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How can a clear object be transparent and visible at the same time?

Category: Physics
Published: July 12, 2013

Clear objects are visible because they bend the light as it passes through. There are four basic things that can happen to light when it hits an object:

refraction by glass of water
Look closely at this glass of water. You are not actually seeing the glass. You are seeing the background image of the railing and house distorted in a certain way because of refraction. Your brain is able to deduce the presence and shape of the glass from the distortion. Public Domain Image, source: Christopher S. Baird.
  1. Specular Reflection: Think of a mirror or metal spoon. The light bounces off the object's surface like a billiard ball, allowing the original image to be seen in the object.
  2. Diffuse Reflection: Think of raw wood, flowers, or non-glossy painted surfaces. The light bounces off the object's surface in all directions, revealing the shape and color of the object.
  3. Absorption: Think of a black piece of coal or ash. The light enters the object where it is absorbed and converted to heat.
  4. Transmission/Refraction: Think of a glass of water. The light travels straight through the object but the direction it is traveling bends when entering and leaving the object.

In reality, all materials interact with light in all four ways. For instance, consider the hood of a red sports car. Some of the light is reflected specularly (leading to the glare spots you see and the image of trees reflected off the car). Some of the light is reflected diffusely (leading to the red color you see). Some of the light is absorbed (leading to the orange, yellow, green, blue, and violet light you don't see because it is absorbed – if these colors were not absorbed, the car would look white and not red). Also, some of the light is transmitted/refracted (very little actually).

For many materials, there may be one dominant way it interacts with light, so that the other ways are so small that they can be ignored. For instance, water does indeed absorb some red light (that is why the ocean is blue), and water does indeed reflect some light (that is why there is glare from the sun on the water's surface), but for the most part we can think of water as a clear material because transmission/refraction dominates.

Now, the interesting part is that each of the four interactions listed above alters the light. Our brains are able to detect this alteration in the light and deduce the presence and shape of an object from this information. Strictly speaking, we never see an "object". We see "light" that has been altered by an object. That is why it is so difficult to build machines that can see the way humans do: there is a great deal of intelligence required to deduce an object's shape and location from a pattern of light that it has altered.

When it comes to clear objects, we see them because we see the way light bends (refracts) as it passes through the objects. Look closely at a glass cup. When you look at the glass cup, what do you see? You just see an image of whatever is behind the cup, but distorted. Refraction bends the light as it passes through the cup and the background image ends up changed. Your brain is smart enough to be able to deduce the shape of the cup simply by how the background image is distorted.

This leads us to an interesting notion. If the refraction of a a clear material can be mostly canceled, the object can be made virtually invisible. One way to cancel refraction effects is to shape a clear material into a very flat slab with parallel surfaces. When light enters the slab, it bends, but when it leaves the slab out the other side it bends back by the same amount. As a result, the image coming out the other side is undistorted and the slab is effectively invisible. This is the principle behind windows. Windows are made out of clear glass and fashioned to be very flat, so that you can't actually see the window. You see the landscape beyond the window as if the window were not there (windows are not completely invisible because they do reflect a small amount of light which can be detected under the right conditions).

Topics: absorption, bending, diffuse, index of refraction, light, reflection, refraction, specular, transmission, transparent