# Reflection (mirrors, seeing your image) Refraction ... ??• Reflection (mirrors, seeing your image) • Refraction (bending light, ... All refraction of light (and reflection) ... • Light waves incident on

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• Physics 1230: Light and Color Geometrical optics - how does light change direction? Reflection (mirrors, seeing your image) Refraction (bending light, light in water)

• Refraction

Ever notice how your leg looks bent as you dangle them in the water from the edge of a pool? Why do fish seem to radically change position as we look at them from different viewpoints in an aquarium? What makes diamonds sparkle so much?

These are all questions that can be addressed with the concept of refraction. Refraction is the bending of light when it goes from one transparent medium to another (e.g. air-to-glass or air-to-water). This meeting place of two different media is called the interface between the media. All refraction of light (and reflection) occurs at an interface.

• Examples of Refraction

Bending of light in glass Mirages Total internal reflection Fiber optics Sparkle of diamonds

• Demonstrations

Refraction e.g. stick in tank of water, pencil in glass of water

Total internal reflection - light pipe

• Changing the speed of light - refraction

Refractions occur because the speed of light changes (slows down) when the light enters a denser material

Applet on Speed of Light http://micro.magnet.fsu.edu/primer/java/speedoflight/index.html

• Optical Density of selected materials

Question: What determines if one material is more or less dense than another from the point of view of light?

Answer: The speed of light is slower in denser materials

We define the index of refraction n of a substance as -

n = speed of light in vacuum speed of light in substance

So n = c/v or v = c/n

• Index of Refraction of Selected Materials

Vacuum Air Water Glass Diamond

1.00000000000..

1.0003

1.333

1.5 (depends on type)

2.4

Optical density is characterized by the Index of Refraction

• Speed of light in different materials

Speed of visible light in different media. The value of 100% refers to the speed of light in vacuum.

http://acept.la.asu.edu/PiN/rdg/refraction/refraction.shtml

• Speed of light in different materials

Applet on Speed of Light http://micro.magnet.fsu.edu/primer/java/speedoflight/index.html

• Light slows down in denser materials

http://acept.la.asu.edu/PiN/rdg/refraction/refraction.shtml

Light waves incident on glass change direction and wavelength when transmitted into the glass because the part of the wave in the medium begins to slow down, causing the light beam to bend.

This is like when a marching band needs to make a turn Applet on Refraction

• Law of Refraction

Light going from a substance of small n to a substance of large n is bent TOWARD the normal

Light going from a substance of large n to a substance of small n is bent AWAY from the normal

Diagram works in either direction

Air (fast medium)

Glass or water (slow)

Normal

Glass or water (slow)

Normal Air (fast medium)

nair < nwater 1.001 < 1.5

• Law of Refraction

Light going from a substance of small n to a substance of large n is bent TOWARD the normal

Light going from a substance of large n to a substance of small n is bent AWAY from the normal

glass or water (large n)

Diagram works in either direction

incident rays

refracted rays

air (small n)

• Ray-bending together with our psychological straight-ray interpretation determine the location of images underwater

The precise amount of bending is determined by the

Law of Refraction (also called Snell's law):

ni sini = nt sint Here, i = angle between incident

ray and normal, and t = angle between transmitted

ray and normal ni and nt are the indices of

refraction in the medium containing the incident ray and in the medium containing the transmitted ray

incident ray

transmitted ray

normal

image of fish for someone above the water

fish

In order to observe the fish from outside the water a transmitted ray must enter your eye. You will think it comes from a point obtained by tracing it backwards, Extend any 2 of the many, many transmitted rays from the nose of the fish backwards to find the image of the nose of the fish (where they intersect). The location of that image will be the same for any observer outside of the water.

Refraction - Real Depth and Apparent Depth

• Material

Vacuum Air Water

Glass Diamond

Index

1.00..

1.0003

1.333

1.5 (depends on type)

2.4

Concept Question: Index of Refraction

normal to surface

The ray shown here will bend most if the second medium is A. Air B. Glass C. Water D. Diamond

ray Air

• APPLET ON REFRACTION

Applet on Refraction http://micro.magnet.fsu.edu/primer/java/scienceopticsu/refraction/refractionangles/index.html

• Underwater at the Great Barrier Reef, Australia

why cant you see the sky?

• TOTAL INTERNAL REFLECTION Total internal reflection happens when light is

incident from a more dense medium to a less dense medium at a large angle of incidence

Examples: light going from glass-to-air or from water-to-air

As the angle on the water side increases, the angle on the air side eventually goes past 90 degrees, which means that the light stays in the water! This happens at the critical angle

Glass or water (slow)

Normal Air (fast medium)

Just below the critical angle for total internal reflection there is a reflected and a transmitted (refracted) ray

Glass or water (slow)

Normal

Just above the critical angle for total internal reflection there is a reflected ray but no transmitted (refracted) ray

For incident angles in glass/water greater than the critical angle, ALL the light is reflected back into the dense substance

• TOTAL INTERNAL REFLECTION

Applet on Critical Angle http://micro.magnet.fsu.edu/primer/java/refraction/criticalangle/index.html

• TOTAL INTERNAL REFLECTION

Total internal reflection occurs when light rays incident at angles to the normal at greater than the critical angle (here, 41 for glass to air) do not leave the material and are reflected at the glass/air interface.

http://acept.la.asu.edu/PiN/rdg/refraction/refraction2.shtml

• DIAMONDS ARE FOREVER. The index of refraction for diamond is very high compared to ordinary glass (2.4 versus 1.5) As a result, the critical angle for light to be reflected totally as it travels from diamond-to-air is small (24 versus 42 for regular glass). Therefore most light is re-reflected back from a diamond, and dispersion separates the colors. This gives the characteristic fire or brilliance to a diamond

• The cut of the diamond determines its brilliance

Marcel Tolkowsky (1919)

• MIRAGES

• MIRAGES Mirages are formed when a layer of hot air forms close to the ground This hot air is less dense than the cold air above it - and therefore has a lower index of refraction (e.g. 1.0002 versus 1.0003)

Light refracts gradually as it enters the hot, less dense layer, continually bending away from the normal

This gives rise to mirages (e.g. water on hot road or in desert)

• Concept Question

Why does the road appear wet in the mirage?

A. Road is wet B. Road surface has melted in the heat C. Rays from sky refract and make image of sky D. Rays from sky reflect and refract from wet road

• Mirages can lead to reflections and double images

• Atmospheric Refraction

Decreasing density of the atmosphere with height causes refraction of light from the Sun and the Moon, delaying the sunset and flattening the heavenly disks. The effect is similar to that of a mirage but with the less dense air now above.

• Mirages can lead to reflections and double images

Fata Morgana in Greenland

• COMMON MIRAGES

http://www.unmuseum.mus.pa.us/mirage.htm

Peary could clearly see the mountain tops of "Crocker Land" across the polar ice pack, but it was only an Arctic Mirage. (Copyright Lee Krystek, 1998)

• MORE MIRAGES

http://www.unmuseum.mus.pa.us/mirage.htm

On June 23, 1744, a phantom army appeared floating above a mountain in Scotland. Twenty seven people, who later gave sworn testimony to what they'd seen, watched the strange vision for two hours till it ended with darkness. In the summer of 1897 in Alaska, an expedition to the wilderness near Mount St. Elias saw a "Silent City" over a glacier. A member of the expedition, C. W. Thornton, wrote, "It required no effort of the imagination to liken it to a city, but was so distinct that it required, instead, faith to believe that it was not in reality a city." Another witness reported, "We could plainly see houses, well-defined streets, and trees. Here and there rose tall spires over huge buildings..."

The above incidents are examples of a startling optical atmospheric effect known as a mirage. Though we associate mirages with the illusion of distant water in a desert, the phantom oasis is actually just the simplest example of this bizarre effect.

• Antarctic Fata Morgana

http://climate.envsci.rutgers.edu/Antarctica/views.html