2
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- At the speed of light:
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---------------------------- E = m * c^2
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-------------------------- E = m * 9*10^16
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-------------------------- mass = (0.1111*10^-6) * Energy
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- Energy is directly proportional to mass and vise versa.
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- You eye is very sensitive to this energy. The eye’s chemistry can detect less than 25 photons per second. There are a lot of photons in sunlight. The sunlight that reaches Earth in 8.3 minutes carries 1400 watts per square meter. How many photons (N) is this?
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- The number of photons equals the Intensity of light divided by the Energy
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-------------------------- N = I / E
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------------------------- N = number of photons
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------------------------- I = Intensity in watts
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------------------------- E = h * f = h * c / w
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-------------------------- w = wavelength = c / f
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--------------------------- N = (I * w) / (h * c)
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------------------- wavelength of redlight = w = 5.6*10^-7 meters
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------------------------- N = 1.4*10^3 * 5.6*10^-7 / 6.64*10^-34 * 3 *10^8 watts * m * sec^2 * m / (m^2 * kg * m^2)
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------------------------- watt = kg * m^2/ sec^3
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------------------------- N = 0.39 *10^22
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- The number of photons entering the eye? The pupil is ½ centimeter. The area is pi*r^2 = 3.14 * (.025)^2 = 2 * 10^-5.
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------------------------ N = 0.39 * 2 * 10^-5 = 0.79 *10^15 photons per second
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----------------------- N = 790,000,000,000,000 photons enter the eye ever second.
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- That is a remarkable range of the eye’s sensitivity to photons, (25) to (790,000,000,000,000). Photons change the chemistry in the eye. Photons are absorbed and emitted, but, they last forever. They were created in the Big Bang. None have died. They just spread out as the Universe expands. They are all over the Universe by now 14 billion years later. Let’s see if we can count how many?
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-------------------- An incandescent light bulb emits 10^20 photons per second.
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--------------------- The Sun is a giant light bulb. It emits 10^45 photons per second.
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- Now there are a few hundred billion suns in the Milky Way Galaxy (10^8). And, there are a few hundred billion galaxies in the Observerable Universe (10^8). That is 10^61 photons per second. But, that is just star light. There are nebulae, supernovae, pulsars, accretion disks around Blackholes, gas, etc that all emit photons. Then there is the oldest light of all, the Cosmic Microwave Background.
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- If your eyes could see microwaves you would see a uniform haze of photons coming from every direction in the sky. 300,000,000 years after the Big Bang the expanding Universe had cooled enough for neutral hydrogen gas to form and photons could escape the charged plasma and expand into the space-time as Gamma Rays, the highest frequency light.
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- After 14 billion years of traveling through expanding space-time the Gamma Rays wavelengths got stretched out to microwave wavelengths.
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- That event released 10^90 photons.
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- So, for every photon that is emitted by the stars in the Universe another 10^30 are traveling as the Cosmic Microwave Background. If the Universe were a single light bulb it would be emitting 10^90 photons. Better start counting that is a big number.
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- Request Reviews # for more to learn:
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- #733 - The Photon, E^2 = c^2*p^2 + m^2 *c^4. Since the photon is massless:
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---------------------------- E = c * p
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- # 1074 - Physics the way I learned it
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- #738 - The science of physics.
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