Eyes and Lenses
Help the Barracuda find its food!
Light rays travel through space in waves. The shortest wavelengths are gamma rays, and the longest are radio waves. The visible light we see is in the middle range of wavelengths:
400 (violet) - 700 (red) nanometers (billionths of a meter)
Light travels fast, 186,000 miles (300,000 kilometers) per second. Even at this speed, the light from the sun takes about eight minutes to reach us. Light from the next nearest star takes four years to reach our eyes on Earth.
Reflection, Refraction, Absorption
When a ray of light falls on the surface of an object, one or a combination of the following three things happen:
Reflection
The light may be reflected (thrown back). When
a ray of light strikes a smooth reflecting object, like a mirror, it returns the ray at an
angle from the surface that is equal to the angle at which it strikes the surface.
It is like a ball that is thrown against a wall. The ball bounces back at the same
angle at which it was thrown. 
Refraction:
Light may be refracted (bent). When a ray of light strikes a transparent object
at an angle, the speed of the light is different inside the object, causing the ray to
bend. A stick seen through a block of plastic looks broken because light travels
slower in plastic than in air and the rays are bent.
Absorption
Light may be absorbed. When a ray of light strikes an object that does not reflect light, such as dark wood, the object absorbs most of the light. Light is more rapidly absorbed by water than by air. As one descends deep in the ocean (or a deep lake) the amount of light present is much less than on the surface of the earth. Animals that live deep in the water rely less on their eyes than on other senses (sound and electricity) to find things to eat, to find their way, and to avoid obstacles.
Index of Refraction
Light travels slower in a liquid or gas than it does in vacuum. The index of
refraction is the ratio of the speed in vacuum to the speed in the material. The
chart below gives some values:
Material |
Index of Refraction |
Speed of Light in the material in miles/second |
| Vacuum (no material) | 1.0000 | 186,282 |
| air | 1.0003 | 186,226 |
| plastic (lenses in exhibits) | 1.5000 | 124,200 |
| glass (as in eye glasses) | 1.5299 | 121,753 |
| fluid in human eyeball | 1.3400 | 139,016 |
| water | 1.3300 | 140,061 |
The index of refraction is the ratio of the speed of light in a vacuum to the speed of
light in a material.. The higher the index of refraction, the more light bends. The
index of refraction of air is about 1.00 and water about 1.33. This means that light
travels 33% more slowly in water than in air. This difference in the speed of light
profoundly affects the different way that animals that live in water see their environment
from those that live in air.
Water travels at different speeds in different fluids. At the boundary between the water and the fluid in the lens of the eye, the velocity of the light will change, and if the light is not perpendicular to the boundary it will bend. This is called refraction.
A java applet to help you understand refraction
from Dr. Fu-Kwun Hwang, National Taiwan Normal University
Eyes and Lenses
A lens is a piece of transparent material with at least one curved side. Lenses refract (bend) light rays and form images. The images can be larger, smaller, or the same size. A lens works (i.e. can focus an image) because of the difference in the speed of light between the lens and what it is immersed in. A slightly curved lens, like that in your eye, works well in the air, where the speed of light in air is much different from the speed of light in the lens in your eye.
The eye
The lens focuses an image on the retina. The brain interprets the signal on the
retina. That is how we, and other animals can see. Humans have eye muscles to adjust
the shape of lens to focus at a wide range of distances.
Fish eyes. A spherical lens, like that in a fish’s eye, works well in the
water. The lens has to be much more curved than the human lens to form an image
underwater. That’s because the speed of light in water is almost identical to
the speed of light in the lens.
The land animal's lens is flatter.
This lens bends the light enough in air to produce a clear image.
This lens can't bend the light enough in water to produce a clear image.
The fish lens is more round.
This lens bends the light a lot more. This is great in the water, but
cannot make a clear image in the air.
Aquatic Vision
In shallow, clear, well-lit waters, however, such as Lake Michigan or the Florida reefs, vision is still an important sense for aquatic animals. Here we see predators with large eyes that are good for hunting, and prey with camouflage patterns (like fake eyes on a tail) to fool those predators. We also see strikingly colored fishes, which communicate species, sex, age, and special behaviors.
Fish eyes are adapted to accommodate the higher degree of refraction (the bending of light) in water than in air. Aquatic animals have eyes and lenses that are more spherical in shape than land animals.
The eye muscles of fish and reptiles move the entire lens back or forward, and have a smaller range of focus distance. Fish eyes protrude slightly to allow fish all-around vision. However, there is little visual overlap, reducing their 3-D vision considerably. Fish lack eyelids since there is no chance of the eye drying up, and they also lack the ability to open and close their pupils to control the amount of light entering the eye.
Help the barracuda find its food!
Barracuda (Sphyraena barracuda.)
Pointed jaws lined with formidable teeth could belong to no other than the “tiger
of the sea”, the great barracuda. These fierce marine predators are
feared. Records of over 30 attacks on humans bear witness to their ferocity.
The fact that the attacks on humans occur mostly in cloudy water implies cases of mistaken
identity. Barracudas locate their food largely by sight, attacking brightly colored
or erratically moving objects. Splashing swimmers wearing jewelry or white sneakers
catch their attention, as do freshly speared fish. They are very inquisitive, and
will follow divers for hours. Silver, torpedo-slim great barracudas reach a length
of six feet. Their flesh, considered excellent eating, may concentrate some rare
poisons from the grunts, jacks, sea basses and other reef fish they eat. Lone
individuals patrol the coral heads and reefs of all tropical seas, as they do in the Coral
Reef Exhibit at the John G. Shedd Aquarium in
Chicago. Parrotfish and barracudas live in the clear waters of coral reefs. They use
their eyes a lot.
Great
underwater photo of barracuda by Phil Shaw
More underwater photos at
Waterworks by Phil Shaw
Parrotfish
Brightly
colored Parrotfish make a meal for the Giant Barracuda. Parrotfish are among a coral
reef’s major herbivores. With teeth fused into a beak, they graze algae down to bare
limestone. Besides converting plant matter into animal tissue, their feeding clears
space for the settlement of coral larvae and keeps algae from overgrowing the reef.
Parrotfish are found in all tropical seas. Parrotfish recognize each other
visually by color and pattern. Barracudas use their big eyes to find them. And use their
big teeth to catch them!
Parrotfish by Eric Martin
More neat underwater photos Eric
Martin's Maui Scuba Gallery
Swimming
When you go swimming it is hard to see under water because you lens is the wrong shape.
If you wear a diver's face mask you can see much better under water. Why?
Fishing
When you go fishing can the fish see you? Try this applet from Dr. Fu-Kwun
Hwang, National Taiwan Normal University
SciTech's Exhibit: "Seeing in Air - Seeing in Water"
Try this!
Brine Shrimp Ballet and Bring Shrine Hatchery
Adult brine shrimp are photo-negative. This means they swim
away from light. Baby brine shrimp (nauplii) are photo-positive. They
swim toward light. This phenomena is not completely understood. One theory
suggests that light separates the adults from the babies, since the adults tend to eat
their children! 
Brine Shrimp
(Artemia). First written about in 1755, Brine Shrimp have since become not only an
important food link in aquaculture projects worldwide but the subject of crustacean
research which embraces all the biological science disciplines. Brine Shrimp have
evolved to thrive in some of the toughest of circumstances. With virtually no
defense mechanisms Brine Shrimp was forced to adapt to harsh environmental conditions in
order to outlast predators. Able to survive in salinities in excess of eight times
ocean strength, Brine Shrimp, along with a few species of bacteria and algae, colonized a
unique niche, unsuitable for Brine Shrimp predators. These harsh habitats are
inland salt lakes (such as the Great Salt Lake, Utah), and the concentration ponds of
solar salt operations as occur around San Francisco Bay. Conditions in these
environments, although suitable for Brine Shrimp, can at times be stressful even on the
Brine Shrimp—stressful in the way of low dissolved oxygen, temperature, and food
supply. As long as satisfactory environmental conditions prevail, adult Brine Shrimp
reproduce by expelling live Brine Shrimp nauplii (babies) from their brood sac.
However, should an environmental stress occur, a shell gland is activated and embryonic
development is stopped. The embryo is wrapped with a hard substance and deposited to
await more favorable conditions before resuming development. These embryos remain in
this arrested state during winter months. When the spring rains come, they rehydrate
and break dormancy. This occurs at the same time when algae starts to bloom,
insuring adequate food supplies.
At SciTech we hatch our own Brine Shrimp 
Here is a recipe:
1. Fill the shallow pan with water. Use the thermometer to make sure the
temperature is between 75? and 85? F.
2. Add 8 tablespoons of salt per gallon of water. Stir well so the salt
dissolves.
3. Add 1 level teaspoon of brine shrimp eggs (Brine Shrimp cysts).
Your eggs will hatch in about 24 hours. Baby brine shrimp are called nauplii. Your nauplii will have plenty of food from their cysts for the first two days. After that, give them a very small pinch of yeast every other day to feed them. If the water gets cloudy, change their water (you may need a brine shrimp net or handkerchief to save the nauplii!). Be careful to keep the temperature the same!
Web references on Brine Shrimp and how to hatch your own brine shrimp
