isns 3371 - phenomena of nature the electromagnetic spectrum most wavelengths of light can not be...

ISNS 3371 - Phenomena of Nature

The Electromagnetic Spectrum

Most wavelengths of light can not be seen by the human eye.

The visible part of the electromagnetic spectrum lies between ultraviolet and infrared light (between about 400 and 700 nm). The higher the frequency (shorter the wavelength), the higher the photon energy. Radio waves are at the long wavelength end of the spectrum and gamma rays are at the short wavelength end of the spectrum.


On the moon, there is no atmosphere to scatter light and the sky is black Atmosphere on Mars too thin to scatter light effectively - sky is reddish from presence in the atmosphere of reddish dust from surface. On Venus, almost all blue light scattered away - atmosphere dimly lit and appears reddish orange.

Moonrise Earthrise

ISNS 3371 - Phenomena of NatureColor

Additive primary colors - adding light of additive primary colors produces complementary colors - all colors produces white

Red Green Blue

Red + Green Yellow

Red + Blue Magenta

Green + Blue Cyan

Red

Blue Green

Magenta

CyanWhite

Yellow

ISNS 3371 - Phenomena of NatureColor

Subtractive primary colors - mixing pigments (that absorb light) of various subtractive primary colors produce complementary colors - added all together, produce black.

Yellow Magenta Cyan

Yellow + Magenta Red

Yellow + Cyan Green

Magenta + Cyan Blue


Only four colors are used to print color illustrations and photos: (a) magenta, (b) yellow, (c) cyan, (e) black. (d) is with all but black, (f) is with all.


Four Ways in Which Light can Interact with Matter

1. emission – matter releases energy as light

2. absorption – matter takes energy from light

3. transmission – matter allows light to pass through it

4. reflection – matter repels light in another direction

ISNS 3371 - Phenomena of NatureISNS 4371 Phenomena of Nature

Properties of Light

Law of Reflection - Angle of Incidence = Angle of reflection

Law of Refraction - Light beam is bent towards the normal when passing into a medium of higher Index of Refraction.

Light beam is bent away from the normal when passing into a medium of lower Index of Refraction.

Index of Refraction -

Inverse square law - Light intensity diminishes with square of distance from source.

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n=Speedof lightin vacuumSpeedof lightin amedium

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Mirror reflects light at angle equal to incoming angle -

Most materials reflect light randomly - scattering. Movie screen scatters light into array of beams that reaches every member of the audience


Index of Refraction

As light passes from one medium (e.g., air) to another (e.g., glass, water, plexiglass, etc…), the speed of light changes. This causes to light to be “bent” or refracted. The amount of refraction is dependent on the index of refraction of the material and the angle of incidence.


Imagine that the axles of a car represent wave fronts. If the car crosses from a smooth to a rough surface at an angle, one tire of the axle will slow down first while the other continues at normal speed. With one tire traveling faster the other, the car will turn in the direction of the slow tire. This is how refraction works.

Refraction


The Prism

A prism separates white light into its component colors - because index of refraction is dependent on wavelength. Blue light bends (is refracted) more than red light

Dispersion - “spreading” of a light beam - dependent on:

- the angle between the two surfaces- the direction of incidence on the first face- the index of refraction of the prism.

Color Violet Blue Green Yellow Orange Red

Index (glass) 1.532 1.528 1.519 1.517 1.514 1.513

ISNS 3371 - Phenomena of NatureRainbows

Water is more dense than air - light is refracted as it enters and leaves a raindrop drop - red is bent less, blue more - the light is dispersed, just like a prism

- some of the light will reflect off the back of the drop if the angle is larger than the critical angle for reflection (48° for water)- the light is then refracted again as it leaves the drop, the colors of white light have been dispersed.- blue light will leave the drop at an angle of 40° from the beam of sunlight- red light will leave the drop at an angle of 42° from the beam of sunlight

This process generates the primary rainbow. Naturally the sun must be behind you and the rain in front of you.


However, you can not see the blue light and red light refracted from the same drop - you only see one color (indeed, only one tiny beam of light!) from each raindrop. So, many drops are involved in producing the rainbow.

The raindrops lower down contribute blue and green light and the raindrops higher up contribute red and yellow light.

Rainbows


antisolar point - the point where the sun is directly opposite of us.

- If we look away from the sun, the shadow of our head on the ground marks the antisolar point

- If the sun is in the sky, the antisolar point is below the horizon. If the sun has set, the antisolar point is above the horizon.

Tells us is the direction of the sun and where we can expect the rainbow to form.

Rainbows


red light gets refracted back at 42° from antisolar line - blue light gets refracted back at about 40° from antisolar line. Every raindrop within 42° of the antisolar line is reflecting red light back to your eyes.

Look at every raindrop within about 40° of the antisolar line - see a circle of raindrops centered on the line - horizon gets in the way of most of the rain, so we only get to see an arc or a bow - full rainbows can be seen from airplanes and mountain-tops.

Rain is falling not in a flat sheet, but in varying distances from you - causes the "rainbow circle" to be formed at varying distances - raindrops that contribute to your rainbow all lie on a cone with its apex at your eye.

Move to the left or the right - looking at new raindrops - a new rainbow.

Two people see different rainbows - - each rainbow is your own.


In science speak - The rainbow is an optical phenomenon caused by the refraction and reflection of light by the locus of raindrops between 40.6 and 42° of the observer's antisolar line.

Rainbows

ISNS 3371 - Phenomena of Nature PolarizationLight emitted by the sun, a lamp in the classroom, a candle flame, etc… is unpolarized light - created by electric charges which vibrate in a variety of directions - an electromagnetic wave (transverse) which vibrates in a variety of directions.

Helpful to picture unpolarized light as a wave which has an average of half its vibrations in a horizontal plane and half of its vibrations in a vertical plane.

Polarized light waves - light waves in which the vibrations occur in a single plane. Polarization - Process of transforming unpolarized light into polarized light.

Most common method of polarization uses a Polaroid filter - made of a special material capable of blocking one of the two planes of vibration of an electromagnetic wave. When unpolarized light is transmitted through a Polaroid filter, it emerges with one-half the intensity and with vibrations in a single plane; it emerges as polarized light.


Two filters with polarization axes perpendicular to each other will completely block the light.Light is polarized upon passage through the first filter - say, only vertical vibrations were able to pass through. These vertical vibrations are then blocked by the second filter since if its polarization filter is aligned in a horizontal direction. Like picket-fence and standing wave on a rope - vibrates in a single plane. Spaces between the pickets of the fence allow vibrations parallel to the spacings to pass through while blocking vibrations perpendicular to the spacings.

Orient two picket fences such that the pickets are both aligned vertically - vertical vibrations will pass through both fences - align pickets of second fence horizontally - the vertical vibrations which pass through the first fence will be blocked by the second fence.


Polarization by Reflection

Unpolarized light can also undergo polarization by reflection off of nonmetallic surfaces - extent dependent upon the angle at which the light approaches the surface and upon the surface material.

Metallic surfaces reflect light with variety of vibrational directions - unpolarized.

Nonmetallic surfaces (asphalt, snow, water, paint on a car) reflect light such that there is a large concentration of vibrations in a plane parallel to the reflecting surface. A person viewing objects by means of light reflected off of nonmetallic surfaces will often perceive a glare if the extent of polarization is large.

Which pair of glasses is best suited for automobile drivers, fishermen, snow skiers?


Adding a third filter with between two filters polarization axis at 45º to the other two will allow light though. How?

Remember, unpolarized light vibrates in all different directions. So not just the light with horizontal vibrations passes through the first filter, but all light with a vibrational component in the horizontal direction - in other words, all but the light with vertical vibrations has some component in the horizontal direction that gets through.


Before the middle filter, the light is horizontally polarized.

The component of horizontally polarized light along 45º gets through the middle filter.

The component of that light in the vertical direction then gets though the last filter.


When light passes through a transparent material such as plastic, internal (and normally invisible) stresses in the material can rotate the angle of polarization - different colors will be rotated by different amounts. Place horizontal polarizer below the object, and a vertical one above it - no light will be transmitted unless there are stresses inside the object that rotate the light.

Useful in the design of engineering structures - build a model out of plastic, and view it with crossed polarizers. Put a force on the model - regions of the model that are stressed the most will show up in color. This way you can determine which parts of the structure are most likely to break, and the design can be changed (if necessary) to relieve some of that stress.


Lenses and Mirrors(and applications to astronomy)


Center of curvature - the center of the circle of which the mirror represents a small arcPrincipal axis - a radius drawn to the mirror surface from the center of curvature of the mirror - normal to mirror surfaceFocus - the point where light rays parallel to principal axis converge; the focus is always found on the inner part of the "circle" of which the mirror is a small arc; the focus of a mirror is one-half the radiusVertex - the point where the mirror crosses the principal axisFocal length - the distance from the focus to the vertex of the mirror

Geometry of a Concave Mirror

Focus

Principal axisVertex

Focal length


Optical axis - axis normal to both sides of lens - light is not refracted along the optical axisFocus - the point where light rays parallel to optical axis converge; the focus is always found on the opposite side of the lens from the objectFocal length - the distance from the focus to the centerline of the lens

Geometry of a Converging (Convex) Lens

Optical axisFocus

Focal length


Fo

cal

Plan

e

L1 L2

D1 D2

L1 D1 _ _L2 D2

=

Image Magnification Using a Simple Lens


The image formed by a single lens is inverted.

ISNS 3371 - Phenomena of NatureThe Eye

The eye consists of pupil that allows light into the eye - it controls the amount of light allowed in through the lens - acts like a simple glass lens which focuses the light on the retina - which consists of light sensitive cells that send signals to the brain via the optic nerve. An eye with perfect vision has its focus on the retina when the muscles controlling the shape of the lens are completely relaxed - when viewing an object far away - essentially at infinity.


When viewing an object not at infinity, the eye muscles contract and change the shape of the lens so that the focal plane is at the retina (in an eye with perfect vision). The image is inverted as with a single lens - the brain interprets the image and rights it.

isns 3371 - phenomena of nature the electromagnetic spectrum most wavelengths of light can not be...

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