Goal: To understand reflection and refraction Objectives: 1)To understand Reflection off of a simple mirror 2)To learn about Scattering 3)To understand Refraction 4)To learn about Mirages 5)To explore Rainbows 6)To learn about Total Internal Reflection

When a photon meets a woman, err a surface A few things can happen, and often times you will get all of these in different amounts. 1) reflection – the light in essence bounces off of the surface. In this case the angle which the light leaves is the same as it hits (θr = θi), as is the wavelength of light (in the perspective of the object it is hitting) The % of light that gets reflected depends on the material it is hitting as well as the wavelength of light.

Plane mirrors These are the sort of mirrors you find in a bathroom. They are straight and flat. (below from a website)

Transmitted 2) Some light will be transmitted. That is it will pass through the material. This will also cause what is called refraction – but more on that later.

Absorption 3) The material can absorb some of the energy. The percentage it absorbs at some wavelength is called the albedo. This can vary greatly with wavelength. A thin layer of water, for example, does not absorb much light in the optical. However, in the infrared it absorbs a LOT of energy. Thus water vapor is a greenhouse gas (and the most abundant greenhouse gas in our atmosphere).

Scattering 4) Scattering. This is similar to reflection. However, most surfaces are not smooth. So, when the light hits the surface, it is not hitting a flat surface. So, different light photons at slightly different positions can hit at a vastly different angle to the surface. Therefore, they will reflect at a wide range of angles. This is scattering. Also, if light hits an electron the electron can absorb the photon and reemit in a random direction – thus scattering the light.

More on Refraction A surface is defined to be a point where the index of refraction changes. The most common form of this is when you have 2 different types of materials – such as air, glass, water, ect. For each the speed of light is different. When the light enters the material the speed changes but the rest of the wave which has not hit the surface is still going at the old pace. This causes an accordion affect (like cyclists hitting a hill and piling up). Also, the index of refraction depends on the wavelength of light – more on this at the end of class

Critical Angle The most the refraction angle can be is 90 degrees (otherwise it would refract back into the substance, which does not make sense and is reflection) That means that if you go from high index of refraction to low index of refraction there is a point where the refraction angle is 90 degrees. This angle is called the Critical Angle. If your initial angle is BIGGER than this, then there is NO refraction (this is called total internal reflection)! Why would this be important?

Now for something completely different But still the same. RAINBOWS! How are rainbows formed? (a lot of scribbling on the board is about to come…)

Conclusion We learned about what light does when it hits a surface or travels into another medium. We now know how rainbows are created – sorry for ruining rainbows forever for all of you.