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Easy Java Simulations (2001- ) => optics => Topic started by: ahmedelshfie on May 12, 2010, 01:25:02 am

Title: Physics of rainbow (EJS version)
Post by: ahmedelshfie on May 12, 2010, 01:25:02 am
This follawing applet is Physics of rainbow
Created by prof Hwang modified by Ahmed
Original project Physics of rainbow (EJS version) (http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=1430.0)
Is excllent simulation

Title: Re: Physics of rainbow (EJS version)
Post by: ahmedelshfie on May 12, 2010, 04:48:46 am
A rainbow is an optical and meteorological  phenomenon that causes a spectrum of light to appear in the sky when the Sun shines onto droplets of moisture in the Earth's atmosphere. They take the form of a multicoloured  arc, with red on the outer part of the arch and violet on the inner section of the arch.

A rainbow spans a continuous spectrum of colours; the distinct bands are an artifact of human colour vision. The most commonly cited and remembered sequence, in English, is Newton's sevenfold red, orange, yellow, green, blue, indigo and violet (popularly memorized by mnemonics like Roy G. Biv). Rainbows can be caused by other forms of water than rain, including mist, spray, and dew.

Title: Re: Physics of rainbow (EJS version)
Post by: ahmedelshfie on May 12, 2010, 04:52:11 am
Rainbows can be observed whenever there are water drops in the air and sunlight  shining from behind at a low altitude  angle. The most spectacular rainbow displays happen when half of the sky is still dark with raining clouds and the observer is at a spot with clear sky in the direction of the Sun. The result is a luminous rainbow that contrasts with the darkened background.

The rainbow effect is also commonly seen near waterfalls or fountains. In addition, the effect can be artificially created by dispersing water droplets into the air during a sunny day. Rarely, a moonbow, lunar rainbow or nighttime rainbow, can be seen on strongly moonlit nights. As human visual perception for colour is poor in low light, moonbows are often perceived to be white.[1] It is difficult to photograph the complete semicircle of a rainbow in one frame, as this would require an angle of view of 84°. For a 35 mm camera, a lens with a focal length of 19 mm or less wide-angle lens would be required. Now that powerful software for stitching several images into a panorama is available, images of the entire arc and even secondary arcs can be created fairly easily from a series of overlapping frames. From an aeroplane, one has the opportunity to see the whole circle of the rainbow, with the plane's shadow in the centre. This phenomenon can be confused with the glory, but a glory is usually much smaller, covering only 5°–20°.

At good visibility conditions (for example, a dark cloud behind the rainbow), the second arc can be seen, with inverse order of colours. At the background of the blue sky, the second arc is barely visible.
Data and images from  http://en.wikipedia.org/wiki/Rainbow

Title: Re: Physics of rainbow (EJS version)
Post by: ahmedelshfie on May 12, 2010, 05:06:02 am
The Physics: How a Rainbow Forms By Tom Field
Rainbow physics answers the question, "How Does A Rainbow Work?" Understanding how rainbows form can help in your quest to find and photograph them, though it's not essential. I'll attempt to explain the physics of how a rainbow is formed without getting too dry and technical. Still, those allergic to physics can skip this section. And those well-versed in physics, please pardon my simplification. As always, corrections and clarifications welcome!
How A Rainbow Forms ?

How A Rainbow Works: Refraction and Reflection

Two physical phenomena are at work within a rainbow: refraction and reflection. Refraction occurs each time light passes across a boundary from one substance to another, such as from air into water. As light crosses that boundary, the rays bend at different angles depending on the wavelength (color) of light. This is the familiar prism effect wherein "white" sunlight is broken into a spectrum of different colors from red to blue-violet.

The same thing that happens in a rainbow: white sunlight enters a raindrop and is broken into different colors heading in slightly different directions. The light is then reflected (and magnified) off the back of the raindrop and passes back into the air again, in the process being further refracted.

Let's pick a single raindrop in the BLUE band of the arc. The blue light is but one part of the spectrum of colors - each shining out from the raindrop at a different angle. Blue is shining our direction, but the other colors shoot out in different directions and therefore we can't see them from where we stand.

Now look at an adjacent raindrop: it's also shining blue light at us. In fact, all of the nearby raindrops appear blue from where we're standing.

But if we look at a single raindrop in the RED band of the arc, only the red light is shining our direction. In between blue and red, we find "all the colors of the rainbow" refracted and reflected from countless raindrops in just such a way that they shine our direction. Beyond the edge of the arc, where we see no color, the raindrops may be emitting colored light but none shines in our direction.
Click to enlarge image
Double Rainbow    
Under certain conditions, some of the light will bounce off the inside of the water droplet more than once, exiting at a different angle. This produces a weaker, secondary arc known as a double rainbow. Bob Peavy has captured this phenomenon in the images above and left. Note the colors are reversed in the secondary rainbow. In theory there are additional rainbows (third, fourth) but those are too faint to see.
Data and images from  http://www.photocentric.net/rainbow_physics.htm
I think is good post information's about Physics of rainbow is really wonderful simulation Prof Hwang  :)

Title: Re: Physics of rainbow (EJS version)
Post by: ahmedelshfie on October 01, 2010, 12:13:12 am
This applet is Physics of rainbow design by prof Hwang
URL applet Physics of rainbow (http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=44.0)

A most charming example of chromatic dispersion is a rainbow.
When white sunlight is intercepted by a drop of water in the atmosphere, some of the light refracts into the drop, reflects from the drop's inner surface, and then refracts out of the drop.
As with the prism, the first refraction separates the sunlight into its component colors, and the second refraction increases the separation. The result is the rainbow.
This applet shows the physics of the rainbow.

 The black circle represents a drop of water in the atmosphere.
Initially, red light is coming from the left; you can click inside the colored blocks to change the color of the incoming light.
 The incoming ray is unpolarized, which can be represented as a mixture of two polarized waves whose planes of polarization are perpendicular to each other.
So the notation " 50%| 50%+" means that half is polarized in the up-down direction and half is perpendicular to the screen.
 Many things can happen to the light.
Part of the incoming ray is reflected back to the atmosphere (indicated by ray number 1). The intensity of each polarized component is shown along the ray path.

Part of the light refracts into the drop, then refracts back to the atmosphere(ray number 2).

Some reflects from the drop's inner surface, and refracts back to the atmosphere (ray number 3). This gives rise to the ordinary rainbow.

Some reflects twice inside the drop, then refracts back to the atmosphere.(ray number 4) This gives rise to the secondary rainbow that is sometimes seen.

You can drag the incoming ray, move it up and down, and watch how the relative intensities change. R is the radius of the water drop; b is the vertical distance of the incoming ray from the center of the circle.
 The intensity of the light coming from rays 3 and 4 is plotted versus viewing angle. Click ^or vto change the scale.
For ray number 3 there is a maximum scattering angle, and for ray number 4 there is a minimum -- this is why there are strong peaks in the scattered intensity.
The rainbow is actually a disk of scattered light, but it is brightest at the edge; the disk for different wavelengths is a different size, and that is why we see the color effects there.
 You can click inside the white box, and see what will happen to white light.
 When light refracts, it follows the law of refraction ni sin(ctai)=nr sin( ctar) where n is the index of refraction.
The number in the left-bottom corneri is the angle of incidence, r is the angle of refraction.
 Most of the light is refracted out by ray
2. When your eyes intercept the separated colors from raindrops, the red you see comes from drops angled slightly higher in the sky than does the blue.
You see a circular arc of color, with red on the outside and blue on the inside.
Click inside the colored box to show this effect. You can drop one of the rain drops,Try it!