refraction in water

The captain of a Navy battlecruiser is standing at the bow of his ship which is moored in very clear waters. He sees a fish in the water and would like to catch it.

 If the captain decides to shoot the fish with his rifle, where should he aim?

     (a) Above the fish

     (b) Below the fish

     (c) Directly at the fish

The answer is (b)he needs to aim below the fish. If he does not and aims directly at the fish, no matter how precise his aiming is , he will notice that he can never hit the fish, he sees the fish, aim the rifle, fires at it, well, and miss, again and again.  This is due to a Science phenomenon called refraction.  What is refraction? Refraction is the change in direction of light when it passes from one medium to another, it is caused by the change of the speed of the light . When light passes from one medium to another medium of different densities, it either speeds up or slows down. If light passes from a optically less dense medium to an optically denser medium, light slows down and light bends towards the normal(  A line that is perpendicular  to the surface .)and vice versa if light travels from an optically denser medium to an optically less dense medium. For example, air is optically less dense than water, so when light enters enter it slows down and bends. Therefore, when you put a straw or rifle in the water, it appears bent.

The thick white line in the middle is the normal. Ans the dotted line is the normal.


So when he looks into the river, refraction happens and the position of the fish is actually lower than he perceives.It appears closer to the surface than it usually is.
The dotted line shows the position of the fish that we see, while the straight line shows where the position of the fish really is. The light coming from the fish is refracting (away from the normal) as it passes into the air to his eyes, hence, the fish looks shallower than it really is.



Question 2.

 If the captain decides instead to zap the fish with a laser, where should he aim?

     (a) Above the fish

     (b) Below the fish

     (c) Directly at the fish

He should aim directly at the fish the answer is (c). This is because laser is a form of light and refraction occurs. So when you shoot the laser directly at the fish, when the laser pass from the air to the water, which is from a less optically dense medium to a optically denser medium, the laser beam slows down and bends towards the normal ( A line that is perpendicular to the surface.) The laser light will refract as it enters the water, following the exact reverse path as the light coming from the fish to his eye, hitting the fish, this is unlike throwing the spear at the fish because the spear is not a form a light , therefore, refraction does occur and we have to aim the spear below the image of the fish  that we see in order to hit the fish. But for the laser, it is a form of light, therefore, refraction occurs and to hit the fish, we have to aim directly at the fish.

Picture on the left: Light travelling through water.(Straight line)* Picture on the right: Light travelling through water and hitting fish(Straight line)*

*Straight line= Path that lights travel.

*Dotted line= What we see