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sparks jump across a gap","JJ Thomson found electrons using cathode ray tubes, leading to the 'plum pudding model' of the atom","Philipp Lenard showed that light can eject electrons from metal, but classical physics couldn't explain it","Albert Einstein proposed that light is made of photons, introducing wave-particle duality",1,{"id":37,"data":38,"type":39,"maxContentLevel":19,"version":35},"2bd76257-6162-43c6-b66e-89df8ccfe20e",{"type":39,"title":40,"intro":41},10,"Electromagnetism and the Photoelectric Effect intro",[42,43],"What did Heinrich Hertz accidentally discover while experimenting with a spark gap?","How did Einstein's theory of light challenge the classical wave theory?",[45,84,99,116],{"id":46,"data":47,"type":35,"maxContentLevel":19,"version":20,"reviews":51},"a4de6406-6fa9-4d33-afac-9fca42faa6db",{"type":35,"title":48,"markdownContent":49,"audioMediaId":50},"Electromagnetism and the Photoelectric Effect page 1","Maxwell’s theory predicted the existence of electromagnetic waves, stating that light itself was one such wave. This greatly excited fellow scientists, who sought to prove his assertions experimentally. In 1887, German physicist Heinrich Hertz was toying around with a ‘spark gap’ when he noticed something peculiar. Hertz’s set-up was a pair of conducting electrodes separated by a tiny gap across which a spark was generated upon detection of electromagnetic waves. After covering his apparatus in a dark box to see the sparks better, he noticed that a glass box decreased the spark length whereas a quartz box had no effect.\n\n![Graph](image://a5d09d39-8914-473f-baa0-af5259337994 \"The photoelectric effect. Image: Ponor, CC BY-SA 4.0, via Wikimedia Commons\")\n\nLittle did he know that it was ultraviolet radiation which was interacting with the electrons in the current and supplying them with energy to jump across the gap. Hertz couldn’t explain his findings, but it soon became clear that the phenomenon occurred because glass absorbs UV and quartz doesn’t. He had accidentally witnessed the first example of the ‘Photoelectric Effect’.","b0861eb0-386c-48e0-95d0-a4f9dbe96ba3",[52,73],{"id":53,"data":54,"type":55,"version":35,"maxContentLevel":19},"49892bdb-7b79-4e10-91a7-ebbea9edb126",{"type":55,"reviewType":19,"spacingBehaviour":35,"collapsingSiblings":56,"multiChoiceQuestion":60,"multiChoiceCorrect":62,"multiChoiceIncorrect":64,"multiChoiceMultiSelect":6,"multiChoiceRevealAnswerOption":6,"matchPairsQuestion":68,"matchPairsPairs":70},11,[57,58,59],"740c625a-2bd5-476f-8db7-92377b5a7136","41cecd66-f60f-4395-b867-524a6cbb273b","4642f563-2780-4715-8042-8835a24dadfe",[61],"Which of the following best describes the Photoelectric Effect?",[63],"The phenomenon of electrons being emitted from a material when light shines on it",[65,66,67],"A form of energy that is propagated as a wave","The energy needed to remove an electron away from a metal","Developed by John Dalton, further refined by other scientists",[69],"Match the pairs below:",[71],{"left":72,"right":63,"direction":19},"Photoelectric Effect",{"id":74,"data":75,"type":55,"version":35,"maxContentLevel":19},"ddae544f-bd00-4afb-a8c1-f958b0f29045",{"type":55,"reviewType":19,"spacingBehaviour":35,"multiChoiceQuestion":76,"multiChoiceCorrect":78,"multiChoiceIncorrect":80,"multiChoiceMultiSelect":6,"multiChoiceRevealAnswerOption":6},[77],"What phenomenon was decreased when using a glass box in Hertz's experiment?",[79],"Spark length",[81,82,83],"Quantum length","Nuclear length","Electron length",{"id":85,"data":86,"type":35,"maxContentLevel":19,"version":20,"reviews":90},"08c9c500-e096-4d3a-a5be-0e21ee326c60",{"type":35,"title":87,"markdownContent":88,"audioMediaId":89},"Electromagnetism and the Photoelectric Effect page 2","In 1897, British physicist J.J. Thomson made one of the most significant discoveries in scientific history, the electron. Using cathode ray tubes, he demonstrated that all atoms contain tiny, negatively charged particles. The electrons he identified inspired Thomson to conceive his ‘plum pudding model’ of the atom, in which negatively charged “plums” were embedded within a positively charged “pudding”.\n\n![Graph](image://32395b01-1dff-4e9d-9e69-0b893b5241f3 \"J.J. Thomson. Image: Bain News Service, publisher, Public domain, via Wikimedia Commons\")\n\nBefore moving on, it’s worth examining what constitutes a metal. At the atomic level, a metal is a closely packed lattice of metal ‘ions’, which are simply atoms with a net positive or negative charge.\n\nThe structure of a solid metal consists of charged ions because each atom has donated at least one of its outermost electrons to form a “sea” of ‘delocalised electrons’. The electrostatic force of attraction between the metal ions and delocalised electrons is what imparts metals with their rigidity and strength. In theory, if given enough energy, these delocalised electrons could become liberated from the metal’s surface.","4719a167-708a-47c6-9757-7754ea890f49",[91],{"id":92,"data":93,"type":55,"version":35,"maxContentLevel":19},"e768bfef-9443-4a05-b3fe-2ae5b2df2d8d",{"type":55,"reviewType":94,"spacingBehaviour":35,"clozeQuestion":95,"clozeWords":97},4,[96],"Metals are closely packed lattices of metal ions and delocalized electrons.",[98],"delocalized",{"id":100,"data":101,"type":35,"maxContentLevel":19,"version":20,"reviews":105},"c56a3ef4-b0b5-49ce-89d0-c294ae33151a",{"type":35,"title":102,"markdownContent":103,"audioMediaId":104},"Electromagnetism and the Photoelectric Effect page 3","In 1902, another German physicist Philipp Lenard demonstrated that illuminating a metal surface with light or another form of electromagnetic radiation could cause electrons in the material to be expelled or ‘liberated’.\n\nThe electrons absorbed incoming ‘radiant energy’, allowing them to break free from the electrostatic forces binding them to the metallic ions. Unfortunately, this ‘photoelectric effect’ represented an interaction between light and matter which couldn’t be explained by classical physics, or by viewing light as an electromagnetic wave.\n\n![Graph](image://ce7c6a71-8746-4f6c-98b8-368ff97e9bf1 \"Philip Lennard. Image: Public domain, via Wikimedia Commons\")\n\nIf light were only a wave, the energy of the released electrons would depend on the light source’s intensity. Except it didn’t! It was actually dependent on the light’s frequency, which according to wave theory should have zero effect. The intensity instead determined the number of electrons released from the metal’s surface. What was made clear is that classical ideas and the wave picture of light simply weren’t cutting it in explaining this puzzling effect.","03cf278d-91ff-474c-b5fd-e3d569988cd8",[106],{"id":107,"data":108,"type":55,"version":35,"maxContentLevel":19},"0d1c0748-ce6f-4983-b353-2f1833005d5a",{"type":55,"reviewType":19,"spacingBehaviour":35,"multiChoiceQuestion":109,"multiChoiceCorrect":111,"multiChoiceIncorrect":113,"multiChoiceMultiSelect":6,"multiChoiceRevealAnswerOption":6},[110],"What phenomenon could not be explained by classical physics or the wave picture of light?",[112],"The Photoelectric Effect",[17,114,115],"The Bohr Photoelectric Effect","The Heisenberg Principle",{"id":117,"data":118,"type":35,"maxContentLevel":19,"version":20,"reviews":122},"821ee30c-c74f-4ce8-984b-4b1f1605b00f",{"type":35,"title":119,"markdownContent":120,"audioMediaId":121},"Electromagnetism and the Photoelectric Effect page 4","Albert Einstein made his mark in 1905 by formulating a startling new theory of light. He did this by expanding upon Planck’s idea of quantization to conjecture that light itself is made up of discrete packets or ‘quanta’. Each of these quanta contained a fixed amount of energy directly related to the light’s frequency. Their name? Photons!\n\nThis is where things took a strange turn, as Einstein was not only suggesting that light is composed of what are essentially particles, but that it also possesses wave-like properties such as frequency and therefore also an effective ‘wavelength’.\n\n![Graph](image://778408f4-026a-4335-93f9-001c6b88c374 \"Albert Einstein. Image: Photograph by Oren Jack Turner, Princeton, N.J., Public domain, via Wikimedia Commons\")\n\nIt became apparent from that point onwards that light isn’t fully described by a wave or a particle. This was the first example of ‘wave-particle duality’, a prominent concept in quantum mechanics stating that every particle may be described as either a particle or a wave. Neither description alone nor their classical interpretations are sufficient to fully describe the behaviour of any quantum system.","99e49045-1e49-4d10-93e9-1846dcdd655c",[123],{"id":124,"data":125,"type":55,"version":35,"maxContentLevel":19},"0f36222f-82c0-42b8-9a46-0f2c5357e02c",{"type":55,"reviewType":35,"spacingBehaviour":35,"activeRecallQuestion":126,"activeRecallAnswers":128},[127],"What is the name of the discrete packets of energy that make up light?",[129],"Photons",{"id":131,"data":132,"type":20,"version":20,"maxContentLevel":19,"summaryPage":134,"introPage":143,"pages":150},"fa77ca64-5266-4c0a-b81f-3eafabb6f7f5",{"type":20,"title":133},"Understanding Photons",{"id":135,"data":136,"type":19,"maxContentLevel":19,"version":35},"80f55c85-e5b5-401e-83d0-685219c9b5ad",{"type":19,"title":137,"summary":138},"Understanding Photons summary",[139,140,141,142],"Light energy travels in discrete units called photons","Photons move at light speed, are neutral, and have zero mass","The energy of a photon is tied to its frequency: higher frequency means more energy","Planck's constant links photon energy to frequency: E = h × f",{"id":144,"data":145,"type":39,"maxContentLevel":19,"version":35},"b7ce4686-fa3f-4f47-ac09-f396ce5f4f3f",{"type":39,"title":146,"intro":147},"Understanding Photons intro",[148,149],"How does the energy of a photon relate to its frequency?","What happens to photons when they encounter matter?",[151,180],{"id":152,"data":153,"type":35,"maxContentLevel":19,"version":35,"reviews":157},"ccd9cb42-3d9e-45f7-b6ac-604cee8c23b7",{"type":35,"title":154,"markdownContent":155,"audioMediaId":156},"Understanding Photons page 1","Like all forms of electromagnetic radiation, light transports energy across space. Einstein was able to convincingly show that the energy transmitted by light arrives at a receiver not continuously but in discrete units called ‘photons’. Instead of the energy being continuously distributed over a wavefront, it was now carried in neat, quantized packages. Therefore, we can view photons as effective “particles” of light! So what do we know about them?\n\nFollowing on from Maxwell’s work, we know that photons always move at the speed of light. We also know that they are electrically neutral and have zero mass, but still carry an energy proportional to the light’s frequency. The higher the frequency or shorter the wavelength, the more energetic the photon. Lastly, photons can be created or destroyed. When a source emits electromagnetic waves, photons are created. When photons encounter matter, they can be absorbed and transfer their energy to the atoms and molecules.","6ed240bd-3bdb-490e-badc-d5f127384908",[158,169],{"id":57,"data":159,"type":55,"version":35,"maxContentLevel":19},{"type":55,"reviewType":19,"spacingBehaviour":35,"collapsingSiblings":160,"multiChoiceQuestion":161,"multiChoiceCorrect":163,"multiChoiceIncorrect":164,"multiChoiceMultiSelect":6,"multiChoiceRevealAnswerOption":6,"matchPairsQuestion":165,"matchPairsPairs":166},[53,58,59],[162],"Which of the following best describes electromagnetic radiation?",[65],[63,66,67],[69],[167],{"left":168,"right":65,"direction":19},"Electromagnetic radiation",{"id":170,"data":171,"type":55,"version":35,"maxContentLevel":19},"af326fb0-5abc-4bd0-859f-b06ba128f151",{"type":55,"reviewType":19,"spacingBehaviour":35,"multiChoiceQuestion":172,"multiChoiceCorrect":174,"multiChoiceIncorrect":176,"multiChoiceMultiSelect":6,"multiChoiceRevealAnswerOption":6},[173],"What is the speed that photons always move at?",[175],"The speed of light",[177,178,179],"The speed of sound","The speed of electricity","The speed of terminal velocity",{"id":181,"data":182,"type":35,"maxContentLevel":19,"version":20,"reviews":186},"f1d4e7ce-69e3-4c87-9ee7-1580964b3914",{"type":35,"title":183,"markdownContent":184,"audioMediaId":185},"Understanding Photons page 2","When Planck derived an equation that described black body radiation curves with tremendous accuracy and resolved the ultraviolet catastrophe, he defined a universal constant ‘h’ in his calculations. This ‘Planck constant’ is incredibly small – approximately 6.63 × 10^-34 joules/second – and just like the speed of light or the charge of an electron, it is a fundamental, unchanging quantity that has profound importance in quantum mechanics. In fact, this constant appears in any equation in which the phenomenon under consideration exhibits quantum mechanical behavior.\n\nIn Einstein’s quantum theory of light, he carried the Planck constant forward, capturing the relationship between the energy of a photon and its frequency via the straightforward equation E = h × f or E = hc / λ where ‘c’ is the speed of light and ‘λ’ is its wavelength. Therefore, according to Einstein’s work, red light photons possess lower energy than blue light photons due to the former having a relatively longer wavelength or lower frequency.","ab26a64f-6f9f-4bbb-b9b1-93fcb2ee5dd4",[187],{"id":188,"data":189,"type":55,"version":35,"maxContentLevel":19},"8af963ae-2929-4497-b01b-2474f81ae2c5",{"type":55,"reviewType":94,"spacingBehaviour":35,"clozeQuestion":190,"clozeWords":192},[191],"According to Einstein’s work, red photons possess lower energy than blue photons.",[193],"lower",{"id":195,"data":196,"type":20,"version":20,"maxContentLevel":19,"summaryPage":198,"introPage":207,"pages":214},"612e6930-fcb1-4fe0-93c7-cc875fd92e3d",{"type":20,"title":197},"Photoelectric Effect and its Implications",{"id":199,"data":200,"type":19,"maxContentLevel":19,"version":35},"6795ec56-52c8-4b40-9445-ef60b5389842",{"type":19,"title":201,"summary":202},"Photoelectric Effect and its Implications summary",[203,204,205,206],"Light's frequency, not intensity, determines if electrons are emitted from a metal","Each material has a unique cut-off frequency below which no electrons are released","The work function is the minimum energy needed to free an electron from a metal","Photoelectric emission is instant when a photon with enough energy hits an electron",{"id":208,"data":209,"type":39,"maxContentLevel":19,"version":35},"85360d17-8c48-4480-a848-0a422fac1f25",{"type":39,"title":210,"intro":211},"Photoelectric Effect and its Implications intro",[212,213],"Why does increasing light intensity not always cause electron emission from a metal?","What determines the maximum kinetic energy of electrons in the photoelectric effect?",[215,231,249],{"id":216,"data":217,"type":35,"maxContentLevel":19,"version":20,"reviews":221},"3b05ceb3-6a10-4718-8cbf-ed6123d4db51",{"type":35,"title":218,"markdownContent":219,"audioMediaId":220},"Photoelectric Effect and its Implications page 1","When light is shone on a metal surface, sometimes no electrons are emitted regardless of the light’s intensity. But if light were a wave, even a low intensity light source should continually transfer energy to the electrons until they absorb enough to escape from their metallic bonds.\n\nIn quantum physics, it’s the light source’s frequency which matters. It turns out that every material has a unique ‘cut-off frequency’ below which no electrons are released and above which photoelectric effects are witnessed.\n\n![Graph](image://2199eb20-0f25-4aac-bec9-0e0e98119a40 \"Particles being reflected from waves\")","ef300803-9569-4ab8-99db-cb316ec9fdd8",[222],{"id":223,"data":224,"type":55,"version":35,"maxContentLevel":19},"83c868c2-0683-4c4f-a33c-b46711b4e020",{"type":55,"reviewType":20,"spacingBehaviour":35,"binaryQuestion":225,"binaryCorrect":227,"binaryIncorrect":229},[226],"What determines whether electrons are emitted from a metal surface undergoing the photoelectric effect?",[228],"The frequency of the light source",[230],"The intensity of the light source",{"id":232,"data":233,"type":35,"maxContentLevel":19,"version":20,"reviews":237},"5768f60c-15e0-4083-8f96-b5703a38bf94",{"type":35,"title":234,"markdownContent":235,"audioMediaId":236},"Photoelectric Effect and its Implications page 2","The quantum view of light states that intensity only affects the rate of electron emission, not whether they will be emitted. In other words, a greater number of photons transmitted per unit area and time means more opportunities for electrons to be liberated, assuming each photon has sufficient energy! When a photon with sufficient energy impacts an electron, it causes it to be liberated. Nothing happens if the photon lacks sufficient energy, regardless of how many there are!\n\nTo liberate an electron from a metal surface the incoming radiation needs to have energy surpassing Φ, which is related to the cut-off frequency fc by Φ = h × fc and known as the ‘work function’ of the metal. This work function is the minimum energy required to induce ‘photoemission’ of electrons, and it depends on the specific metal being illuminated.","098612a5-d804-45f2-b46d-6512f6fbbb3a",[238],{"id":58,"data":239,"type":55,"version":35,"maxContentLevel":19},{"type":55,"reviewType":19,"spacingBehaviour":35,"collapsingSiblings":240,"multiChoiceQuestion":241,"multiChoiceCorrect":243,"multiChoiceIncorrect":244,"multiChoiceMultiSelect":6,"multiChoiceRevealAnswerOption":6,"matchPairsQuestion":245,"matchPairsPairs":246},[53,57,59],[242],"Which of the following best describes the work function?",[66],[63,65,67],[69],[247],{"left":248,"right":66,"direction":19},"Work Function",{"id":250,"data":251,"type":35,"maxContentLevel":19,"version":20,"reviews":255},"73c7e509-e835-4508-a52d-6ff200766520",{"type":35,"title":252,"markdownContent":253,"audioMediaId":254},"Photoelectric Effect and its Implications page 3","If an incoming photon’s energy exceeds the work function of the material, an electron can be freed, and any excess energy appears as kinetic energy of the electron. It was established that the maximum kinetic energy of liberated electrons was independent of the light’s intensity, which only makes sense when light is quantized into photons. Each photon has a fixed energy based on frequency only, meaning that electrons will have a maximum kinetic energy – calculated as the difference between the energy of the photon and the work function of the metal – that doesn’t vary with intensity.\n\nAccording to classical wave theory, a measurable time lag should exist between the time when light first starts to illuminate a surface and the subsequent ejection of electrons. However, no detectable time lag has ever been measured in the photoelectric experiment. In other words, when a single photon with sufficient energy interacts with a single electron the photoelectric emission process is instantaneous.\n\nClassically, the time scale of the interaction should be measurable as the incoming wave continuously supplies energy until the electron has enough to escape, but this is not what was observed. The lack of a time lag in the photoelectric effect was another finding in favor of the quantum view.","c6655d2a-aea7-45f2-b66d-7d5f5bdf677a",[256,263],{"id":257,"data":258,"type":55,"version":35,"maxContentLevel":19},"a16b8c27-3e72-485b-834b-8261c2e8a734",{"type":55,"reviewType":94,"spacingBehaviour":35,"clozeQuestion":259,"clozeWords":261},[260],"The maximum kinetic energy of liberated electrons is independent of the light’s intensity.",[262],"independent",{"id":264,"data":265,"type":55,"version":35,"maxContentLevel":19},"40985fa0-cc4a-4182-a2fd-e9457257ac1e",{"type":55,"reviewType":19,"spacingBehaviour":35,"multiChoiceQuestion":266,"multiChoiceCorrect":268,"multiChoiceIncorrect":270,"multiChoiceMultiSelect":6,"multiChoiceRevealAnswerOption":6},[267],"What was the unexpected finding in the photoelectric experiment that favored the quantum view?",[269],"No time lag",[271,272,273],"Measurable time lag","Gamma radiation","Covalent bonding",[275,399,435],{"id":23,"data":24,"type":20,"version":20,"maxContentLevel":19,"summaryPage":26,"introPage":36,"pages":276},[277,309,339,369],{"id":46,"data":47,"type":35,"maxContentLevel":19,"version":20,"reviews":51,"parsed":278},{"data":279,"body":282,"toc":307},{"title":280,"description":281},"","Maxwell’s theory predicted the existence of electromagnetic waves, stating that light itself was one such wave. This greatly excited fellow scientists, who sought to prove his assertions experimentally. In 1887, German physicist Heinrich Hertz was toying around with a ‘spark gap’ when he noticed something peculiar. Hertz’s set-up was a pair of conducting electrodes separated by a tiny gap across which a spark was generated upon detection of electromagnetic waves. After covering his apparatus in a dark box to see the sparks better, he noticed that a glass box decreased the spark length whereas a quartz box had no effect.",{"type":283,"children":284},"root",[285,292,302],{"type":286,"tag":287,"props":288,"children":289},"element","p",{},[290],{"type":291,"value":281},"text",{"type":286,"tag":287,"props":293,"children":294},{},[295],{"type":286,"tag":296,"props":297,"children":301},"img",{"alt":298,"src":299,"title":300},"Graph","image://a5d09d39-8914-473f-baa0-af5259337994","The photoelectric effect. Image: Ponor, CC BY-SA 4.0, via Wikimedia Commons",[],{"type":286,"tag":287,"props":303,"children":304},{},[305],{"type":291,"value":306},"Little did he know that it was ultraviolet radiation which was interacting with the electrons in the current and supplying them with energy to jump across the gap. Hertz couldn’t explain his findings, but it soon became clear that the phenomenon occurred because glass absorbs UV and quartz doesn’t. He had accidentally witnessed the first example of the ‘Photoelectric Effect’.",{"title":280,"searchDepth":20,"depth":20,"links":308},[],{"id":85,"data":86,"type":35,"maxContentLevel":19,"version":20,"reviews":90,"parsed":310},{"data":311,"body":313,"toc":337},{"title":280,"description":312},"In 1897, British physicist J.J. Thomson made one of the most significant discoveries in scientific history, the electron. Using cathode ray tubes, he demonstrated that all atoms contain tiny, negatively charged particles. The electrons he identified inspired Thomson to conceive his ‘plum pudding model’ of the atom, in which negatively charged “plums” were embedded within a positively charged “pudding”.",{"type":283,"children":314},[315,319,327,332],{"type":286,"tag":287,"props":316,"children":317},{},[318],{"type":291,"value":312},{"type":286,"tag":287,"props":320,"children":321},{},[322],{"type":286,"tag":296,"props":323,"children":326},{"alt":298,"src":324,"title":325},"image://32395b01-1dff-4e9d-9e69-0b893b5241f3","J.J. Thomson. Image: Bain News Service, publisher, Public domain, via Wikimedia Commons",[],{"type":286,"tag":287,"props":328,"children":329},{},[330],{"type":291,"value":331},"Before moving on, it’s worth examining what constitutes a metal. At the atomic level, a metal is a closely packed lattice of metal ‘ions’, which are simply atoms with a net positive or negative charge.",{"type":286,"tag":287,"props":333,"children":334},{},[335],{"type":291,"value":336},"The structure of a solid metal consists of charged ions because each atom has donated at least one of its outermost electrons to form a “sea” of ‘delocalised electrons’. The electrostatic force of attraction between the metal ions and delocalised electrons is what imparts metals with their rigidity and strength. In theory, if given enough energy, these delocalised electrons could become liberated from the metal’s surface.",{"title":280,"searchDepth":20,"depth":20,"links":338},[],{"id":100,"data":101,"type":35,"maxContentLevel":19,"version":20,"reviews":105,"parsed":340},{"data":341,"body":343,"toc":367},{"title":280,"description":342},"In 1902, another German physicist Philipp Lenard demonstrated that illuminating a metal surface with light or another form of electromagnetic radiation could cause electrons in the material to be expelled or ‘liberated’.",{"type":283,"children":344},[345,349,354,362],{"type":286,"tag":287,"props":346,"children":347},{},[348],{"type":291,"value":342},{"type":286,"tag":287,"props":350,"children":351},{},[352],{"type":291,"value":353},"The electrons absorbed incoming ‘radiant energy’, allowing them to break free from the electrostatic forces binding them to the metallic ions. Unfortunately, this ‘photoelectric effect’ represented an interaction between light and matter which couldn’t be explained by classical physics, or by viewing light as an electromagnetic wave.",{"type":286,"tag":287,"props":355,"children":356},{},[357],{"type":286,"tag":296,"props":358,"children":361},{"alt":298,"src":359,"title":360},"image://ce7c6a71-8746-4f6c-98b8-368ff97e9bf1","Philip Lennard. Image: Public domain, via Wikimedia Commons",[],{"type":286,"tag":287,"props":363,"children":364},{},[365],{"type":291,"value":366},"If light were only a wave, the energy of the released electrons would depend on the light source’s intensity. Except it didn’t! It was actually dependent on the light’s frequency, which according to wave theory should have zero effect. The intensity instead determined the number of electrons released from the metal’s surface. What was made clear is that classical ideas and the wave picture of light simply weren’t cutting it in explaining this puzzling effect.",{"title":280,"searchDepth":20,"depth":20,"links":368},[],{"id":117,"data":118,"type":35,"maxContentLevel":19,"version":20,"reviews":122,"parsed":370},{"data":371,"body":373,"toc":397},{"title":280,"description":372},"Albert Einstein made his mark in 1905 by formulating a startling new theory of light. He did this by expanding upon Planck’s idea of quantization to conjecture that light itself is made up of discrete packets or ‘quanta’. Each of these quanta contained a fixed amount of energy directly related to the light’s frequency. Their name? Photons!",{"type":283,"children":374},[375,379,384,392],{"type":286,"tag":287,"props":376,"children":377},{},[378],{"type":291,"value":372},{"type":286,"tag":287,"props":380,"children":381},{},[382],{"type":291,"value":383},"This is where things took a strange turn, as Einstein was not only suggesting that light is composed of what are essentially particles, but that it also possesses wave-like properties such as frequency and therefore also an effective ‘wavelength’.",{"type":286,"tag":287,"props":385,"children":386},{},[387],{"type":286,"tag":296,"props":388,"children":391},{"alt":298,"src":389,"title":390},"image://778408f4-026a-4335-93f9-001c6b88c374","Albert Einstein. Image: Photograph by Oren Jack Turner, Princeton, N.J., Public domain, via Wikimedia Commons",[],{"type":286,"tag":287,"props":393,"children":394},{},[395],{"type":291,"value":396},"It became apparent from that point onwards that light isn’t fully described by a wave or a particle. This was the first example of ‘wave-particle duality’, a prominent concept in quantum mechanics stating that every particle may be described as either a particle or a wave. Neither description alone nor their classical interpretations are sufficient to fully describe the behaviour of any quantum system.",{"title":280,"searchDepth":20,"depth":20,"links":398},[],{"id":131,"data":132,"type":20,"version":20,"maxContentLevel":19,"summaryPage":134,"introPage":143,"pages":400},[401,418],{"id":152,"data":153,"type":35,"maxContentLevel":19,"version":35,"reviews":157,"parsed":402},{"data":403,"body":405,"toc":416},{"title":280,"description":404},"Like all forms of electromagnetic radiation, light transports energy across space. Einstein was able to convincingly show that the energy transmitted by light arrives at a receiver not continuously but in discrete units called ‘photons’. Instead of the energy being continuously distributed over a wavefront, it was now carried in neat, quantized packages. Therefore, we can view photons as effective “particles” of light! So what do we know about them?",{"type":283,"children":406},[407,411],{"type":286,"tag":287,"props":408,"children":409},{},[410],{"type":291,"value":404},{"type":286,"tag":287,"props":412,"children":413},{},[414],{"type":291,"value":415},"Following on from Maxwell’s work, we know that photons always move at the speed of light. We also know that they are electrically neutral and have zero mass, but still carry an energy proportional to the light’s frequency. The higher the frequency or shorter the wavelength, the more energetic the photon. Lastly, photons can be created or destroyed. When a source emits electromagnetic waves, photons are created. When photons encounter matter, they can be absorbed and transfer their energy to the atoms and molecules.",{"title":280,"searchDepth":20,"depth":20,"links":417},[],{"id":181,"data":182,"type":35,"maxContentLevel":19,"version":20,"reviews":186,"parsed":419},{"data":420,"body":422,"toc":433},{"title":280,"description":421},"When Planck derived an equation that described black body radiation curves with tremendous accuracy and resolved the ultraviolet catastrophe, he defined a universal constant ‘h’ in his calculations. This ‘Planck constant’ is incredibly small – approximately 6.63 × 10^-34 joules/second – and just like the speed of light or the charge of an electron, it is a fundamental, unchanging quantity that has profound importance in quantum mechanics. In fact, this constant appears in any equation in which the phenomenon under consideration exhibits quantum mechanical behavior.",{"type":283,"children":423},[424,428],{"type":286,"tag":287,"props":425,"children":426},{},[427],{"type":291,"value":421},{"type":286,"tag":287,"props":429,"children":430},{},[431],{"type":291,"value":432},"In Einstein’s quantum theory of light, he carried the Planck constant forward, capturing the relationship between the energy of a photon and its frequency via the straightforward equation E = h × f or E = hc / λ where ‘c’ is the speed of light and ‘λ’ is its wavelength. Therefore, according to Einstein’s work, red light photons possess lower energy than blue light photons due to the former having a relatively longer wavelength or lower frequency.",{"title":280,"searchDepth":20,"depth":20,"links":434},[],{"id":195,"data":196,"type":20,"version":20,"maxContentLevel":19,"summaryPage":198,"introPage":207,"pages":436},[437,462,479],{"id":216,"data":217,"type":35,"maxContentLevel":19,"version":20,"reviews":221,"parsed":438},{"data":439,"body":441,"toc":460},{"title":280,"description":440},"When light is shone on a metal surface, sometimes no electrons are emitted regardless of the light’s intensity. But if light were a wave, even a low intensity light source should continually transfer energy to the electrons until they absorb enough to escape from their metallic bonds.",{"type":283,"children":442},[443,447,452],{"type":286,"tag":287,"props":444,"children":445},{},[446],{"type":291,"value":440},{"type":286,"tag":287,"props":448,"children":449},{},[450],{"type":291,"value":451},"In quantum physics, it’s the light source’s frequency which matters. It turns out that every material has a unique ‘cut-off frequency’ below which no electrons are released and above which photoelectric effects are witnessed.",{"type":286,"tag":287,"props":453,"children":454},{},[455],{"type":286,"tag":296,"props":456,"children":459},{"alt":298,"src":457,"title":458},"image://2199eb20-0f25-4aac-bec9-0e0e98119a40","Particles being reflected from waves",[],{"title":280,"searchDepth":20,"depth":20,"links":461},[],{"id":232,"data":233,"type":35,"maxContentLevel":19,"version":20,"reviews":237,"parsed":463},{"data":464,"body":466,"toc":477},{"title":280,"description":465},"The quantum view of light states that intensity only affects the rate of electron emission, not whether they will be emitted. In other words, a greater number of photons transmitted per unit area and time means more opportunities for electrons to be liberated, assuming each photon has sufficient energy! When a photon with sufficient energy impacts an electron, it causes it to be liberated. Nothing happens if the photon lacks sufficient energy, regardless of how many there are!",{"type":283,"children":467},[468,472],{"type":286,"tag":287,"props":469,"children":470},{},[471],{"type":291,"value":465},{"type":286,"tag":287,"props":473,"children":474},{},[475],{"type":291,"value":476},"To liberate an electron from a metal surface the incoming radiation needs to have energy surpassing Φ, which is related to the cut-off frequency fc by Φ = h × fc and known as the ‘work function’ of the metal. This work function is the minimum energy required to induce ‘photoemission’ of electrons, and it depends on the specific metal being illuminated.",{"title":280,"searchDepth":20,"depth":20,"links":478},[],{"id":250,"data":251,"type":35,"maxContentLevel":19,"version":20,"reviews":255,"parsed":480},{"data":481,"body":483,"toc":499},{"title":280,"description":482},"If an incoming photon’s energy exceeds the work function of the material, an electron can be freed, and any excess energy appears as kinetic energy of the electron. It was established that the maximum kinetic energy of liberated electrons was independent of the light’s intensity, which only makes sense when light is quantized into photons. Each photon has a fixed energy based on frequency only, meaning that electrons will have a maximum kinetic energy – calculated as the difference between the energy of the photon and the work function of the metal – that doesn’t vary with intensity.",{"type":283,"children":484},[485,489,494],{"type":286,"tag":287,"props":486,"children":487},{},[488],{"type":291,"value":482},{"type":286,"tag":287,"props":490,"children":491},{},[492],{"type":291,"value":493},"According to classical wave theory, a measurable time lag should exist between the time when light first starts to illuminate a surface and the subsequent ejection of electrons. However, no detectable time lag has ever been measured in the photoelectric experiment. In other words, when a single photon with sufficient energy interacts with a single electron the photoelectric emission process is instantaneous.",{"type":286,"tag":287,"props":495,"children":496},{},[497],{"type":291,"value":498},"Classically, the time scale of the interaction should be measurable as the incoming wave continuously supplies energy until the electron has enough to escape, but this is not what was observed. The lack of a time lag in the photoelectric effect was another finding in favor of the quantum view.",{"title":280,"searchDepth":20,"depth":20,"links":500},[],{"left":4,"top":4,"width":502,"height":502,"rotate":4,"vFlip":6,"hFlip":6,"body":503},24,"\u003Cpath fill=\"none\" stroke=\"currentColor\" stroke-linecap=\"round\" stroke-linejoin=\"round\" stroke-width=\"2\" d=\"m9 18l6-6l-6-6\"/>",{"left":4,"top":4,"width":502,"height":502,"rotate":4,"vFlip":6,"hFlip":6,"body":505},"\u003Cpath fill=\"none\" stroke=\"currentColor\" stroke-linecap=\"round\" stroke-linejoin=\"round\" stroke-width=\"2\" d=\"M4 5h16M4 12h16M4 19h16\"/>",1778228330825]