The sun or other source of light is usually behind the person seeing the rainbow. In fact, the center of a primary rainbow is the antisolar point , the imaginary point exactly opposite the sun. Rainbows are the result of the refraction and reflection of light. Both refraction and reflection are phenomena that involve a change in a wave 's direction. A refracted wave may appear "bent", while a reflected wave might seem to "bounce back" from a surface or other wavefront.
Light entering a water droplet is refracted. It is then reflected by the back of the droplet. As this reflected light leaves the droplet, it is refracted again, at multiple angles. The radius of a rainbow is determined by the water droplets' refractive index. A refractive index is the measure of how much a ray of light refracts bends as it passes from one medium to another—from air to water, for example.
A droplet with a high refractive index will help produce a rainbow with a smaller radius. Saltwater has a higher refractive index than freshwater, for instance, so rainbows formed by sea spray will be smaller than rainbows formed by rain.
Rainbows are actually full circles. The antisolar point is the center of the circle. Viewers in aircraft can sometimes see these circular rainbows. Viewers on the ground can only see the light reflected by raindrops above the horizon. Because each person's horizon is a little different, no one actually sees a full rainbow from the ground.
In fact, no one sees the same rainbow—each person has a different antisolar point, each person has a different horizon. Someone who appears below or near the "end" of a rainbow to one viewer will see another rainbow, extending from his or her own horizon.
A rainbow shows up as a spectrum of light: a band of familiar colors that include red, orange, yellow, green, blue, and violet. The name " Roy G. Biv " is an easy way to remember the colors of the rainbow, and the order in which they appear: red, orange, yellow, green, blue, indigo, and violet. Many scientists, however, think " indigo " is too close to blue to be truly distinguishable.
White light is how our eyes perceive all the colors of the rainbow mixed together. Sunlight appears white. When sunlight hits a rain droplet, some of the light is reflected. The electromagnetic spectrum is made of light with many different wavelength s, and each is reflected at a different angle. Thus, spectrum is separated, producing a rainbow. Red has the longest wavelength of visible light, about nanometer s. It usually appears on the outer part of a rainbow's arch.
The next thing to know is that when light enters water at an angle, it changes direction. You can see this every time you look into a pond. Ripples on the water make everything under the water look wonky. This is because the light bends as it goes between the air and the water. The amount the light bends depends on its colour. The shorter the wavelength of the colour, the more it changes direction.
So violet light changes direction more than green light. And yellow light changes direction more than red light. Refraction is why all the colours in the sunlight end up separating when it hits the water drop, and we are then able to see all the colours of the rainbow. The distance from your eye to the far edge is the same at all points and the far edge is circular.
So is your rainbow. Tim Bowra, Rozelle. Why do dogs have wet noses? The wet nose helps cool dogs off, as they don't have sweat glands and can lose moisture only through the pads of their feet. Jaya Seethamraju, Gladesville. Why are do dogs have wet noses: Why are human noses dry when cats, bears, weasels, shrews, koalas and most other animals have wet noses? Elizabeth Latimer Hill, Denistone Well, the answer I like is because when Noah's Ark developed a leak, the hole was plugged by the nose of one of the dogs on board.
That is why dogs' noses are always wet and cold! Mary Milton, Kenthurst. Guessing that sea salt comes from the sea, where does table salt come from? Table salt is scooped off the top of the water table. Adrian Cooper, Queens Park. Table salt is found as rock deposits, then it is crushed and processed. Salt lakes are another source, where it is obtained through natural evaporation.
During processing, iodine can be added, which helps to prevent cretinism, along with magnesium carbonate to make it free-running.
The locality of salt determines the colour; some is pink, for example, and is usually sold as more expensive gourmet salt. Some salts can be saltier than others and are used more sparingly, depending on taste. Helen Triggs, Katoomba. The sea. Michael Morton-Evans, Mosman The Dutch Salt industry of the 15 to 16th century developed to such a degree that it and the herring trade became the backbone of the Dutch economy.
The sea salt and salt from springs that they used were so contaminated by impurities that the salt was often highly corrosive. With the introduction of highly purified salt, it could be presented upon the table in salt cellars of great beauty, we recall Cellini's name in this regard. In brief then, a salt on the table was highly purified and non-corrosive. Steve Barrett, Glenbrook Maybe from where I live: the tablelands. Bob Dengate, Bathurst. How does insect spray kill insects?
Insects breathe through small perforations along their body, which are more easily blocked than a nose or mouth. Insecticides also generally contain additional substances, such as pyrethrins and cholesterinase inhibitors, which interfere with the insect's nervous system. Other poisons may induce catastrophic changes to the insect's chemical balance, for example, by displacing potassium or calcium from body fluids. Peter R. Green, Marrickville. Most insect sprays contain neurotoxins poisons which act on nerve cells called pyrethrins, which come from the seeds of the pyrethrum plant.
Spraying an insect with a pyrethrin will ususally cause paralysis, as the nerve cells stop working, which will immobilise and then kill the insect. Although deadly to insects and fish , pyrethrins are fairly non-toxic to mammals and birds because their bodies can transform them into harmless substances.
However, human contact with pyrethrins can trigger skin allergies or cause breathing difficulty if inhaled pyrethroids, made from chemicals rather than pyrethrum seeds, are less allergenic.
Also, industrial insecticides, such as those used in commerical pest control, are often extremely toxic to humans, so should be treated with more caution. Greg Turner, Surry Hills Insecticides work by stopping the reabsorbtion of the chemical which allows contact between neurons in the insect's brain. The neurons cannot stop firing, thus we see the uncontrollably twitching limbs of a poisoned insect. In effect their brains are burnt out.
They all break white light into distinct beams of red, orange, yellow, green, blue, indigo and purple light.
But your eyeballs will never see more than one color per drop if that. All the others exit the drop at the wrong angle to meet your pupils. Purple is the lowest color on a rainbow because purple light exits water beads at the sharpest angle: 40 degrees relative to its entry point. Meanwhile, red light — which sits at the top of a rainbow — gets sent back in your general direction at a degree angle.
A key factor here is the location of the antisolar point. This is the spot in the sky — or on the ground — that's exactly degrees away from the sun relative to your perspective. On a bright, sunny day, the head of your shadow marks the antisolar point. Every rainbow is a perfectly circular ring centered around this very spot. Yet if you're standing at ground level, you won't be able to see the circle's lower half. Indeed, from this vantage point, basically any part of a rainbow that dips below the horizon is rendered invisible.
One of the reasons for this is that the close proximity of Earth's surface limits the amount and concentration of raindrops within your line of sight. As such, the percentage of a rainbow that's visible to most people is directly correlated with the sun's position.
When our solar neighbor is just barely peeking over the horizon , the antisolar point will be fairly high up, affording you the chance to see a much bigger rainbow than you would when the sun climbs higher. Conversely, if the sun is more than 42 degrees above the horizon, it becomes impossible for ground-based observers to see any portion of a rainbow whatsoever.
But when you're soaring in an aircraft, things get more interesting.
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