Journal 2/12/15 Tell me 1 way you might be able to use all this stuff you're learning about light later in life Objective Tonight’s Homework To learn about the quantum nature of light none

The Quantum Nature of Light We’ve seen how light acts like a wave. We can create beams and bounce them. We can see how they bend through glass and we can create lasers. However, if we imagine light acting like a particle instead, all the things we’ve talked about still work experimentally. Why? So which is it? Particle or wave? 200 years ago, this was the big question.

The Quantum Nature of Light We’ve seen light act as a wave. It can diffract, refract, interfere, and reflect. All of these are things waves do. But 200 years ago, some questions were arising. There were rumors that in a few cases, light was doing things only particles could do. For example, if you shine a very short burst of light through a small opening, it will make a spot on the wall. A wave should spread out and make a big, dim area.

The Quantum Nature of Light So does light interfere and diffract like a wave? Or does it behave like a particle? In 1801, a scientist named Young performed an experiment to find out. He fired a beam of light at two very thin slits in a piece of thick material.

The Quantum Nature of Light He reasoned that if light is a particle, it should go straight through the slits and make a pattern of two lines. If light is a wave, it should diffract as it goes through the slits and make a pattern of many lines.

The Quantum Nature of Light So which result did Young see?

The Quantum Nature of Light So which result did Young see? Both! Huh?! When he ran the experiment, it spread like a wave, so everyone was happy at first. But then Young had an idea. He fired the light from just one wave at a time, slowly. When he did that, it made a pattern of 2 lines, as if each “wave” of light was actually a particle. But if he covered one slit and fired slowly, he saw an interference pattern again! Somehow just one particle could go through and interfere with itself as if it was a wave. Weeeeird.

The Quantum Nature of Light As confusing as it may seem, this is the basis for the modern quantum theory. Light is both a particle and a wave at the same time. We can fire individual particles that act like waves at certain times. Since then, we’ve found out that ALL atomic particles do this. Everything is both particles and waves at the same time! This is a basic principle of quantum mechanics and is called wave-particle duality.

The Quantum Nature of Light Since then, we’ve come up with many things. Light is said to be “quantized”. That means the energy in a beam of light can only exist in exact steps. A beam could have an energy of “1” or “2”, but not “1.5”. So how do we measure this? With stuff more complex than we really want to cover in this class. Rest assured that energy in a beam of light goes as: E = hf where f is frequency and h is 6.626x J/Hz

Mythbusters This is fairly heavy lesson so let’s let our brains relax with a bit of mythbusters.

Exit Question How do we know light is both a particle and a wave? a) It bends like a wave, but can form patterns like a particle. b) Because we’ve seen little particles of light. c) Because if we look at laser up close, it looks all grainy. Those are the particles. d) Because math. e) Because the Bible says so. f) None of the above