The Photoelectric Effect – A Level Physics

Published on February 23, 2015 by

Something to do with photons and electricity. The emission of electrons from a metal surface when illuminated by certain colours of light has long been known about, but explaining why was always a challenge. This video shows why this can only explained by the interaction of individual packets of energy.

Watch Part 2 of this video for an explanation of Einstein's equation:

Thanks for watching,



Find even more videos organised by exam board and topic at:

GCSE Physics Online
A Level Physics Online

Your support in watching this video has been invaluable! To contribute towards the free videos on YouTube, make a small donation at:


#photoelectric #alevelphysics #physicsonline

Category Tag


Advertise here. Telegram, Signal - Call/message +1-868-308-4028.


  • uke22 7 years ago

    Your videos are really helpful! Deserve more views!

    • Physics Online 7 years ago

      Thanks, hopefully as more people share and like them they’ll be seen by more. Have a look at other videos I’ve made too. More being produced every week.

  • Ieuan Gundy 7 years ago

    OH MY GOSH….. YOUR A LEVEL VIDEOS ARE AMAAAAAAAAAAAAAZING. Please… PLEASE DON’T STOP MAKING THEM! They are helping me SO much. I consider myself a visual learner, and my physics teacher SUCKS! These videos are teaching me more this past week than any book/teacher ever has! Keep up the good work!

    Also, any chance you take requests? I am having major problems with electricity/resistance/DC circuits and it makes up like a third of the entire WJEC exam board! :'(

  • ginga2122 7 years ago


  • Snowflakes Queen 7 years ago

    can i get your email i kinda worke on progect and i need help can you help me ?

  • Sang Wangle 7 years ago

    Very good examples , thank you!

  • Alex Osborne 7 years ago

    This is Great, I will now be thinking of Lego in my exams!

  • Sarah Young 6 years ago

    This is a brilliant way to explain this. Couldn’t understand what my teacher was saying and now everything is so clear! Thank you!

  • RNHRK 6 years ago

    Nicely done

  • Akashascosset 6 years ago

    great lessons and examples.
    some Stop-Go Animation would bring your videos to the next level!

  • Hola Mania 6 years ago

    A brilliant analogy, Well done to your Sir.

  • ibgmarx 6 years ago

    very well explained

    • Physics Online 6 years ago

      +ibgmarx Thanks

  • Samie Mohammed 6 years ago

    Hi there I’m a little confused on this. If you increase the intensity of a low frequency light it still won’t make a difference as the energy of these photons will be lower than the work function however if you increase the intensity of light with a frequency more than or equal to the work function/ threshold frequency will it still emit photoelectrons or not? are we assuming the low frequency light hasn’t enough energy to release a photo electron. If you could help with this is would be very grateful

    • Physics Online 6 years ago

      +Samie Mohammed If the light is below the threshold frequency then the intensity does not matter since no photoelectrons can be emitted. Above the threshold frequency the intensity of photoelectrons is proportional to the intenisty of the light. More light = more electron emitted.

    • Samie Mohammed 6 years ago

      +A Level Physics Online thanks for replying loving the revision videos

  • Samie Mohammed 6 years ago

    Also if the electrons only gain energy of one photon won’t that photon have to have an frequency equal to or more than the threshold frequency. so by increasing the intensity how does it make a difference to the amount of photoelectrons released as for example a photon has a lower frequency than the threshold it will only give that amount of energy relating its frequency to that electron however its not enough so by increasing the intensity won’t every photon be giving the electrons energy less than the work function every time it collides? wont the electron stay put in its energy level.

    • Physics Online 6 years ago

      +Samie Mohammed One incoming photon can release one electron provided it is above the threshold frequency.

  • Abrar Istiak Akib 6 years ago

    Does photoelectric effect applicable for hydrogen?Is it only applicable for metal?

    • Physics Online 6 years ago

      +Abrar Akib Good question. I think it generally only occurs in metals because it is relatively easy to free the electrons from the ions, whereas in gases like hydrogen it takes more energy to liberate the electrons (with x-rays etc). Have a look here for a bit more information:

  • Izzy Manuel 6 years ago

    How do you calculate the number of photons that are being emitted when you are given the intensity, the area, and the wavelength of light? 🙂

    • A Level Physics Online - Year 13 6 years ago

      +Izzy Manuel I = P / A so if you know the intensity and the area you multiply them together to find the power – which is the energy per second. If you know the wavelength you can find the energy per photo (because E = hc / lambda). You now know the energy per photon and the total energy per second so you can divide one by the other to find the total number of photons. Does that work?

  • Jon Keevash 6 years ago

    I love you ruse of props with the diagrams. they really make it clearer

  • Rebecca Snell 6 years ago

    this is one thing i have really been struggling on, and you have really helped get a grasp of it

  • SaltyShrooms 6 years ago

    I didn’t really understand, as the red photon strikes the electron, why is its energy not transferred into the electron and if you shine enough high intensity red light into an electron the it should eventually have enough energy to escape the atom right? Why do low energy photons go straight through electrons?

    • vertonical 6 years ago

      It’s because in that particular sample of gold leaf, the wavelength and energy of red light did not correspond to the energy difference between any 2 principal quantum shells ( energy levels). You have to know that elements are very picky, unless they receive the EXACT amount of energy from the incoming photons thay EXACTLY correspond to one of the energy gaps between their shells, they will not absorb those photons.

    • SaltyShrooms 6 years ago

      +vertonical thanks

  • paritosh raikar 5 years ago

    nice video…made my concepts clear..Thanks!!

  • Waldo Buitendag 5 years ago

    This was incredible and helped so much. Thank you!

    • Physics Online 5 years ago


  • Thomas Drowry 5 years ago

    Why did Einstein only win one Nobel prize, for the Photo electric effect and neither the Special or General theory of relativity ? Other scientists like Marie Curie have won two Nobels so that can’t be the reason. Is it because Einstein didn’t actually do the hard maths himself but relied on others like Lorentz and Minkowski ?

    • Physics Online 5 years ago

      Very good question and I don’t have an answer – but I did find this article from the Guardian.

  • Yolia Ese 5 years ago

    this helped a lot wooow, bravo and thank you 😀

  • Ronii _9811 5 years ago

    Excellent explanation ! This channel never disappoints 😀

  • Shakif Uddin 5 years ago

    Night before the exam, time to cram in last minute revision

    • Moh k 4 years ago

      how did the exam go?

    • YeetusMC 4 years ago

      shakif uddin im in the same position right now

  • sara anderson 5 years ago

    Look for the new difinition of the colorfulness of  photon 
    on SALEH THEORY group’s youtube chanel: Saleh Theory-com

  • Sean Thrasher 5 years ago

    At 2:50 where does the charge go? I understand that when you discharge it by touching it, the electrons flow through your body and so it is effectively earthed. That doesn’t appear to be the case here- so what happens? Thank you!

  • gamer zone 5 years ago


  • Ankit Sahu 5 years ago

    ty for uploading the video

  • Tasin Al-Hassan 5 years ago

    The Legos explanation was great. Thank you.

  • Reevee 4 years ago

    If you increase the intensity would that increase the rate at which electrons left the electroscope?

    • Physics Online 4 years ago

      Reevee yes

    • Adaora Elliott 3 years ago

      A year later, increasing intensity of radiation on a metal surface, increases the number of electrons emitted per second.

  • Joe Hindley 4 years ago

    when you talk about ‘Energy wells’ Are you refering to orbitals? Great video

  • Evelyn Lau 4 years ago

    The photon will get absorbed by the electron and then the photon particle just disappears? How about there is elastic collision happening and the photon is still there but just left with less energy? Because photon also behaves like a particle.

  • Shon Wuls 3 years ago

    This is a really good way of explaining the PE visually, although I think there’s a small error in the explanation of how the PE works. A couple of times in the middle of the video, you describe electrons as escaping from atoms, but the electrons which become photoelectrons are initially already delocalised electrons within the atom; the energy well is a potential well keeping each delocalised electrons within the metal as a material, not keeping an electron in an atom. Otherwise photoelectric emission wouldn’t be an exclusive behaviour to metal materials.

    Aside from that minor detail this is a really well-made video though.

    • BazookaKid 2 years ago

      I second that, but great video nonetheless

    • Shon Wuls 2 years ago

      @BazookaKid retrospectively I think that I was *sort-of* wrong having studied solid state physics further at university since writing the original comment. The ‘delocalised’ electrons are actually in a sense still within the atoms, but within higher energy bands than the other electrons.. however these bands are actually shared between atoms. So in a sense the photoelectrons do leave the atoms.

    • BazookaKid 2 years ago

      @Shon Wuls Oh ok, I’m not at uni yet so have not covered solid state physics, so I’m still being taught that free delocalised electrons are not within the atoms, but are attracted to their nuclei from the outside

    • BazookaKid 2 years ago

      @Shon Wuls Did you choose to specialise in physics at uni then?

  • mhhndeable 3 years ago

    Can’t these be explained by resonance in the wave model? Cuz, “not by intensity, but by using some specific frequency..” does sound like it

  • Terry Scaddon 2 years ago

    I am really sorry to say, this does not help at all

  • Shelby's garage 2 years ago

    Your a very smart man, legos really did help me to understand.

  • Elite a20 2 years ago

    Sir, say you increased the intensity of the UV light, will the electrons still have the same velocity?

    • Physics Online 2 years ago

      Yes, but there would just be more of them.

  • Shineth Rev Navales 2 years ago

    Pls let me know what materials you’ve used.

    • User 1 2 years ago


  • Shineth Rev Navales 2 years ago

    For academic purposes only

  • Edrei 2 years ago

    thank u for this explanation!

  • Tufty Tom 2 years ago

    nice pen bro

  • Sultana Shah Jalal 2 years ago

    Sometimes i wonder if he has an endless supply of lego or something

    • Physics Online 2 years ago

      If only I did! I’m about to start work on a fission reactor for the summer!

  • 999999 suscrubers with no video challenege lets go 2 years ago

    Very useful

Comments are closed.