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Merriam Webster defines transparent as:

Having the property of transmitting light without appreciable scattering so that bodies lying beyond are seen clearly.

And translucent as:

Transmitting and diffusing light so that objects beyond cannot be seen clearly.

Now if you see any object through a lens or a bottle filled with water then most of the times whatever you see is not at all clear. What does this signify? Are these two transparent or translucent? Or is their behaviour conditional to how we see through them?

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up vote 7 down vote accepted

Lenses and glass bottles are transparent. As you quoted above, the different has to do with diffusion.

Here is an example of an image through a transparent object: transparent water glasses

Here is an example of a translucent object: frosted glass

This is an example of how diffusion causes translucency:

diffusion

As light passes through a translucent object, it either enters or exists a rough surface that causes light to reflect and refract at a bunch of different angles. This causes the image through the glass to be very blurry.

When you look through a glass or lens and object isn't clear, that's because it isn't focused, not because of diffusion. There are many reasons why images won't be focused but most have to do with the lens not being shaped perfectly or different behavior for different colors of light. See Wikipedia on optical aberrations for more information on this.

Here is an example of a perfect lens (top) versus a lens with a spherical aberration (bottom):

spherical aberration

The word transparent is used in all cases where diffusion isn't involved. Even if the lens is poor and causes images to not focus properly, as long as the issue is due to aberrations. The word translucent gets applied when there is significant diffusion of light to the point where the object looks "cloudy" or "frosted" and a sharp image can never be formed.

When you look through glasses with water and see an out-of-focus image, the glasses are still transparent.

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So translucent objects always diffuse ? And what about plastic bottles ? –  rijul gupta Jan 12 at 9:16
    
There isn't an exact, objective delineation between the two terms. I've seen plastic bottles that are clearly transparent and I've seen some that are clearly translucent. It's a judgment call. –  Brandon Enright Jan 12 at 9:19
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@rijulgupta One "easy" way to tell if something is translucent or not is to shine a laser pointer through it. If the whole object lights up with the color of the laser then there is significant diffusion scattering the laser light. –  Brandon Enright Jan 12 at 9:22
    
Thanks, that clears it up ! –  rijul gupta Jan 12 at 9:26
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