For many purposes, you can consider light to propagate as rays, explaining phenomena such as shadows or imaging with a pin-hole camera. Even refraction of light at interfaces between transparent matter or vacuum can be understood in the framework of ray optics by introducing an index of refraction n and using Snell's laws of refraction. This explains lenses, prisms, or effects such as mirages.  However, light is an electromagnetic wave.  You can only neglect the wave nature of light and use a ray description if all sizes are much larger than the wavelength of light. And even this is only true if you have completely "incoherent" light, for example from the sun or from a light bulb. Things entirely change if sizes become small, or if you have pure light of a single color as it can be generated by a laser.

In today's lab we will explore both of these cases and explore the wave nature of light. We will also use light to carry out two precision measurements. We will measure the index of refraction of air, and the tiny displacement of a piezo actuator.