Why are rainbows in an arc-shaped?

118th Discovery Posting

I am sure that many of you have seen a rainbow in the skies above you before. But have you ever wondered why are rainbows shaped in an arc? Personally, I am not sure of it myself and I was made curious about the answer by one of my participants in a recently guided inter tidal walk who asked his son this question when they saw a rainbow over at Big Sisters' island not long ago. As a result of this curiosity, I decided to do some simple research on the internet and do up this blog post.
The first thing to note is that rainbows are caused sunlight passing through raindrops and by refraction, which is the bending of light as it passes from one medium to another. I would not go through the exact science of how to explain this as I have found websites that does this explanation better than I can. You can find the websites below.

It is interesting to note that a rainbow is really circle in shape than the arc shaped or semi circle shape we usually see. The reason why we don't see the whole circle is because the earth blocks our view. And the closer the sun is to the horizon, the more of the circle we see. So this means that the best time to see a longest rainbow would be either during early morning after sunrise and evening time before sunset. =D
Great websites that explains how a rainbow is formed and why is it in a arc.
a) http://science.howstuffworks.com/rainbow.htm
b) http://www.scientificamerican.com/article.cfm?id=why-do-rainbows-form-inst

Side note:
Rainbow Connection by Kermit the Frog. Enjoy! =D

Inter Tidal walk @ Big Sister's Island on 30 May 2010

117th Discovery Posting

Personally, I have been to Big Sister's Island for less than the fingers on one hand. So it was a great chance to both visit and introduce other people to this small little island to the south of Singapore through a guided inter tidal trip.

Sidenote:
If you would like to read more about Big Sister's Island, you may visit http://www.wildsingapore.com/places/sisters.htm
I need to mention that this was also the first public guided trip on Big Sisters organised by RMBR and I was guiding a group of rather experienced participants as most of them have to been other shores with us.
To start off, the first 'discovery' is something which I really like, feather stars (picture below)!
As their name suggests, their arms are a little like feathers, handle with care as they may break easily. I have seen them in many different colours besides red. They include black, yellow, white, etc. They are also related to the sea stars being in the same echinoderm phylum.

As we moved towards the coral reefs, a participant spot this school of juvenile catfish, second 'discovery' (picture below).Like other fishes you may find in the sea, juvenile catfishes are often seen in a group or what we call as a school of fish. They do this as a predator will find it harder to pounce on a group for food than compared to an individual, this is otherwise known as safety in numbers. Interestingly, as the cattail grows bigger, their group gets smaller and they are usually found alone when they are adults. As they have poisonous spines, it is not advised to touch them. One reason, I guess, why they are called catfish is because of their whiskers, these are usually to help them find food in murky waters.

If in case you are wondering, a cat's whiskers help it to find its way and move around. A little similar to the function of the catfish's whiskers.

Now, it is to be noted that Big Sister's Island has one of the nicest looking coral reefs where one can come really close up into (picture below). And my group, coral reefs, did spent a certain amount of time admiring at it. As we moved on, we came across this leaf slug which our hunter seeker has found for us, third 'discovery' (picture below).The leaf slug is named so mainly because of its colour which resembles a leaf. I reckon another reason which it is named as such is because that it is able to retain the chloroplasts of the seaweed it eats. These chloroplasts which it manages to retain will continue to carry out photosynthesis inside the slug and provide it with extra nutrients. Cool, isn't it?

Just to sidetrack a bit, we had this little joke about if we could do the same, would we be green in colour as well...hahaha

Moving on, our hunter seekers also found us a flatworm, fourth 'discovery' (picture below). As their name suggest, flatworms are really flat. This can help them to squeeze or move into narrow and small spaces to find food and at the same time to hide from their predators. At the same time, being really flat means that their bodies are easily tore when handled, so please handle them with them or don't handle them at all. One really interesting thing about them is that flatworms are hermaphrodites. In really simple terms, this means that a flat worm is both a guy and gal. In specific terms, flatworms have both the male and female reproductive organs.

And the last organism I am going to feature is this bohol nudibranch, fifth 'discovery' (picture below).First off, they are sea slugs. They are called nudibranchs because they have naked gills. Notice the flowery like thing near the center of the picture? That's the gills for this nudibranch. They may look like easy prey, they are however not. They protect themselves in different ways like producing distasteful substances, toxins and even acids. It is important to take note that these protective methods vary from species to species. Some of them will advertise themselves to be 'not nice to eat' with their really colourful bodies while some, like this bohol nudibranch will try to camouflage themselves.

As we were still wondering around the shore to look for more things, dark clouds and strong winds came in quickly and threaten to drench us, so we had no choice but to end the walk prematurely and leave the shore quickly.
Luckily, it did not rain eventually and the visitors has some time to take a short picnic and relax on the island.
Before I sign off, I would like to say thanks to all Coral Reefs, my group for the day. I hope the 3 first timers enjoyed themselves as I did not really give much details about the things we've seen that day and of course not forgetting the rest of my group.

Last note:
As a result of the passing rain clouds, we caught sight of a really beautiful rainbow which I would blog about in the next post, so do come back soon to check that posting out. =D

Discovery @ Big Sister's Island on 30 June 2007

Thirteenth Discovery Posting:

Time: 420am
Place (picture below): Marina South Ferry Terminal
Destination: Big Sister's Island
Purpose: Exploration
Quite a number of first-s on this trip (read on to discover why), my first trip to Big Sister's Island, so even though i had to forgo my sleep for the night, i had to go.

The cool breezes, a full moon and reflections of moonlight against the water along the way to our destination ensured that sleep never had a chance to crawl upon me.

Reaching the island, our first stop was the small lagoon as we wanted to check if there were any common sea stars around. But the first 'discovery' of the 'morning' was a land hermit crab (picture below). My first time seeing this!
Discovery Note:
1. Land Hermit Crabs need dry land to survive, unlike their very similar cousins the sea hermit crabs.
2. Sea Hermit Crabs will die in taken out of water for too long while a Land Hermit Crab will die if left in water for too long.
3. But like the sea hermit crabs, as it grows bigger, it will have to move out of its shell and move into a bigger one.
4. So please don’t not remove any shells from the shores as they may be potential homes for either land or sea hermit crabs.

As we explored on, someone spotted two anemone shrimps on a carpet anemone (picture below)! Second 'discovery'!
Discovery Note:
1. Difficult to spot due to its translucent appearance (can you spot the anemone shrimps in the picture above?)
2. They are able to live in or with anemones just like clown fishes.
3. The anemone shrimp and the anemone share a symbiotic relationship of mutualism; the shrimp attracts prey to the anemone for food while the anemone provides the shrimp with a shelter.

When you are exploring our shores during low tides, you might hear clicks and if you ever wondered what made them. Here's the answer, a snapping shrimp (picture below). Third 'discovery' of the morning.
Discovery Note:
1. One of their pincers is greatly enlarged. Sometimes, the enlarged pincer can be as long as its entire body.
2. The enlarged pincer has one movable ‘finger’ held apart with a catch. When the catch is released, an explosive sound occurs.
3. When colonies of the shrimp snap their claws, the cacophony is so intense that submarines can take advantage of it to hide from sonar.

Fourth 'discovery' was a brown egg crab (picture below).
Discovery Note:
1. The Brown egg crab is listed among the threatened animals of Singapore due to habitat lost.
2. Egg crabs are highly poisonous and contain toxins which are not destroyed by cooking.
3. Since other animals don't particularly want to eat this crab as it is poisonous, it is slow moving and doesn't really bother to hide.

As i stood taking photos of the crab, it seem to feeding on shrimps found nearby itself (picture below). Fifth 'discovery'!
Note:
Could anyone help me ID this shrimp? Thanks!

Alas but as further exploration yielded no sights of the common sea star, we decided to move towards the big lagoon and along the way was another first encounter for me. A blue spotted stingray (picture below) spotted by Ron. Sixth 'discovery'!
Discovery Note:
1. The position of the eyes allows the blue spot stingray to see almost behind it.
2. The gills and mouth are found on the underside of the body.
3. The blue spot stingray doesn't really have teeth—instead, the mouth is outfitted with two food-crushing plates.
4. Rays dart away when they sense trouble approaching. When caught off guard, these fish fend off predators with a flick of the tail, which is equipped with two venomous spines. Since its tail is so long, the blue spot stingray can even strike at animals directly in front of it.
5. The large tail spine of the blue spot stingray is dangerous and even deadly. The barbs in the tail are so large; people have bled to death from a sting.

Helen soon exclaimed that something was very beautiful as i was photographing some corals. Thinking of it to be another coral, i slowly approached the 'site' of 'sight' only to find out that it was a grapsus crab (picture below). Seventh 'discovery' and another first encounter!
Discovery Note:
1. Adults are quite variable in colour. Some are muted brownish-red, some mottled or spotted brown, pink, or yellow.
2. It feeds on algae primarily, sometimes sampling plant matter and dead animals. It is a quick-moving and agile crab, and hard to catch, but not considered very edible by humans.

And as we walked on, Ron spotted a carpet eel blenny on a omelette leathery soft coral (picture below). Another first encounter plus it's the eighth 'discovery'.
Discovery Note:
1. They are not eels, but eel look alike fish.
2. These well-camouflaged, eel-shaped fishes lurk among the coral rubble and snatch at other smaller fishes that may pass by.

Ninth 'discovery' was a brittle star that Helen spotted (picture below) in the big lagoon, it's about 25cm in diameter by the way.
Discovery Note:
1. Related to and like the sea stars, they have five arms and a central disc.
2. As the name suggests, the arms of the brittle stars are rather liable to break. This is actually an escape mechanism. They can regenerate their arms, but slowly.
3. Brittle stars use their arms for locomotion. They do not, like sea stars, depend on tube feet.
4. Brittle stars move fairly rapidly by wriggling their arms which are highly flexible and enable the animals to make either snake-like or rowing movements.
5. Brittle stars can reproduce asexually by self-division.

As i scanned the rocks and corals hoping to find flatworms, nudibranchs or any other interesting stuff, a fast moving small creature darted across my sight. A closer look reveal it to be a marine spider (picture below). Tenth 'discovery'.
Discovery Note:
1. The marine spider can live in the sea. During high tides, it waits in air pockets among the submerged rocks or coral rubble. And when it’s low tide, it comes out to hunt.
2. As its furry feet are able to repel water, it can actually 'walk' on water!
3. Do you know that this is a threaten species? Yes, it is!

Eleventh 'discovery' was an unknown sea cucumber (picture below). Looks a bit like the unknown sea cucumber Luan Keng discovered at Semakau recently.

Question:
Discoverers , can you remember or do you knowwho sea cucumbers are related to? =)

Answer:
The sea stars!

Helen then pointed out a toadfish she spotted (picture below). Twelfth 'discovery'.Discovery Note:
1. Toadfish are well known for their ability to "sing", males in particular using the swim bladder as a sound-production device used to attract mates.
2. Instead of scales, toadfishes have skin which is covered with a thick layer of slimy mucus.
3. Its teeth are large and blunt; and it has two short spines at the upper angle of each gill cover, hidden however, in the thick skin.

"Basket!" Chay Hoon yelled.

Oh, if you're wondering, she was not scolding anyone or stung by anything. She was shouting "basket" because she found a basket star (picture below). Another first encounter and this is the thirteenth 'discovery'! Discovery Note:
1. Basket Stars are one of the stranger cousins of starfish. These creatures look very plant-like and are a specialized type of brittle stars.
2. They usually attach themselves to a coral head or sea fan or rocks.
3. During the day it coils itself into a knotted ball, but at night it stretches out all of its legs into a basket-like shape, fishing for plankton and other microscopic critters.
4. This animal does not usually inhabit the inter-tidal zone.

Side Note:
1. This was also the first basket star that most of us encounter on the inter-tidal zone. So naturally most of us were excited about it.
2. Further checking revealed that the last sighting of basket sea stars in Singapore was back in 1896! That's exactly 111 years ago!

Corals found so near our urban land scape. Yes, it 's possible. Here's a photo (picture below), check out our CBD in the background.

Our nature reserves and the beautiful corals found along some of shorelines and the southern islands are living proves that Singapore is not just about air-conditioned shopping malls and land scape gardens.

From the garden city to "City in a garden" is a wonderful step, but please not let these gardens be just man made, land-scape gardens but also nature gardens of tree and coral forests.Turning my attention back to the lagoon pools, a bohol nudibranch was found! Fourteenth 'discovery'.Discovery Note:
1. In scientific terms, this nudibranch is called the 'Discodoris-boholiensis'.

Note:
Wondering why was there a 'disco' in its name? Did it like to party? =p

Anyway, that about warps everything up for the morning. And here's something to prove that we were on Big Sister's Island (pictures below).
So here's a 'bye' to the Big Sister's Island and thanks to Ria for planning this trip and all others for spotting all the interesting things and animals. =)