Let's Talk Physics

A stochasticly updated blog about interesting topics in Physics & Astronomy

The Photoelectric Effect

I’m sorry about the delay on this article, making the gif’s took a lot longer than I anticipated. – Mark

Today, I’m taking a bit of a divergence to my plan. Initially, I was going to be talking about the first few seconds of our Universe using the concepts of Plasma and Annhilation, but I realised this week that I can’t fully explain it without one of the most significant discoveries of the early 20th century – the Photoelectric Effect.

First of all, what is the photoelectric effect? Well, let’s start with a simple setup. Imagine there’s a baseball attached to the ground by a wooden stick, and you go to hit the baseball with a bat. Now, there’s obviously an energy (determined by the speed of the bat) at which if you swing the bat and hit the ball, then the stick connecting the ball to the ground will break, but the ball won’t go anywhere. Let’s call this the threshold energy. If we swing the bat such that the energy the ball gets it’s less than the threshold energy, then the wooden stick won’t break. If we swing the bat such that the energy is greater than the threshold energy, the wooden stick will break, and the ball will move off with a velocity determined by how much energy above the threshold energy we are.

Now, replace the ball with an electron, the ground with the nucleus of an atom, the wooden stick with the bonding energy of the electron to the nucleus and the bat with a photon of light. And that is the photoelectric effect.

Below, there are 3 gif’s demonstrating the photoelectric effect. Say, for example, the threshold energy (the energy associated with the binding of the electron to the nucleus) is 5 joules (these are unrealistic numbers, but just got with it). If a photon of energy 4 joules strikes the electron, nothing happens! The bond isn’t broken and the photon continues on its way (example 1). Now, imagine the photon is of energy 5 joules. The bond will break, but the electron won’t go anywhere – all of the energy of the photon went into breaking the bond (example 2). Now, imagine the photon has 6 joules of energy – 5 joules of energy from the photon will go into breaking the bond, and then 1 more joule will go into giving the electron a velocity (example 3)!

Example 1: Photon is below Threshold Energy

Example 1: Photon is below Threshold Energy (click to enlarge and play)

Example 2: Photon is at Threshold Energy. Bond is broken, but electron remains stationary.

Example 2: Photon is at Threshold Energy. Bond is broken, but electron remains stationary. (Click to enlarge and play)

Example 3: Photon is above Threshold Energy. Bond is broken and electron moves away.

Example 3: Photon is above Threshold Energy. Bond is broken and electron moves away. (Click to enlarge and play)

This all seems like a very simple idea – but it wasn’t back in the early 20th century! Firstly, back then, people didn’t know for certain that light traveled as photons (this is known as the quantization of light). In our theory, if we replaced a photon of light with a continuous stream of energy (which is how people assumed light traveled then) then shining light on any object for long enough would eventually break the bonds of electrons to the nucleus, and atoms wouldn’t exist! Secondly, it required the idea that electrons and photons can act as either waves or particles – a fundamental idea of quantum mechanics, and something I’ll talk about in weeks to come (it’s called wave-particle duality if you want to get ahead!). If electrons and photons didn’t behave this way, then they couldn’t react with one another.

So, it was this idea – using wave-particle duality and the quantization of light – that won Einstein the Noble Prize in 1921.

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About irishphysicist

I'm a PhD student with the Departments of Physics in University College Cork, Ireland and University of Notre Dame, Indiana. I want to try and bring astrophysics to the public, and also would like world domination. But that's a story for another day.

One comment on “The Photoelectric Effect

  1. Pingback: The First Few Minutes | Let's Talk Physics

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This entry was posted on June 18, 2013 by in Astrophysics, Physics and tagged , , , , , , , , , , .
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