{"id":40,"date":"2022-04-27T07:12:53","date_gmt":"2022-04-27T11:12:53","guid":{"rendered":"https:\/\/pressbooks.library.upei.ca\/danceofphotons\/chapter\/what-photons-are\/"},"modified":"2022-05-12T17:20:00","modified_gmt":"2022-05-12T21:20:00","slug":"what-photons-are","status":"publish","type":"chapter","link":"https:\/\/pressbooks.library.upei.ca\/danceofphotons\/chapter\/what-photons-are\/","title":{"raw":"WHAT PHOTONS ARE","rendered":"WHAT PHOTONS ARE"},"content":{"raw":"
\r\n

p. 2<\/p>\r\n

\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 simple answer: traveling<\/em><\/strong> packets<\/em><\/strong> of<\/em><\/strong> vibrating<\/em><\/strong> energy<\/em><\/strong><\/p>\r\n

Electromagnetic<\/em><\/strong> radiation<\/em><\/strong> is emitted by all matter above absolute zero (0o<\/sup> Kelvin or -273o<\/sup> Celsius).<\/p>\r\n

It is streams of energy bundles - photons<\/em><\/strong>.<\/p>\r\n

A photon\u2019s energy depends on its frequency of vibration:<\/p>\r\n\r\n

Energy = frequency x Planck\u2019s constant<\/h3>\r\n

Planck\u2019s<\/strong> constant<\/strong> is 6.625 x 10-34 joule-second). It is the smallest amount of power possible.<\/p>\r\n

Therefore photons only come in discrete (quantal<\/em><\/strong>) steps.<\/p>\r\n

Multiplying their frequency<\/strong> in vibr<\/em>a<\/em>ti<\/em>o<\/em>ns<\/em> p<\/em>er<\/em> sec<\/em>o<\/em>nd<\/em> by Planc<\/strong>k<\/strong>\u2019s<\/strong> j<\/em>ou<\/em>le-seco<\/em>nd<\/em>s<\/em>, cancels the<\/p>\r\n

seconds<\/em>, leaving Energy<\/em><\/strong> as joules<\/em> -<\/em> the unit of measurement for pure energy.<\/p>\r\n

N.B. This is the only equation - except for one on page 5, 6, on 17 & 18, 25, and in a Figure on page 35 - no calculus. They\u2019re only to show that such matters can be calculated. You don\u2019t need to use them to get the point.<\/p>\r\n \r\n

p. 3<\/p>\r\nIt takes more energy for matter to release higher frequency photons. Therefore, as temperature increases, more high frequency photons are emitted:\r\n\r\n\"\"\r\n\r\nTo picture the relationship between energy, frequency and wavelength, consider this: It takes more effort to shake a rope, carpet runner or bed sheet faster thereby increasing how many waves occur.\r\n\r\nFor ideal matter, the relation between temperature and energy of the emitted photons is called the Black<\/em><\/strong> Body<\/em><\/strong> Radiation<\/em><\/strong> Law<\/em><\/strong>:<\/em>\r\n\r\nIt\u2019s only an average. Real matter deviates from this ideal, but it\u2019s a good place to start.\r\n\r\n<\/div>\r\n

At room temperature, a square meter of ordinary stuff (stairs, spoons, etc.) emits one photon in about every 42 seconds - virtually none of them visible.2<\/sup> That\u2019s why we need lamps - even if we turn up the thermostat.<\/p>\r\n

Photons vibrate transversely to their direction of travel. This results in sinusoidal-like envelops of energy around each photon\u2019s path:<\/p>\r\n \r\n

p. 4<\/p>\r\n\"image\"\r\n\r\n \r\n\r\nTheir vibration expresses two kinds of energy: electric and magnetic\r\n

energy: electric<\/em><\/strong> and magnetic<\/em><\/strong><\/p>\r\n

It is a stable mutually reinforcing association:<\/p>\r\n

The varying electric field induces a corresponding varying magnetic field at a right angle to itself, The varying magnetic field similarly induces a varying electric field at a right angle to itself.<\/p>\r\n

Having<\/strong> no<\/strong> mass<\/strong> and<\/strong> vibrating<\/strong> so<\/strong> fast<\/strong> is<\/strong> why<\/strong> photons<\/strong> for<\/strong> many<\/strong> purposes<\/strong> can<\/strong> be<\/strong> successfully<\/strong> treated<\/strong> as<\/strong> traveling<\/strong> along<\/strong> the<\/strong> average<\/strong> location<\/strong> of<\/strong> their<\/strong> combined<\/strong> electric<\/strong> and<\/strong> magnetic<\/strong> fields.<\/strong><\/p>\r\n

For most lighting, chemistry, and vision work, attending to the electric wave is sufficient. So to simplify this glance, the magnetic component will not be discussed further.<\/p>\r\n \r\n

p. 5<\/p>\r\n

The speed of a photon depends on the medium through which it travels.<\/p>\r\n

(Sho<\/em>rtly,<\/em> we<\/em> s<\/em>ha<\/em>ll<\/em> learn<\/em> w<\/em>h<\/em>y<\/em> this<\/em> is<\/em> very<\/em> f<\/em>o<\/em>rt<\/em>un<\/em>ate<\/em>.<\/em>)<\/p>\r\nPhotons travel 300,000,000 meters per second in vacuum and nearly as fast in air. This speed is generally called \u201cc<\/em><\/strong>\u201d.\"\"\r\n

<\/div>\r\n
They slow substantially in media such as glass and water. How much they slow is specified by a medium\u2019s refractive<\/em><\/strong> index<\/em><\/strong> (n):<\/em><\/strong><\/div>\r\n

n <\/em><\/strong> = c <\/strong>\/ speed in medium<\/p>\r\n

For example:Photons entering glass (n<\/em><\/strong> = 1.5) slow down to:<\/p>\r\n

speedglass<\/sub> = 3 x 108<\/sup> m\/s\/ 1.5 = 2 x 108<\/sup> m\/s.<\/p>\r\n\r\n

\r\n\r\nWhy? Their vibrating electric field is hindered by fields of the electrons in the medium - an effect I\u2019ll call \u201cdrag<\/em><\/strong>\u201d. Denser media, tend to have stronger fields, larger refractive indices and exert more drag<\/em>.\r\n

Traveling in the opposite direction from right to left above, the photons are released from the drag<\/em> and resume their speed and wavelength.<\/p>\r\n \r\n

p. 6<\/p>\r\n

The distance a photon has traveled during one complete cycle of vibration is called its \u201cwavelength<\/em><\/strong>\u201d. A photon\u2019s wavelength therefore depends on its velocity:<\/p>\r\n\r\n

wavelength = speed \/ frequency<\/h3>\r\n

\"image\"<\/p>\r\n

When photons go slower in a medium denser than air, their frequency stays the same.<\/p>\r\n

Their wavelength shortens because they travel a shorter distance during one cycle of vibration. They do not loss energy when slowed, because that is determined by frequency (page 2).<\/p>\r\n

Nor do they gain energy when they speed up on leaving a denser medium.<\/p>\r\n

<\/p>\r\n\r\n<\/div>","rendered":"

\n

p. 2<\/p>\n

\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 simple answer: traveling<\/em><\/strong> packets<\/em><\/strong> of<\/em><\/strong> vibrating<\/em><\/strong> energy<\/em><\/strong><\/p>\n

Electromagnetic<\/em><\/strong> radiation<\/em><\/strong> is emitted by all matter above absolute zero (0o<\/sup> Kelvin or -273o<\/sup> Celsius).<\/p>\n

It is streams of energy bundles – photons<\/em><\/strong>.<\/p>\n

A photon\u2019s energy depends on its frequency of vibration:<\/p>\n

Energy = frequency x Planck\u2019s constant<\/h3>\n

Planck\u2019s<\/strong> constant<\/strong> is 6.625 x 10-34 joule-second). It is the smallest amount of power possible.<\/p>\n

Therefore photons only come in discrete (quantal<\/em><\/strong>) steps.<\/p>\n

Multiplying their frequency<\/strong> in vibr<\/em>a<\/em>ti<\/em>o<\/em>ns<\/em> p<\/em>er<\/em> sec<\/em>o<\/em>nd<\/em> by Planc<\/strong>k<\/strong>\u2019s<\/strong> j<\/em>ou<\/em>le-seco<\/em>nd<\/em>s<\/em>, cancels the<\/p>\n

seconds<\/em>, leaving Energy<\/em><\/strong> as joules<\/em> –<\/em> the unit of measurement for pure energy.<\/p>\n

N.B. This is the only equation – except for one on page 5, 6, on 17 & 18, 25, and in a Figure on page 35 – no calculus. They\u2019re only to show that such matters can be calculated. You don\u2019t need to use them to get the point.<\/p>\n

 <\/p>\n

p. 3<\/p>\n

It takes more energy for matter to release higher frequency photons. Therefore, as temperature increases, more high frequency photons are emitted:<\/p>\n

\"\"<\/p>\n

To picture the relationship between energy, frequency and wavelength, consider this: It takes more effort to shake a rope, carpet runner or bed sheet faster thereby increasing how many waves occur.<\/p>\n

For ideal matter, the relation between temperature and energy of the emitted photons is called the Black<\/em><\/strong> Body<\/em><\/strong> Radiation<\/em><\/strong> Law<\/em><\/strong>:<\/em><\/p>\n

It\u2019s only an average. Real matter deviates from this ideal, but it\u2019s a good place to start.<\/p>\n<\/div>\n

At room temperature, a square meter of ordinary stuff (stairs, spoons, etc.) emits one photon in about every 42 seconds – virtually none of them visible.2<\/sup> That\u2019s why we need lamps – even if we turn up the thermostat.<\/p>\n

Photons vibrate transversely to their direction of travel. This results in sinusoidal-like envelops of energy around each photon\u2019s path:<\/p>\n

 <\/p>\n

p. 4<\/p>\n

\"image\"<\/p>\n

 <\/p>\n

Their vibration expresses two kinds of energy: electric and magnetic<\/p>\n

energy: electric<\/em><\/strong> and magnetic<\/em><\/strong><\/p>\n

It is a stable mutually reinforcing association:<\/p>\n

The varying electric field induces a corresponding varying magnetic field at a right angle to itself, The varying magnetic field similarly induces a varying electric field at a right angle to itself.<\/p>\n

Having<\/strong> no<\/strong> mass<\/strong> and<\/strong> vibrating<\/strong> so<\/strong> fast<\/strong> is<\/strong> why<\/strong> photons<\/strong> for<\/strong> many<\/strong> purposes<\/strong> can<\/strong> be<\/strong> successfully<\/strong> treated<\/strong> as<\/strong> traveling<\/strong> along<\/strong> the<\/strong> average<\/strong> location<\/strong> of<\/strong> their<\/strong> combined<\/strong> electric<\/strong> and<\/strong> magnetic<\/strong> fields.<\/strong><\/p>\n

For most lighting, chemistry, and vision work, attending to the electric wave is sufficient. So to simplify this glance, the magnetic component will not be discussed further.<\/p>\n

 <\/p>\n

p. 5<\/p>\n

The speed of a photon depends on the medium through which it travels.<\/p>\n

(Sho<\/em>rtly,<\/em> we<\/em> s<\/em>ha<\/em>ll<\/em> learn<\/em> w<\/em>h<\/em>y<\/em> this<\/em> is<\/em> very<\/em> f<\/em>o<\/em>rt<\/em>un<\/em>ate<\/em>.<\/em>)<\/p>\n

Photons travel 300,000,000 meters per second in vacuum and nearly as fast in air. This speed is generally called \u201cc<\/em><\/strong>\u201d.\"\"<\/p>\n

<\/div>\n
They slow substantially in media such as glass and water. How much they slow is specified by a medium\u2019s refractive<\/em><\/strong> index<\/em><\/strong> (n):<\/em><\/strong><\/div>\n

n <\/em><\/strong> = c <\/strong>\/ speed in medium<\/p>\n

For example:Photons entering glass (n<\/em><\/strong> = 1.5) slow down to:<\/p>\n

speedglass<\/sub> = 3 x 108<\/sup> m\/s\/ 1.5 = 2 x 108<\/sup> m\/s.<\/p>\n

\n

Why? Their vibrating electric field is hindered by fields of the electrons in the medium – an effect I\u2019ll call \u201cdrag<\/em><\/strong>\u201d. Denser media, tend to have stronger fields, larger refractive indices and exert more drag<\/em>.<\/p>\n

Traveling in the opposite direction from right to left above, the photons are released from the drag<\/em> and resume their speed and wavelength.<\/p>\n

 <\/p>\n

p. 6<\/p>\n

The distance a photon has traveled during one complete cycle of vibration is called its \u201cwavelength<\/em><\/strong>\u201d. A photon\u2019s wavelength therefore depends on its velocity:<\/p>\n

wavelength = speed \/ frequency<\/h3>\n

\"image\"<\/p>\n

When photons go slower in a medium denser than air, their frequency stays the same.<\/p>\n

Their wavelength shortens because they travel a shorter distance during one cycle of vibration. They do not loss energy when slowed, because that is determined by frequency (page 2).<\/p>\n

Nor do they gain energy when they speed up on leaving a denser medium.<\/p>\n

\n<\/div>\n","protected":false},"author":28,"menu_order":2,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-40","chapter","type-chapter","status-publish","hentry"],"part":3,"_links":{"self":[{"href":"https:\/\/pressbooks.library.upei.ca\/danceofphotons\/wp-json\/pressbooks\/v2\/chapters\/40","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/pressbooks.library.upei.ca\/danceofphotons\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/pressbooks.library.upei.ca\/danceofphotons\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/pressbooks.library.upei.ca\/danceofphotons\/wp-json\/wp\/v2\/users\/28"}],"version-history":[{"count":32,"href":"https:\/\/pressbooks.library.upei.ca\/danceofphotons\/wp-json\/pressbooks\/v2\/chapters\/40\/revisions"}],"predecessor-version":[{"id":358,"href":"https:\/\/pressbooks.library.upei.ca\/danceofphotons\/wp-json\/pressbooks\/v2\/chapters\/40\/revisions\/358"}],"part":[{"href":"https:\/\/pressbooks.library.upei.ca\/danceofphotons\/wp-json\/pressbooks\/v2\/parts\/3"}],"metadata":[{"href":"https:\/\/pressbooks.library.upei.ca\/danceofphotons\/wp-json\/pressbooks\/v2\/chapters\/40\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/pressbooks.library.upei.ca\/danceofphotons\/wp-json\/wp\/v2\/media?parent=40"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/pressbooks.library.upei.ca\/danceofphotons\/wp-json\/pressbooks\/v2\/chapter-type?post=40"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/pressbooks.library.upei.ca\/danceofphotons\/wp-json\/wp\/v2\/contributor?post=40"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/pressbooks.library.upei.ca\/danceofphotons\/wp-json\/wp\/v2\/license?post=40"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}