Refraction & Lenses

Refraction• The change in direction of a wave as is

crosses the boundary between two media in which the wave travels at different speeds.

Index of Refraction

materialinlightofspeed

vacuuminlightofspeedn

Snell’s Law

2211 sinsin nn

Index of RefractionMaterial Index of RefractionVacuum 1.0000

Air 1.0003Ice 1.3100

Water 1.3330Ethyl Alcohol 1.3600

Plexiglas 1.5100Crown Glass 1.5200

Light Flint Glass 1.5800Dense Flint Glass 1.6600

Zircon 1.9230Diamond 2.4170

Rutile 2.9070Gallium phosphide 3.5000

Lenses

Converging Lens

Rules For Converging Lenses

1) Any incident ray traveling parallel to the principal axis of a converging lens will refract through the lens and travel through the focal point on the opposite side of the lens.

2) Any incident ray traveling through the focal point on the way to the lens will refract through the lens and travel parallel to the principal axis.

3) An incident ray which passes through the center of the lens will in effect continue in the same direction that it had when it entered the lens.

Image Formation by Converging Lens

Diverging Lens

Rules For Diverging Lenses1) Any incident ray traveling parallel to the

principal axis of a diverging lens will refract through the lens and travel in line with the focal point (i.e., in a direction such that its extension will pass through the focal point).

2) Any incident ray traveling towards the focal point on the way to the lens will refract through the lens and travel parallel to the principal axis.

3) An incident ray which passes through the center of the lens will in effect continue in the same direction that it had when it entered the lens.

Diverging Lens Image Formation

Always Virtual, Smaller, and Right-Side Up

Optical phenomena

• Dispersion

• Total Internal Reflection (TIR)

• Critical angle and periscopes

• Fiber optics

• Chromatic abberation

Dispersion

• The separation of light into colors arranged according to their frequency, by interaction with a prism or diffraction grating.

Rainbows

• Perhaps the most spectacular and best known atmospheric optical phenomenon is the rainbow.

• Sunlight and water droplets are necessary for the formation of a rainbow.

• Furthermore, the observer must be between the Sun and rain.

• When a rainbow forms, the water droplets act as prisms and refraction disperses the sunlight into the spectrum of colors, a process called dispersion.

• The curved shape of the rainbow results because the rainbow rays always travel toward the observer at an angle between 40 and 42° from the path of the sunlight.

Rainbows• White light separates into different colors

(wavelengths) on entering the raindrop because red light is refracted by a lesser angle than blue light. On leaving the raindrop, the red rays have turned through a smaller angle than the blue rays, producing a rainbow.

Why is a Rainbow Curved?Why is a Rainbow Curved?

Total internal reflection (TIR)

Fiber optics

Chromatic aberration

Occurs for converging lenses and arises because the index of refraction of the lens material varies with wavelength. --colors pass through lens and refract at different angles--this is related to dispersion--a blurred image like below is produced

Aurora borealis• Optical phenomena in the

atmosphere is varied and can be awe inspiring.

• Can range from auroras to halos, sun pillars to green flashes.

• Aurora borealis is caused by the collision of energetic charged particles with atoms in the high altitude atmosphere

Auroras - interaction of the Sun’s particles (solar wind) and the magnetosphere of Earth.

Green Flash(not the comic-book character)

• Seen on upper rim of rising or setting sun (more common). There is so much atmosphere to penetrate when sun is on horizon, sunlight is refracted. Purple, blue bend the most, red the least. Blue light should appear at top of sun, but blue light is scattered selectively, so green light is seen.