How could you direct the laser towards the transparent tank full of water so that 100% of the laser light was reflected?
Asked by: James McDougall
Answer
Almost all of the laser light will be reflected off of the air-glass interface if the incident angle is close to 90 deg. Note that the incident angle is the angle between the laser beam and the normal to the interface.
The figure below shows:
i - incident angle; r - reflected angle, and t - transmitted angle.
When the incidence angle is high (i.e. i 90 deg) then we call it a grazing or glancing angle - because the incident light beam is approaching the interface almost parallel to it. Almost all surfaces reflect almost all of the incident light at this angle. You can just try holding a book at the level of the middle of your eyes and face a bright light; you will see the light from the source reflected nicely in the cover.
Another example is when you are driving on a straight road and the Sun is just setting in front of you (or you are sitting on a beach facing the ocean and the sunset). The road or the surface of the ocean will become almost completely reflective.
This total reflection at grazing angles is used in x-ray telescopes to reflect x-rays toward the detector. By making a shallow angle conically shaped mirror one can create an x-ray focusing lens.
Answered by: Anton Skorucak, M.S. Physics, PhysLink.com Creator
'The strength and weakness of physicists is that we believe in what we can measure. And if we can't measure it, then we say it probably doesn't exist. And that closes us off to an enormous amount of phenomena that we may not be able to measure because they only happened once. For example, the Big Bang. ... That's one reason why they scoffed at higher dimensions for so many years. Now we realize that there's no alternative... '