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Visible light

The range of photon energies for visible light    from red to violet is 1.63 to 3.26 eV, respectively (left for this chapter’s Problems and Exercises to verify). These energies are on the order of those between outer electron shells in atoms and molecules. This means that these photons can be absorbed by atoms and molecules. A single photon can actually stimulate the retina, for example, by altering a receptor molecule that then triggers a nerve impulse. Photons can be absorbed or emitted only by atoms and molecules that have precisely the correct quantized energy step to do so. For example, if a red photon of frequency f size 12{f} {} encounters a molecule that has an energy step, Δ E , size 12{ΔE} {} equal to hf , size 12{ ital "hf"} {} then the photon can be absorbed. Violet flowers absorb red and reflect violet; this implies there is no energy step between levels in the receptor molecule equal to the violet photon’s energy, but there is an energy step for the red.

There are some noticeable differences in the characteristics of light between the two ends of the visible spectrum that are due to photon energies. Red light has insufficient photon energy to expose most black-and-white film, and it is thus used to illuminate darkrooms where such film is developed. Since violet light has a higher photon energy, dyes that absorb violet tend to fade more quickly than those that do not. (See [link] .) Take a look at some faded color posters in a storefront some time, and you will notice that the blues and violets are the last to fade. This is because other dyes, such as red and green dyes, absorb blue and violet photons, the higher energies of which break up their weakly bound molecules. (Complex molecules such as those in dyes and DNA tend to be weakly bound.) Blue and violet dyes reflect those colors and, therefore, do not absorb these more energetic photons, thus suffering less molecular damage.

Photograph of a worn-out movie advertisement poster on a wall.
Why do the reds, yellows, and greens fade before the blues and violets when exposed to the Sun, as with this poster? The answer is related to photon energy. (credit: Deb Collins, Flickr)

Transparent materials, such as some glasses, do not absorb any visible light, because there is no energy step in the atoms or molecules that could absorb the light. Since individual photons interact with individual atoms, it is nearly impossible to have two photons absorbed simultaneously to reach a large energy step. Because of its lower photon energy, visible light can sometimes pass through many kilometers of a substance, while higher frequencies like UV, x ray, and γ size 12{γ} {} rays are absorbed, because they have sufficient photon energy to ionize the material.

How many photons per second does a typical light bulb produce?

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, calculate the number of visible photons emitted per second.

Strategy

Power is energy per unit time, and so if we can find the energy per photon, we can determine the number of photons per second. This will best be done in joules, since power is given in watts, which are joules per second.

Solution

The power in visible light production is 10.0% of 100 W, or 10.0 J/s. The energy of the average visible photon is found by substituting the given average wavelength into the formula

E = hc λ . size 12{E = { { ital "hc"} over {λ} } } {}

This produces

E = ( 6 . 63 × 10 –34 J s ) ( 3.00 × 10 8 m/s ) 580 × 10 –9 m = 3.43 × 10 –19 J . size 12{E = { { \( 6 "." "63 " times " 10" rSup { size 8{"–34"} } " J " cdot " s" \) \( 3 "." "00 " times " 10" rSup { size 8{8} } " m/s" \) } over {"580 " times " 10" rSup { size 8{"–9"} } " m"} } =" 3" "." "43 " times " 10" rSup { size 8{"–19"} } " J"} {}

The number of visible photons per second is thus

photon/s = 10.0 J/s 3 . 43 × 10 –19 J/photon = 2.92 × 10 19 photon/s . size 12{"photon/s "= { {" 10" "." "0 J/s"} over {3 "." "43" times " 10" rSup { size 8{"–19"} } " J/photon"} } =" 2" "." "92 " times " 10" rSup { size 8{"19"} } " photon/s"} {}

Discussion

This incredible number of photons per second is verification that individual photons are insignificant in ordinary human experience. It is also a verification of the correspondence principle—on the macroscopic scale, quantization becomes essentially continuous or classical. Finally, there are so many photons emitted by a 100-W lightbulb that it can be seen by the unaided eye many kilometers away.

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Questions & Answers

what is nuclear reaction?
Velina Reply
In a chemical reaction, you have atoms being rearranged in different patterns, so you end up with the same atoms but different molecules. In a nuclear reaction, you go one step deeper: you rearrange protons and neutrons, do you end up with different atoms.
Adrian
So, for example, stars fuse 2 Hydrogen atoms into 1 Helium atom (that's called nuclear fusion). Nuclear reactors break down heavy unstable atoms into smaller ones (that's nuclear fission)
Adrian
whats drag force?
Muhsin Reply
who can explain me about the connecting between energy and work? I don't understand about the equation of the formula
The Reply
Energy can be defined as the ability an object has to do work. so lets say if u want to move something from one place to another you would need energy to do it. a table a certain distance, but if u dont have energy that means you will not be able to move it. Work=Force×distance.
Mario
from work-kinetic energy theorem we get that work=change in kinetic energy
Mario
what is thermo electric thermometer
Undie Reply
Who can help me with dynamics?
ivan Reply
can someone enumerate the First and second law of thermodynamics
oladele Reply
radiation of phones kept amazing me
Okugbesan Reply
f=m(v-u)/t
Adeleke Reply
why acceleration due to gravity varies from place to place
Adeleke
I understand light is a range of wavelenghts from em spectrum, but Where do photons come from in particular, how it is emitted from the sun?
Ian Reply
please what is the formula for calculating Newton second law of motion?
Ogodo Reply
F=ma
pierre
what is emotion
Lilian Reply
properties of transverse waves
Abiodun Reply
is visible light electromagnetic wave?
akash Reply
Visible light is a range of wavelengths within the electro magnetic spectrum.
Robert
It is electro magnetic radiation from the sun.
Robert
please what is the formula for coefficient of kinetic friction
Seyi Reply
What is work
Sunbomustaphar Reply
Practice Key Terms 9

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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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