What factors determine the diameter of a rainbow?
961441
Rainbows are always the same size.  The diameter of a rainbow is best expressed in degrees as it is not at a fixed distance away.  Rainbows can be thought of as occupying the surface of a thin cone with its pointy end at your eye, and the angle of the cone is fixed.  This angle depends only on the refractive index of water.\n\nYes - you can get one bigger than a semi-circle, but only if you get well above the earth's surface (perhaps in an aeroplane) so that the ground doesn't get in the way!
655753
I'll try to explain here, but it would be a lot easier if I could draw you a picture.  Here goes... \n\nThe shape of the rainbow actually has to do merely with your own perspective.  Each droplet of water acts like a prism, deflecting different colors of light at slightly different angles.  All the rays of sunlight are more or less parallel when they enter air full of water droplets.  So each color is, on average, deflected a certain number of degrees, varying by wavelength.  But you can only see the colors if they were deflected towards you.  So if, say, green light is deflected by 10 degrees on average, then most of the green light that you see is coming from areas where your line of sight is 10 degrees off from the direction of the sun's rays.  Geometrically speaking, the source of the light has a cone shape, but from your perspective (at the "point" of the cone) it looks like a circle.  If you have ever seen a rainbow while in an airplane or up on a mountian, you will notice that it is actually a full circle.  Usually, we can only see the top half, because the ground absorbs the light instead of deflecting it, and because the "cone" of light is far away (thus its radius is very large, and the bottom part goes "into the ground").  However, you can sometimes see circular rainbows in the mist from a waterfall or sprinkler - anytime you are high up or the water source is very close.\n\nAargh, this would be so much easier to explain with a pencil and paper... Oh well, hope it was useful anyway.
143433
Prismatic refraction through water vapors in the atmosphere.\n\nAs light shines through the atmosphere, certain things like temperature, barometric pressure, and humidity (the amount of water vapor in the air) come together to produce the right conditions for prismatic refraction.  This is the same process by which rainbows are produced, but can also produce halos around the sun or moon.
380553
When we draw a circle with a compass, we keep the angle between the two legs constant.\n\nA ray from the top most point on the rainbow to our eye makes an angle to the horizontal direction. Theory says this angle is a constant.\n\nImagine the ray as one leg of long compass and the horizontal direction as another leg which points the center. Keeping the angle constant when the compass is rotated an arc is formed. \n\nIf we go high up we can see the whole circle.
374573
The diameter is unimportant, it is the length that matters.\n\nA frequency is related to a wavelength by:\nThe speed of light = wavelength x frequency.\n\nThis wavelength is the length of the aerial (Simplisticly) there are a number of modifiers that affect the length too, but this is a basic start
125458
Well I think they appear as lines; fat, multicolored, curved lines.  But they aren't really "there" where we see them.  That is, other people, say a mile away from you, will see the rainbow shifted from where you see it, and that's because the rainbow's apparent direction is relative to the sun's rays that are near you.  But is this the answer to your question?  Are you asking why they're not thin? straight?  Give us something, Bea.
543022
In order to measure the diameter of a star you really need three pieces of information about the star (1) distance (2) brightness (3) and color.\nStep 1: Scientists can figure out the total power output of the star by knowing its brightness at Earth and its distance. \n\nStep 2: Figure out the surface temperature. The universe really gave scientists a lucky break because stars are fairly predictable objects. What I mean is that, for most stars once a scientist know one property of the star they can generally figure out everything else they want to know about it. This is because stars behave like "black bodies". This is a physics term which describes how an object of certain temperature glows at different wavelength (thus influencing its color). Most ordinary matter resembles a black body (has nothing to do with being black, it's just terminology). You probably already have an intuitive understanding of this from your everyday experience. For example, when you heat up an object, at first it might start to glow red. And as it gets hotter it might start to glow yellow, blue, and eventually white. Because stars approximate black bodies very well, by knowing the color of the star scientist can figure out what its surface temperature is quite accurately. \n\nStep 3: Another great thing about black bodies is that, for any specific amount of surface area astronomers can predict exactly how much light it will radiate (hotter objects are brighter). And since they already know the temperature and the total brightness of the star they can figure out its surface area, and thus it's diameter :)
786396
hi, try this link .. its a nice one ..\n\nhttp://eo.ucar.edu/rainbows/
16143
Rainbows are formed because of the process of bending light knows as Refraction. different colors of light bend at different angles when they refract. When light from the sun enters a droplet of rain, it refracts a tiny bit causing the colors of light to spread apart, the light then bounces, or reflects, off of the back of the droppled and refracts once again as it leave the dropplet on the same side it entered. By the time the light reaches your eyes the refraction is so noticeable that you see all the colors of sunlight spread apart - creating a Rainbow
73801
When white light (all colors) hits a rain drop, each color refracts at a different angle.  The color is governed by what frequency it is.  Red has the lowest frequency and violet has the highest.  This is what causes the specific pattern of the rainbow.  The angle is with respect to your eye.  Hope this helped.
427036
Rainbows are caused by sunlight reflecting off and refracting through raindrops. There is a very specific angle that each different wavelength of light will take so all parts of the rainbow of that color are always seen at exactly that angle away from the Sun, about 42 degrees for visible light. In math, the collection of all points at a certain distance from a point is a circle, so rainbows are seen as a circle around the sun.
52478
When light is refracted off of droplets of water it enables you to see all the colors that make up white light (sunlight, even light from bulbs.) creating what people more commonly refer to as a rainbow. \n  Go outside on a sunny day and spray the waterhose if you have one. If you look very carefully, you can see that the rainbow you created is not just a semi-circle, but a full circle! The bottom half is harder to see, but it is there. The reason rainbows don't look like that when we see them outside is because we are too low to the ground to see the rest.
655891
It doesn't matter if the rainbow appears around the sun or on the ground after a rainshower. The two rainbows are called primary and secondary bows. There are two because the sunlight is relected off the inside of a water droplet twice instead of once; once would result in only one rainbow. This all depends on the angle in which the sunlight enters the water droplet.\nThe secondary bow is fainter and  the colors are reversed.
507580
A rainbow is an optical and meteorological phenomenon that causes a nearly continuous spectrum of light to appear in the sky when the Sun shines onto droplets of moisture in the Earth's atmosphere. It takes the form of a multicoloured arc, with red on the outside and violet on the inside. Even though a rainbow spans a continuous spectrum of colours, traditionally the full sequence of colours is most commonly cited as red, orange, yellow, green, blue, indigo, and violet. It is commonly thought that indigo was included due to the different religious connotations of the numbers six and seven at the time of Isaac Newton's work on light, despite its lack of scientific significance and the poor ability of humans to distinguish colours in the blue portion of the visual spectrum.\n\n\nThe rainbow effect can be observed whenever there are water drops in the air and sunlight shining from behind the observer at a low altitude or angle. The most spectacular rainbow displays when half of the sky is still dark with draining clouds and the observer is at a spot with clear sky overhead. The rainbow effect is also commonly seen near waterfalls or fountains. Rainbow fringes can sometimes be seen at the edges of backlit clouds and as vertical bands in distant rain or virga. The effect can also be artificially created by dispersing water droplets into the air during a sunny day.\n\nIn a very few cases, a moonbow, or night-time rainbow, can be seen on strongly moonlit nights. As human visual perception for colour in low light is poor, moonbows are perceived to be white.\n\nThe rainbow's appearance is caused by dispersion of sunlight as it is refracted by (approximately spherical) raindrops. The light is first refracted as it enters the surface of the raindrop, reflected off the back of the drop, and again refracted as it leaves the drop. The overall effect is that the incoming light is reflected back over a wide range of angles, with the most intense light at an angle of about 40°–42°, regardless of the size of the drop. Since the water of the raindrops is dispersive, the amount that the sunlight is bent depends upon the wavelength, and hence colour, of the light's constituent parts. Blue light is refracted at a greater angle than red light, but because the area of the back of the droplet has a focal point inside the droplet, the spectrum crosses itself, and therefore the red light appears higher in the sky, and forms the outer colour of the rainbow. Contrary to popular belief, the light at the back of the raindrop does not undergo total internal reflection; however, light that emerges from the back of the raindrop does not create a rainbow between the observer and the Sun. The 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 and do not form a rainbow.\n\nA rainbow does not actually exist at a location in the sky, but rather is an optical phenomenon whose apparent position depends on the observer's location. All raindrops refract and reflect the sunlight in the same way, but only the light from some raindrops reaches the observer's eye. These raindrops are perceived to constitute the rainbow by that observer. The position of a rainbow in the sky is always in the opposite direction of the Sun with respect to the observer, and the interior is always slightly brighter than the exterior. The bow is centred on the shadow of the observer's head, or more exactly at the antisolar point (which is below the horizon during the daytime), appearing at an angle of approximately 40°–42° to the line between the observer's head and its shadow. As a result, if the Sun is higher than 42°, then the rainbow is below the horizon and cannot be seen unless the observer is at the top of a mountain or a similar vantage point. Similarly, it is difficult to photograph the complete arc of a rainbow, which would require an angle of vie
445187
A rainbow is a prismatic effect, a kind of optical illusion, because of water droplets in the air and the way the sun shines through them.
985228
You can see a rainbow when the light is behind you.  It's reflected by the raindrops.  Light of a particular colour is refected by the raindrops that are at the correct angle between the straight line from the sun to the raindrop and the straight line from the raindrop to your eye.  The raindrops at that angle are in a circle, although of course there aren't any under the ground so you only see an arc.  If you're in a plane and the sun and a raincloud are in the right places, sometimes it's possible to see the whole circle.
436348
A rainbow is seen when the Sun is behind you and water is in the air in front of you. You can make one with a water hose or maybe see one in the air with rain in the area. Natural rainbows appear very large and several miles away. The top is all you can see because the ground in front of you blocks the lower portion of the rainbow. However, when seen from an airplane, rainbows are complete circles.