It is known that birds can detect the earth's magnetic field, but the mechanism of how they do this is not known. it has been suggested that perhaps they detect a motional emf as they fly north to south, but it turns out that the induced voltages are small compared to the voltages normally encountered in cells, so this is probably not the mechanism involved. to check this out, calculate the induced voltage for a wild goose with a wingspan of 1.2 m flying due south at 13 m/s at a point where the earth's magnetic field is 5 × 10-5t directed downward from horizontal by 40°. the expected voltage would be about

Explanation:

Here we know that the EMF induced in this Field is given as

here B = perpendicular component of magnetic field

v = speed of the bird

L = length of the wings

now we have

now we have

as we know our atmosphere or air have different levels. every level has other gasses, and every gas has another permeability. when the light comes from lower permeability to higher permeability, it changes its direction. so that means that when the light comes from the moon to the earth, it changes it's direction many more times. it basically means that the shape of the light part of the moon is changed on the earth.

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