Snell’s Law Snell’s Law describes refraction as light strikes the boundary between two media n1 sin q1 = n2 sin q2 The index of refraction of a pure vacuum and of air is n = 1. The index of refraction of every other substance is greater than 1. incidence reflected refracted q

What is the index of refraction of the glass? Example: Light traveling through air enters a block of glass at an angle of 30° and refracts at an angle of 22°. What is the index of refraction of the glass? incidence reflected refracted q

(ok, actually violet refracts the most…) Different frequencies (colors) refract slightly different amounts. This means that the index of refraction, “n”, for blue light is slightly different than “n” for red light. This results in a dispersions of colors as seen in a prism or a rainbow. Blue Bends Best! (ok, actually violet refracts the most…)

Rainbows! Sunlight refracts as it enters a raindrop. Different colors refract different amounts. This spreads out the colors. The light reflects off the back of the raindrop. The light refracts again, spreading out the colors even more. We see the rainbow!

The Critical Angle and Total Internal Reflection When light passes from a material that is MORE dense to one that is LESS dense, its refracts AWAY from the Normal line. As the angle of incidence increase, the angle of refraction also increases. q refracted incidence

The Critical Angle and Total Internal Reflection At some Critical Angle of incidence, the angle of refraction is 90°. Beyond that critical angle, no light that is refracted! All of the light is reflected back into the original medium. This is called Total Internal Reflection n2 reflected incidence qcritical n1

The most useful application of the phenomenon of Total Internal Reflection is in Fiber Optics

When wavefronts pass through a narrow slit they spread out When wavefronts pass through a narrow slit they spread out. This effect is called diffraction.

The amount of diffraction depends upon the size of the slit. If the slit is comparable in size to the wavelength of the wave then maximum diffraction occurs.

The number of slit openings also determines what the diffraction pattern looks like.

Thomas Young, in1801, first established that light was a wave by demonstrating that light diffracted. He also provided the first measurement of the wavelength of light.

Thomas Young’s Double-Slit Experiment He allowed sunlight to fall on two slits. He knew that if light was a wave, it would diffract as it passed through the slits. The diffracted waves would have areas of both constructive and destructive interference. This interference would produce bright and dark areas on a screen.

If the wavelength of light is longer, the pattern on the screen is more spread out. (700 nm- 400 nm) Red light spreads out more than violet. If the screen is farther, the pattern on the screen is more spread out. If the slits are CLOSER to each other, the pattern on the screen is more spread out.

Optical diffraction effects can be seen with eye - in fact most of us when children have noticed it, but ignored it when becoming adults. Look through a narrow slit between your fingers. If you look carefully you should see the objects behind are distorted and that blackish bands parallel to the slit appear in the gap. The bands are diffraction patterns.

In the atmosphere, diffracted light is actually bent around atmospheric particles -- most commonly, the atmospheric particles are tiny water droplets found in clouds. An optical effect that results from the diffraction of light is the silver lining sometimes found around the edges of clouds

The pattern of bright and dark fringes did appear on a screen. The brightest area, in the center, he called the “central bright spot”. He was able to mathematically determine the wavelength by measuring the distance from the central bright spot to each fringe.

ml = d(x ÷ L) = dsinq m- “order” (m = 0 is the central bright spot) l- wavelength of light d- distance between the slits x- distance from central bright spot to another bright fringe L- distance from the slits to the screen q- the angle between the line to the central bright spot and the observed bright fringe.

Different frequencies (colors) of light diffract by different amounts

The more slits there are, the narrower the fringes become. The fringes on top are from two slits. The fringes on bottom are from eight slits. A “diffraction grating” has hundreds of slits per millimeter.