L 31 Light and Optics [1] Measurements of the speed of light The bending of light – refraction Dispersion Total internal reflection Dispersion Rainbows Atmospheric scattering Blue sky and red sunsets Light: electromagnetic waves in the visible part of the spectrum: wavelengths of 400 nm (red) to 700 nm (violet) (nm = m)

Galileo’s attempt to measure the speed of light c Galileo was first to even consider that the speed was finite Galileo turns on his flashlight and starts his clock When Sagredo sees Galileo’s light, he turns on his flashlight When Galileo sees the light from Sagredo’s flashlight, he notes the time the actual delay would be t = 2D/c = 10 microseconds, so the attempt failed. D = 1 mile Galileo Galileo’s assistant Sagredo

The speed of light inside matter The speed of light c = 300,000,000 m/s = 3 x 10 8 m/s = 186,000 mi/s in vacuum In any other medium such as water or glass, light travels at a lower speed. The speed of light in a medium can be found by using v = c/n, where c is the speed in vacuum (300,000,000 m/s) and n is a number (greater than 1) called the index of refraction.

MEDIUM INDEX OF REFRACTION (n) SPEED OF LIGHT (m/s) (v) Vacuum air Exactly ,000,000 water ,564,000 glass ,368,000 diamond ,967,000 v(speed in medium ) = c(speed in vacuum) / n

VISIBLE LIGHT Color  WAVELENGTH OR FREQUENCY Wavelength X Frequency = c

COLORCOLOR Any color can be made by combining primary colors  Red, Green and Blue A color TV uses mixtures of the primary colors to produce “full color” images Perceived color is a physiological effect  depends on how our eyes work

Refraction  the bending of light One consequence of the fact that light travels more slowly in say water compared to air is that a light ray must bend when it enters water  this is called refraction the amount of refraction (bending) that occurs depends on how large the index of refraction (n) is, the bigger n is, the more bending that takes place

Reflection and refraction at a surface Incident Light ray reflected Light ray refracted Light ray Normal line

Refraction of light Water n= 1.33 Glass (n=1.5) Incident ray refracted ray The refracted ray is bent more in the glass

Refraction from air into water water n = 1.33 n = 1.0 normal When a light ray goes from air into water, the refracted ray is bent away toward the normal.

Refraction works both ways: water into air water n = 1.33 n = 1.0 normal When a light ray goes from water into air, the refracted ray is bent away from the normal.

Windows can be mirrors

Some interesting effects due to refraction Underwater objects appear to be closer to the surface than they actually are Total internal reflection  fiber optics Seeing through a window Dispersion  rainbows

What does it mean to “see” something? To “see” something, light rays from the object must get into your eyes. unless the object if a light bulb or some other luminous object, the light rays from some light source (like the sun) reflect off of the object and enter our eyes. BEER!

Where is the fish? Closer than you think! fish Apparent location Of the fish

Total internal reflection n 1 > n 2 n2n2 When the incident angle is too big, the refracted ray disappears and the incident ray is totally reflected back.

Fiber optics  A fiber optic cable is a bunch (thousandths) of very fine (less than the diameter of a hair) glass strands clad together.  The light is guided through the cable by successive internal reflections.

fiber optic communications can carry more info with less distortion over long distances not affected by atmospheric conditions or lightning and does not corrode copper can carry 32 telephone calls, fiber optics can carry 32,000 calls takes 300 lbs of copper to carry same info as 1 lb of fiber optics downside  expensive

The index of refraction (n) depends of the color (wavelength) of the light colorWavelength (nm)n Red orange yellow green blue violet nm = m

Different colors are refracted (bent) by different amounts Glass prism White light red violet contains all wavelengths (colors)

The rainbow Rainbows are caused by dispersion of sunlight from water droplets which act as tiny prisms

Why is it a rain BOW ? The drops must be At just the right Angle (42 degrees) Between your eyes And the sun to see The rainbow. This Angle is maintained Along the arc of a Circle. Wow!