Why rainbows are arc shaped

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.

These chemicals are known as anti-colinesterase agents. Dave Standfield, Kandos Many of the current insect sprays destroy the wax coating of the insect's respiratory system. When applied directly, the insect suffocates and die instantly. Jaya Seethamraju, Gladesville They don't. The sprays are formulated to trigger a self-destruction response, which causes the insects to hang themselves. Matt Petersen, Randwick All contain poisons that interfere with the nervous system in varying ways, usually by prolonging or shutting down the activity of neurotransmitters.

Pyrethroids, alkaloids, nicotine and rotenone, based on the natural insecticides found in chrysanthemum, sabadilla, tobacco and derris root plants, all kill insects in this way. Paul Roberts, Lake Cathie. When does ancient history stop and become modern history? It's not that simple. The world then slipped into the Dark Ages, until about AD or until the beginning of the European Renaissance in the 14th century, depending on which historical authority you side with.

Modern history starts with the Renaissance, or in - again depending on which historical authority you side with. Jim Dewar, North Gosford. According to my daughter, ancient history is everything that happened before she was born. Peter Ryall, Lake Haven. It doesn't - at least not in Europe, which is the only place where these historical divisions really mean anything.

The sun is setting behind you in the west. White sunlight a combination of all the colors in the rainbow travels through the atmosphere, flies eastward past your head, and hits the water droplets as they fall from a passing storm. When a beam of light hits one of these droplets, one of two things can happen: The light could simply pass through, or, more interestingly, it could hit the front of the droplet, bend as it enters, then reflect off the back of the droplet and leave through the front, back toward us.

This is the light that forms rainbows. Different colors emerge from the droplets at different angles, ranging from red to violet by around two degrees. As seen in the figure below, the red light observed by an observer comes from droplets slightly higher in the atmosphere than the droplets that scatter violet light towards the observer:.

Although the human eye is capable of seeing a wide variety of colors, it is still popular to think of a rainbow as consisting of seven colors. For the rainbow to appear, water droplets must be floating in the air and clouds must have cleared the sun.

For a rainbow to occur, the sun and raindrops must be in the exact right positions with respect to the observer:. A compete rainbow is actually a full circle but from the ground level, we are just able to see a portion of it. However, complete rainbow can be seen from an airplane, under right conditions. We have already discussed that how is rainbow formed. Rarely, it is possible to see another fainter secondary rainbow above the primary rainbow. Since primary rainbow is caused by the reflection of light inside the water droplet, similarly, secondary rainbow is due to the double reflection inside the water droplet.

This twice reflected ray exits from drop at an angle of 50 degrees instead of This is the reason due to which secondary rainbow is always above the primary rainbow. Moreover, the order of colors in secondary rainbow is also reversed. Your email address will not be published. Save my name, email, and website in this browser for the next time I comment. Harsh asked Ramu is a shoe manufacturer and he wants to directly exchange shoes for rice without the use of money.

A double coincidence of wants B good exchange with money C good exchange with goods D none of these. Arun 26 Jul 1 0. Priyanka Arya 08 Apr 0 0. Present tense ending words Recommended Articles. Class 10 Tuition in Delhi. Class 10 Tuition in Bangalore. Class 10 Tuition in Hyderabad. Class 10 Tuition in Kolkata. Class 10 Tuition in Mumbai. Class 10 Tuition in Chennai. Class 10 Tuition in Pune. Class 10 Tuition in Noida. Class 10 Tuition in Ghaziabad. Class 10 Tuition in Gurgaon.

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