Question: A bird is flying down vertically towards the surface of water in a pond with constant speed. There is a fish inside the water. If that fish is exactly vertically below the bird, then the bird will appear to the fish to be?

A bird is flying down vertically towards the surface of water in a pond with constant speed. There is a fish inside the water. If that fish is exactly vertically below the bird, then the bird will appear  to the fish to be?

1. farther away than its actual distance.

2. closer than its actual distance.

3. moving faster than its actual speed.

4. moving slower than its actual speed.

Sol. According to the image formula for refraction at plane surface. Here river water surface acts as a plane surface.

We have n2/v = n1/u

Let x be the height of the bird at O above the water surface and y be the height of the image at I.

Here n1 = 1 (refractive index of Air)

          n2 = n (refractive index of water)

According to sign convention,

Height measured vertically above from the point of axis is taken as positive and below is taken as negative.

Therefore u = x (object distance vertically above)

                    V = y(image distance vertically above)

n/(y) = 1/(x)

y = nx from above expression which we got, we can say that n is greater than 1 and therefore y is greater than x.

So from this we can say that bird appears to fish to be farther away from the actual distance and it appears to fish that the bird covers more distance i.e y distance than the actual x distance. It appears that the speed of the bird is greater than the actual speed.

Refraction

1.We can observe a coin in the empty vessel up-to a certain distance. After moving certain distance from the vessel.You cannot see the coin in the vessel.

2. But when you fill the vessel with water up-to the brim of the vessel.

3 As we know light always travels in a straight line path in a medium.

4. So we get a question why in second condition we are able to see the coin in the filled

vessel ?

5. We can observe clearly that bending of the light rays has been occurred.

6. According to Fermat's principle that light ray always travel between the two points which needs the shortest possible time to travel.

7. If you observe the light ray in the above picture, the light ray changes its

Direction at the INTERFACE (separating region of two media. Here region between water and air).

8.This path is chosen by light ray to minimize time of travel.

9. Thus we can say that speed of light changes when light propagates from one medium to another medium.

Therefore,

   Refraction of light is defined as the process of changing speed at an interface when light travels from one medium to another medium resulting in a change in direction.

                        Or

     Bending of light is simply known as Refraction.

There are two types of medium . They are

1. Denser medium.

2. Rarer medium.

When light travels from rarer medium to denser medium ,

          Speed of light increases and light ray bend away from the normal.

When light travels from denser medium to rarer medium,

    

             Speed of light decreases and light ray bend towards the normal.

The extent of bending depends on a constant known as Refractive index.