 A great white shark attacks from below and behind, taking its victim by surprise. To do this it has to come up fast and keep its quarry in sight. Its eyes must react rapidly to the change in light if its hunt is to be successful.
Like all other sharks, the great white relies on its array of senses to locate prey such as seals and sea lions, but its sight is crucial to pinpointing a victim at the surface, silhouetted against the light.
Just as a human, entering a brightly lit room from the darkness outside, can be briefly dazzled and take a few seconds to adjust to the change, so a shark, rising to the surface from the murky depths, must cope with a rapid increase in light. It cannot afford to wait for its eyes to adjust - a lost second and the prey could be gone. The secret of this ability lies in the tapetum, a layer of mirror-like plates at the back of the eyeball. It is the tapetum that eerily lights up a cats eyes when it is caught in car headlamps at night. By reflecting light back through the retina, it effectively doubles the amount of light for the eye to use. This is essential for night hunters, like the cat if they are to see with clarity, especially in faint moonlight.
The great white shark's tapetum greatly enhances its vision in the murky depths, but it has no iris to protect its eyes when it surfaces rapidly. In order to compensate, it has developed a 'curtain' of cells containing pigment. As the shark moves into the bright light, these automatically expand over each tapetal plate, and then contract as it returns to the depths. While it is swimming in shallow water, a shark needs to be able to see into the light above it and into the darkness below. In this case the tapetal curtain reacts differently in each half of the eye. The lower half of the tapetum, which reflects light from above, is covered to protect the retina. The upper half of the tapetum, which reflects light from below, is exposed to make the most of the light hitting the retina and so gives as clear a picture as possible of the murky depths below.
Besides its eyes, a shark has remarkable senses, it uses receptors which are located in its snout, it can detect minute electrical currents caused by contracting muscles - such as a beating heart - in its victim's body. It is the predator's ultimate attack sense, guiding it in for the last short spurt to the kill.
This may explain why sharks sometimes attack boats. They are not trying to reach the occupants inside, but are attracted by the weak electrical currents produced by metal fittings immersed in sea water. In response to the stimulus, the shark behaves instictively - it opens its jaws and attacks the source of the signals.  Attacks on humans have been suggested that sharks mistake them for seals, sea lions or even fish, although this is not a proven fact, it does seem to be true in the way sharks hunt, after all a seal or sea lion, can seem to be a human swimming or surfing, tests have been done as regards to the latter, where the sharks have shyed away from the board when first striking it. Which brings me to believe that shark's are not as bad as some make them out to be, although I would not want to be near one unless I am well protected. I suppose that makes me out to be a hypocrite...
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