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• M: In this week’s assignment we will do a virtual lab and it is important to take a break from all this physics and ponder what makes physics a science.
• S: Ok and what is that?
• M: Physics is a science, and a powerful one at that, because it is based on experimental evidences. It all starts with measurements. Here is a quote from Lord Kelvin:

I often say that when you can measure something and express it in numbers, you know something about it. When you cannot measure it, when you cannot express it in numbers, your knowledge is of a meager and unsatisfactory kind.

• S: That's not so nice to say for all the non-scientific knowledge out there.
• M: True. It was Kelvin’s personal opinion that scientific knowledge based on measurements expressible by numbers (mathematics) is the surest, highest form of knowledge. This is an opinion that is shared by many people nowadays and while you may not agree with the statement, you would be hard pressed not to acknowledge how experimental science has changed our view of the world and our society in the last thousand years.
• M: We have measured the Earth's diameter, we have measured the speed at which the Earth orbits the Sun, we have measured the size of atoms, we have measured the concentration of carbon dioxide in the atmosphere. All of these numbers are not “debated” by anyone. We all agree on the size of the Earth. These numbers are the starting point from which science can be done.
• S: Well isn’t there room for errors in measurements?
• M: Oh, yes, and this is really important. We will talk about that next week. A big part of the job of being an experimental physicist is to understand the errors and to try to minimize them.
• S: I always think of physicists as being like Einstein, just thinking and figuring out how the world works.
• M: This is a common misconception and really not true. Here, let's look at an example. Lets look at how physicists tried to find out whether light is a wave or a particle... the first time.

Is Light a Wave or a Particle? Newton's test

Isaac Newton did the following experiment with light. He shined light on a prism and he observed that the outgoing light was dispersed in different rainbow colors. He could measure exactly (with numbers) the spread of the dispersed light. He realized that this dispersion occurred when light went through any lens (for example in your camera or even in your eyeglasses), and he found ways to polish lenses to correct for this dispersion (this is called chromatic aberration in this context). This led him to build the first functional reflecting telescope which he could then use to study the cosmos!

This is typical of experimental physics. A precise, measurable, understanding of the world leads to technological breakthroughs sometimes unexpected!

Now Newton had a hypothesis about light (he just came up with it). He thought that light was made of a bunch of particles of different colors (bunches of blue, red, purple, … light particles). His idea was that different colored particles would accelerate (go faster) when going through a denser material. With this hypothesis, he was able to explain his experimental result on the prism ... but he was wrong.

His ideas about light as a particle failed to correctly (again with numbers) describe other phenomena like interference (to be discussed next). Since Newton was famous, his ideas about light lasted longer than it should have, but eventually, in physics, we don’t care about how famous you are. If your hypothesis does not fit the data (does not match the numbers we measure), it is just wrong and it goes to the garbage can. No sentiments involved!