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Phet explorations: color vision

Make a whole rainbow by mixing red, green, and blue light. Change the wavelength of a monochromatic beam or filter white light. View the light as a solid beam, or see the individual photons.

Color Vision

Section summary

  • Photon energy is responsible for many characteristics of EM radiation, being particularly noticeable at high frequencies.
  • Photons have both wave and particle characteristics.

Conceptual questions

Why are UV, x rays, and γ size 12{γ} {} rays called ionizing radiation?

How can treating food with ionizing radiation help keep it from spoiling? UV is not very penetrating. What else could be used?

Some television tubes are CRTs. They use an approximately 30-kV accelerating potential to send electrons to the screen, where the electrons stimulate phosphors to emit the light that forms the pictures we watch. Would you expect x rays also to be created?

Tanning salons use “safe” UV with a longer wavelength than some of the UV in sunlight. This “safe” UV has enough photon energy to trigger the tanning mechanism. Is it likely to be able to cause cell damage and induce cancer with prolonged exposure?

Your pupils dilate when visible light intensity is reduced. Does wearing sunglasses that lack UV blockers increase or decrease the UV hazard to your eyes? Explain.

One could feel heat transfer in the form of infrared radiation from a large nuclear bomb detonated in the atmosphere 75 km from you. However, none of the profusely emitted x rays or γ size 12{γ} {} rays reaches you. Explain.

Can a single microwave photon cause cell damage? Explain.

In an x-ray tube, the maximum photon energy is given by hf = qV . size 12{ ital "hf"= ital "qV"} {} Would it be technically more correct to say hf = qV + BE, size 12{ ital "hf"= ital "qV"+"BE"} {} where BE is the binding energy of electrons in the target anode? Why isn’t the energy stated the latter way?

Problems&Exercises

What is the energy in joules and eV of a photon in a radio wave from an AM station that has a 1530-kHz broadcast frequency?

6.34 × 10 9 eV size 12{6 "." "34" times "10" rSup { size 8{ - 9} } " eV"} {} , 1.01 × 10 27 J size 12{6 "." "34" times "10" rSup { size 8{ - 9} } " eV"} {}

(a) Find the energy in joules and eV of photons in radio waves from an FM station that has a 90.0-MHz broadcast frequency. (b) What does this imply about the number of photons per second that the radio station must broadcast?

Calculate the frequency in hertz of a 1.00-MeV γ size 12{γ} {} -ray photon.

2 . 42 × 10 20 Hz size 12{2 "." "42" times "10" rSup { size 8{"20"} } " Hz"} {}

(a) What is the wavelength of a 1.00-eV photon? (b) Find its frequency in hertz. (c) Identify the type of EM radiation.

Do the unit conversions necessary to show that hc = 1240 eV nm, size 12{ ital "hc"="1240"" eV" cdot "nm"} {} as stated in the text.

hc = 6.62607 × 10 34 J s 2.99792 × 10 8 m/s 10 9 nm 1 m 1.00000 eV 1.60218 × 10 19 J = 1239.84 eV nm 1240 eV nm alignl { stack { size 12{ ital "hc"= left (6 "." "62607" times "10" rSup { size 8{ - "34"} } `J cdot s right ) left (2 "." "99792" times "10" rSup { size 8{8} } `"m/s" right ) left ( { {"10" rSup { size 8{9} } `"nm"} over {1`m} } right ) left ( { {1 "." "00000"`"eV"} over {1 "." "60218" times "10" rSup { size 8{ - "19"} } `J} } right )} {} #="1239" "." "84 eV" cdot "nm" {} # approx "1240 eV" cdot "nm" {}} } {}

Confirm the statement in the text that the range of photon energies for visible light is 1.63 to 3.26 eV, given that the range of visible wavelengths is 380 to 760 nm.

(a) Calculate the energy in eV of an IR photon of frequency 2.00 × 10 13 Hz. size 12{2 "." "00" times "10" rSup { size 8{"13"} } " Hz"} {} (b) How many of these photons would need to be absorbed simultaneously by a tightly bound molecule to break it apart? (c) What is the energy in eV of a γ size 12{γ} {} ray of frequency 3 . 00 × 10 20 Hz? size 12{3 "." "00" times "10" rSup { size 8{"20"} } " Hz"} {} (d) How many tightly bound molecules could a single such γ size 12{γ} {} ray break apart?

(a) 0.0829 eV

(b) 121

(c) 1.24 MeV

(d) 1 . 24 × 10 5 size 12{2 "." "24" times "10" rSup { size 8{5} } } {}

Prove that, to three-digit accuracy, h = 4 . 14 × 10 15 eV s, size 12{h=4 "." "14" times "10" rSup { size 8{ - "15"} } " eV" cdot s} {} as stated in the text.

(a) What is the maximum energy in eV of photons produced in a CRT using a 25.0-kV accelerating potential, such as a color TV? (b) What is their frequency?

(a) 25.0 × 10 3 eV size 12{" 25 " times " 10" rSup { size 8{3} } " eV"} {}

(b) 6 . 04 × 10 18 Hz size 12{" 6" "." "04 " times " 10" rSup { size 8{"18"} } " Hz"} {}

What is the accelerating voltage of an x-ray tube that produces x rays with a shortest wavelength of 0.0103 nm?

(a) What is the ratio of power outputs by two microwave ovens having frequencies of 950 and 2560 MHz, if they emit the same number of photons per second? (b) What is the ratio of photons per second if they have the same power output?

(a) 2.69

(b) 0.371

How many photons per second are emitted by the antenna of a microwave oven, if its power output is 1.00 kW at a frequency of 2560 MHz?

Some satellites use nuclear power. (a) If such a satellite emits a 1.00-W flux of γ size 12{γ} {} rays having an average energy of 0.500 MeV, how many are emitted per second? (b) These γ size 12{γ} {} rays affect other satellites. How far away must another satellite be to only receive one γ size 12{γ} {} ray per second per square meter?

(a) 1 . 25 × 10 13 photons/s size 12{" 1" "." "25 " times " 10" rSup { size 8{"13"} } " photons/s"} {}

(b) 997 km

(a) If the power output of a 650-kHz radio station is 50.0 kW, how many photons per second are produced? (b) If the radio waves are broadcast uniformly in all directions, find the number of photons per second per square meter at a distance of 100 km. Assume no reflection from the ground or absorption by the air.

How many x-ray photons per second are created by an x-ray tube that produces a flux of x rays having a power of 1.00 W? Assume the average energy per photon is 75.0 keV.

8.33 × 10 13 photons/s size 12{" 8" "." "33 " times " 10" rSup { size 8{"13"} } " photons/s"} {}

(a) How far away must you be from a 650-kHz radio station with power 50.0 kW for there to be only one photon per second per square meter? Assume no reflections or absorption, as if you were in deep outer space. (b) Discuss the implications for detecting intelligent life in other solar systems by detecting their radio broadcasts.

Assuming that 10.0% of a 100-W light bulb’s energy output is in the visible range (typical for incandescent bulbs) with an average wavelength of 580 nm, and that the photons spread out uniformly and are not absorbed by the atmosphere, how far away would you be if 500 photons per second enter the 3.00-mm diameter pupil of your eye? (This number easily stimulates the retina.)

181 km

Construct Your Own Problem

Consider a laser pen. Construct a problem in which you calculate the number of photons per second emitted by the pen. Among the things to be considered are the laser pen’s wavelength and power output. Your instructor may also wish for you to determine the minimum diffraction spreading in the beam and the number of photons per square centimeter the pen can project at some large distance. In this latter case, you will also need to consider the output size of the laser beam, the distance to the object being illuminated, and any absorption or scattering along the way.

Questions & Answers

A golfer on a fairway is 70 m away from the green, which sits below the level of the fairway by 20 m. If the golfer hits the ball at an angle of 40° with an initial speed of 20 m/s, how close to the green does she come?
Aislinn Reply
cm
tijani
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John Reply
what is physics
Siyaka Reply
A mouse of mass 200 g falls 100 m down a vertical mine shaft and lands at the bottom with a speed of 8.0 m/s. During its fall, how much work is done on the mouse by air resistance
Jude Reply
Can you compute that for me. Ty
Jude
what is the dimension formula of energy?
David Reply
what is viscosity?
David
what is inorganic
emma Reply
what is chemistry
Youesf Reply
what is inorganic
emma
Chemistry is a branch of science that deals with the study of matter,it composition,it structure and the changes it undergoes
Adjei
please, I'm a physics student and I need help in physics
Adjanou
chemistry could also be understood like the sexual attraction/repulsion of the male and female elements. the reaction varies depending on the energy differences of each given gender. + masculine -female.
Pedro
A ball is thrown straight up.it passes a 2.0m high window 7.50 m off the ground on it path up and takes 1.30 s to go past the window.what was the ball initial velocity
Krampah Reply
2. A sled plus passenger with total mass 50 kg is pulled 20 m across the snow (0.20) at constant velocity by a force directed 25° above the horizontal. Calculate (a) the work of the applied force, (b) the work of friction, and (c) the total work.
Sahid Reply
you have been hired as an espert witness in a court case involving an automobile accident. the accident involved car A of mass 1500kg which crashed into stationary car B of mass 1100kg. the driver of car A applied his brakes 15 m before he skidded and crashed into car B. after the collision, car A s
Samuel Reply
can someone explain to me, an ignorant high school student, why the trend of the graph doesn't follow the fact that the higher frequency a sound wave is, the more power it is, hence, making me think the phons output would follow this general trend?
Joseph Reply
Nevermind i just realied that the graph is the phons output for a person with normal hearing and not just the phons output of the sound waves power, I should read the entire thing next time
Joseph
Follow up question, does anyone know where I can find a graph that accuretly depicts the actual relative "power" output of sound over its frequency instead of just humans hearing
Joseph
"Generation of electrical energy from sound energy | IEEE Conference Publication | IEEE Xplore" ***ieeexplore.ieee.org/document/7150687?reload=true
Ryan
what's motion
Maurice Reply
what are the types of wave
Maurice
answer
Magreth
progressive wave
Magreth
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Muhammad Reply
fine, how about you?
Mohammed
hi
Mujahid
A string is 3.00 m long with a mass of 5.00 g. The string is held taut with a tension of 500.00 N applied to the string. A pulse is sent down the string. How long does it take the pulse to travel the 3.00 m of the string?
yasuo Reply
Who can show me the full solution in this problem?
Reofrir Reply
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Source:  OpenStax, Basic physics for medical imaging. OpenStax CNX. Feb 17, 2014 Download for free at http://legacy.cnx.org/content/col11630/1.1
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