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Jack Brooks
Jack Brooks

Why Do Fine Mists Of Water Create Rainbows !!INSTALL!!


On a sunny day, students can go outside and use sunlight through a prism to project a color spectrum on a piece of white paper. Or, cut a slit 1 cm wide and 15 cm long in a piece of black construction paper. Place the paper over the glass of an overhead projector so that just a slit of light is projected in a darkened room. Have students position a prism close to the projector, within the light beam, and experiment to find the most optimal placement for projecting a color spectrum. Another alternative is to have students spray a fine mist of water in bright sunlight to produce a rainbow, which is a particularly useful rainbow demonstration since, as with natural rainbows, water droplets are used. This can also be done in the classroom using an overhead projector projecting a slit of light. Spray a fine mist in front of the projector across the light beam. A color spectrum can be seen within the mist.




Why do fine mists of water create rainbows



Reading/Writing - Choose poems (there are 70) from Rainbows are Made by Carl Sandburg (ISBN 015265481X) that relate to natural phenomena. Have students react to the poems and then write their own about rainbows, rain, or water.


Nice question! Rainbows are a product of sunlight passing through small droplets of water suspended in the atmosphere (or even falling through it!). Not only are they beautiful, but they are usually rare because you need a rainy day and a sunny day at the same time to make one appear -- the sun has to be shining from one part of the sky, and the rain in another part of the sky before a rainbow can appear.The sunlight takes a complicated path through each water droplet. It comes in the side closest to the sun, bends because the index of refraction in water is bigger than that of air (you can see this effect by putting a pencil in a glass of water so that some of it sticks out and looking at it from different angles -- the pencil will apppear "broken" at the place it crosses the water surface). The sunlight, passing through the water droplet, bounces off the back surface of the droplet, travels back to the other side, and bends once again on its way out.The reason why the rainbow is curved is because all the angles in the water drop have to be just right for the drop to send some sunlight to you, standing on the ground. So, with the sun *behind* you, only those water droplets that have the same angle formed by you, the drop, and the sun (this angle happens to be approximately 42 degrees) will contribute to the rainbow. Other droplets send their light somewhere else, and if you move to a different location, new droplets are needed to make the rainbow you see in the new location. This is why you can't go to the end of a rainbow to find the mythical leprechauns and pots of gold; anywhere you stand, the rainbow is formed by faraway drops of water reflecting and bending sunlight. The rainbow is curved because the set of all the raindrops that have the right angle between you, the drop, and the sun lie on a cone pointing at the sun with you at one tip. The rainbow may look semicircular if the sun is setting or rising (a good time to see a rainbow because the sunlight at that time can get under rain clouds because it is traveling horizontally). If the sun is higher in the sky, the earth gets in the way and you may see less than a semicircular rainbow.The rainbow is colored because the water drops act like little prisms -- how much the light bends when it enters and exits the drop depends on its color, and light from the sun contains contributions from light of all colors. So that 42 degrees above is a bit different for red light and different still for blue light -- you have to look in a different place to see the red rainbow arc and the blue rainbow arc, so you see them as arcs in the sky of different sizes and the rainbow is striped with colors.You can make your own rainbows with a lawn sprinkler or even a water spray-bottle that can make a fine mist. On a bright, sunny day with the sun at your back, spray some water in front of you in different directions to see where the best rainbows can be seen. Can you make one go all the way around in a circle? Rainbows crop up in the nicest places -- you can see them sometimes at the bottoms of waterfalls and even briefly in the splashes of divers at the swimming pool.


With the help of a harden hose, you can make your own rainbow. During the early morning or late afternoon, put the sun at your back and spray a fine mist of water into the air in front of you. A circular rainbow should be reflected by the water droplets. Increase the size of the water droplets and notice whether the width of the bands of color changes. Mist from a plunging waterfall will produce a similar rainbow on a sunny day.


Rainbows can be observed whenever there are water drops in the air and sunlight shining from behind the observer at a low altitude angle. Because of this, rainbows are usually seen in the western sky during the morning and in the eastern sky during the early evening. The most spectacular rainbow displays happen when half 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. During such good visibility conditions, the larger but fainter secondary rainbow is often visible. It appears about 10 outside of the primary rainbow, with inverse order of colours.


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.[4]


The light at the back of the raindrop does not undergo total internal reflection, and some light does emerge from the back. However, light coming out the back of the raindrop does not create a rainbow between the observer and the Sun because spectra emitted from the back of the raindrop do not have a maximum of intensity, as the other visible rainbows do, and thus the colours blend together rather than forming a rainbow.[21]


Unlike a double rainbow that consists of two separate and concentric rainbow arcs, the very rare twinned rainbow appears as two rainbow arcs that split from a single base.[29] The colours in the second bow, rather than reversing as in a secondary rainbow, appear in the same order as the primary rainbow. A "normal" secondary rainbow may be present as well. Twinned rainbows can look similar to, but should not be confused with supernumerary bands. The two phenomena may be told apart by their difference in colour profile: supernumerary bands consist of subdued pastel hues (mainly pink, purple and green), while the twinned rainbow shows the same spectrum as a regular rainbow.The cause of a twinned rainbow is believed to be the combination of different sizes of water drops falling from the sky. Due to air resistance, raindrops flatten as they fall, and flattening is more prominent in larger water drops. When two rain showers with different-sized raindrops combine, they each produce slightly different rainbows which may combine and form a twinned rainbow.[30]A numerical ray tracing study showed that a twinned rainbow on a photo could be explained by a mixture of 0.40 and 0.45 mm droplets. That small difference in droplet size resulted in a small difference in flattening of the droplet shape, and a large difference in flattening of the rainbow top.[31]


A circular rainbow should not be confused with the glory, which is much smaller in diameter and is created by different optical processes. In the right circumstances, a glory and a (circular) rainbow or fog bow can occur together. Another atmospheric phenomenon that may be mistaken for a "circular rainbow" is the 22 halo, which is caused by ice crystals rather than liquid water droplets, and is located around the Sun (or Moon), not opposite it.


In certain circumstances, one or several narrow, faintly coloured bands can be seen bordering the violet edge of a rainbow; i.e., inside the primary bow or, much more rarely, outside the secondary. These extra bands are called supernumerary rainbows or supernumerary bands; together with the rainbow itself the phenomenon is also known as a stacker rainbow. The supernumerary bows are slightly detached from the main bow, become successively fainter along with their distance from it, and have pastel colours (consisting mainly of pink, purple and green hues) rather than the usual spectrum pattern.[37] The effect becomes apparent when water droplets are involved that have a diameter of about 1 mm or less; the smaller the droplets are, the broader the supernumerary bands become, and the less saturated their colours.[38] Due to their origin in small droplets, supernumerary bands tend to be particularly prominent in fogbows.[39]


A sleetbow forms in the same way as a typical rainbow, with the exception that it occurs when light passes through falling sleet (ice pellets) instead of liquid water. As light passes through the sleet, the light is refracted causing the rare phenomena. These have been documented across United States with the earliest publicly documented and photographed sleetbow being seen in Richmond, Virginia on December 21, 2012.[59] Just like regular rainbows, these can also come in various forms, with a monochrome sleetbow being documented on January 7, 2016 in Valparaiso, Indiana.[citation needed]


The circumzenithal and circumhorizontal arcs are two related optical phenomena similar in appearance to a rainbow, but unlike the latter, their origin lies in light refraction through hexagonal ice crystals rather than liquid water droplets. This means that they are not rainbows, but members of the large family of halos.


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