Photoelectric Effect Demonstration

one of the big ideas in physics is that lights can behave as waves or particles and this demonstration of the photoelectric effect is a really great way of introducing that idea what I've got here is a zinc plate and if you want the demonstration to work it's absolutely essential to give it a good scrub on both sides with a bit of iron wool and that just removes a surface oxide layer I then place the plate on top of this goldleaf electroscope I've also got an ordinary desk lamp and a UV lamp and an order for the demonstration to work this plate has to be negatively charged and I can do that using the method of charging by induction now let me show you that again I've got a perspex rod here which I can make positively-charged by rubbing with this bit of cloth I bring the rod close to the plate touch the plate with my finger remove my finger then remove the rod now watch what happens when I shine visible light onto the plate absolutely nothing that's actually a really important part of the demonstration your students need to notice that with visible light nothing happens but watch what happens when I shine ultraviolet light you can see that the gold leaf immediately starts to fall and that must mean that the UV light is somehow discharging the electroscope we know that we started with negative charge on the plate so the UV light must somehow be releasing the electrons and the important thing to notice is that is the frequency of the lights that makes a difference remember with a visible light it doesn't matter how bright it is or how long it shines for it doesn't release any electrons now this phenomenon cannot be explained using a wave model of light but we can understand it if we adopt the idea that radiation arrives at the plate as packets of energy which we can call quanta or photons a single photon of UV light can release a single electron from the plate however a single photon of visible light has less energy and so cannot release an electron now this is a relatively simple demonstration to do but it took the genius of Albert Einstein to explain what was going on and it was for that work that he was awarded the 1921 Nobel Prize not his work on relativity you

Channel: National STEM Centre

Views: 271,148

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Keywords: Albert Einstein (Academic), physics demonstration, Photoelectric Effect, gold leaf electroscope, wave particle duality of light, ultra violet light, 1921 nobel prize winner, National STEM Centre, Physics resources, Post 16 physics demonstration, Post 16, science demonstration, Physics (Field Of Study), Alom Shaha

Id: v-1zjdUTu0o

Length: 3min 4sec (184 seconds)

Published: Mon Mar 16 2015