One of the greatest mysteries of light

Einstein in 1905 wrote a very beautiful

paper for which he subsequently got the

Nobel Prize. This is the same year that

he wrote the special theory of

relativity and a very famous paper on

Brownian motion. But it's a the theory

for which he was most I suppose well

known at the time. Um it's a theory of

something called the photoelectric

effect. So the photoelectric effect is

again it's an experimental observation

that uh if you shine light on on a a

piece of metal let's say some substance

then the light can hit the substance and

cause electrons to be emitted from the

substance from the from the metal. But

what was observed was that if the light

is too long a wavelength or too too low

a frequency then no matter how bright

the light then you can you can make it

brighter and brighter and brighter and

brighter no electrons are emitted. This

is a a mystery. If you think of light as

just delivering energy to something and

just a and not like a stream of little

particles, then it would seem that if

you turn the intensity up, brighter

light, then you'd supply enough energy

to knock the electrons out of the metal.

That's not what happens. So, you find

that there's a a minimum frequency.

Remember, the frequency is the color.

So, it's almost like saying if I shine

red light on this thing, um then no

matter how bright the red light is,

nothing happens. But if I make the light

a little bluer, you get to a point where

electrons start coming off and then that

light can be quite dim, but it will

still emit electrons. So this was a

mystery. It's called the photoelectric

effect. Einstein explained it following

plank by saying that actually you can

think of light as a stream of particles,

photons. And then if the photons don't

have enough energy to knock the

electrons out of the material, then no

electrons will emerge. So you could

imagine just having one single photon,

but if it has enough energy, then it

will come in and knock an electron out.

And so that's essentially the

photoelectric effect. It's important

because it's the first time that we get

the sense that this so-called

quantization of the electromagnetic

field, this chopping up of light into

little packets, is not to do with the

way that matter emits light. It's to do

with light itself. The idea, it's going

all the way back hundreds of years to

pictures that Newton would have had of

light being a stream of particles that

you can really think of these things

called photons almost like little

bullets that go and hit things and and

interact with them. So that's a that's

1905.

It's worth saying actually historically

that that was very controversial at the