Answer 1:
Good question. There are two main concepts
involved in the answer. The first is that when you
see a perfect image on the ocean or another
surface, you are seeing a reflection of a
reflection. All objects reflect some wavelengths of light and absorb others. When sunlight (or another source of light) strikes objects such as clouds, mountains, etc., light that is not absorbed is reflected off of the object in all directions. Some of it travels to your eye directly, while some rays first reach other surfaces; these surfaces can, in turn, either absorb or reflect the light again.
It is the rays that are reflected twice (once off of the object itself, and again off of another surface) that you see as an image of the original object.
The second part of the answer to your question
involves the reflecting surface itself, because not all surfaces are capable of producing a perfect reflected image. As you might have observed, a lake surface will reflect the surroundings beautifully on a calm day, but there will be almost no clear images at all on a windy day. The amount of light that is reflected depends largely on how smooth the surface is. Surfaces that are very smooth (meaning that tiny grooves and bumps on the surface are smaller than the wavelength of the incident light) will follow the law of reflection for a perfect plane mirror. In
other words, angle of incidence = angle of
reflection when measured from the normal of the surface. (This is sometimes called specular
reflection).
Thus incoming light rays that started
out side by side will remain so after being
reflected, and what reaches your eye is a
point-by-point reflected image of the original
object. If the surface is not smooth (the more
common scenario), light that hits the surface will
scatter in all directions and the original order will be lost. This is known as diffuse reflection.
See here for more detail and an illustration of diffuse vs. specular reflection. You might then wonder, if light is being reflected off of a cloud and scattered in all directions, why do you see the cloud only at one point on the surface of the ocean and not at multiple places? Why isn't everything in the sky blurred together, for that matter? While it is true that reflected light rays are hitting all parts of the ocean surface, only the ones that bounce off at exactly the right angle can reach your eye. If you think of connecting the dots from the cloud to your eye and drawing the incident rays coming off of each point of the cloud and reflecting off of the surface of
the ocean, all the while obeying the law of
reflection (angle of incidence = angle of
reflection), there really are only a few
rays that can come off of a particular point of
the cloud and still reach your eye. It is this
precision that allows you to see a single, clear
image on the surface. |
Answer 2:
When light reflects off a sufficiently smooth
surface, the reflection is coherent. This means that parallel wave fronts of the energy remain parallel after the reflection.
A different kind of reflection is diffuse reflection in which the surface is rough enough that large parts of the surface contribute to the 'image'. Such a surface averages the light that falls on it rather well.
A way to think about this is that you
have a set of features in the sky that compose the
image you see from the ground. From your point of
view, each point in the image is casting a larger
or smaller portion of light into your eyes. Now if
you look at the surface of a smooth lake, you can
make a map of points where the light leaving the
image bounces off the lake into your eye. There is
only one such point for each image point because
the light reflecting off the surface has to have
the same angle as the entering light coming from the image, where the angle is measured to a line perpendicular to the surface. Thus you see a reversed image of the sky on the lake. If the lake is sufficiently rough, many points will satisfy this condition and the image will be averaged to a uniform smear.
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