How does light pass through a drop of water?

[John Salerno]

I was reading about how rainbows are formed in Richard Dawkins' book "The Magic of Reality," and basically it was described as such: light passes through the upper part of the outside of the drop of water (that is in the air) and then reflects off the interior of the back of the drop and then exits the lower part of the drop.

 

My question is, why does the light seem to react differently to different parts of the water drop? In other words, why does the light pass through (rather than reflect off of) the upper part of the outside of the drop? Why does the light reflect off the inside of the back of the drop (rather than pass through the drop, in the same way that it entered it)? Finally, why does the light then pass through the lower part of the drop (rather than reflect off of it, as it did off the back of the drop)? To put it one more way, why doesn't the light either always pass through the water drop, or always reflect off of it, rather than do both at different parts of the drop?

 

Hope that makes sense!

 

Thanks,

John

Edited August 21, 2012 by John Salerno

[JMJones0424]

The answer to your questions is more difficult than it may seem. It is helpful to think of light as a wave that is partially reflected and refracted by the surface of each side of the rain drop. Similar to how light is both partially reflected and refracted when you look into the surface of a pond. Especially on a bright day, you can see a faint reflection in water, but you also still see through the water. However, this is an oversimplification and leads to intuitive problems such as those you have identified.

 

What is actually going on is a little more difficult to explain. The photons interact with the electrons throughout the water, and the probability of the path each photon takes can be understood using quantum electrodynamics. I highly suggest that you read Feynman's QED: The Strange Theory of Light and Matter, particularly chapter 2. You should be able to find it at a local library or on Amazon, or alternatively, you can read it free online at Scribd.

 

If you wish, you can also watch videos of the four lectures that inspired the book. The second video corresponds to the second chapter in the book.

Edited August 21, 2012 by JMJones0424

[John Salerno]

The answer to your questions is more difficult than it may seem. It is helpful to think of light as a wave that is partially reflected and refracted by the surface of each side of the rain drop. Similar to how light is both partially reflected and refracted when you look into the surface of a pond. Especially on a bright day, you can see a faint reflection in water, but you also still see through the water. However, this is an oversimplification and leads to intuitive problems such as those you have identified.

 

What is actually going on is a little more difficult to explain. The photons interact with the electrons throughout the water, and the probability of the path each photon takes can be understood using quantum electrodynamics. I highly suggest that you read Feynman's QED: The Strange Theory of Light and Matter, particularly chapter 2. You should be able to find it at a local library or on Amazon, or alternatively, you can read it free online at Scribd.

 

If you wish, you can also watch videos of the four lectures that inspired the book. The second video corresponds to the second chapter in the book.

 

Thanks. I've actually been searching for a good intro book to the subject of light, and so far I found this one: http://www.amazon.com/Introduction-Light-Physics-Vision-Color/dp/048642118X/ref=wl_it_dp_o_pC_S_nC?ie=UTF8&colid=2SSP0D0OTC6WF&coliid=I2KL719QS7RFGJ

 

Feynman's book was an option, but I felt it might be a little too heavy for a basic introduction, although I do plan to read it eventually as well. I'll at least check out the videos for now, but I have to admit, as much as I've read about quantum mechanics, I still have a little "huh?" question mark over my head every time.

[Ronald Hyde]

JMJones0424 gave a very good description of this, but FYI the Feynman book is written so that anyone who understands Algebra can comprehend it.

 

As far as Quantum mechanics is concerned, it's a bunch of more or less arbitrary seeming rules for calculating what can happen in a physical situation.

 

Even Feynman has said that, he may have even said it in the book. So your finding it mysterious and opaque isn't that surprising, so does everyone else.

 

But it does work in every case, and everything that suggests that it's wrong, when tested, proves to be false. So it's how the World really works.

[swansont]

I think a classical description works just fine here. At every interface there is some transmission and reflection; the amount depends on the index of refraction. Some light that is transmitted at the first surface and some of that is reflected at the second*, and all of that undergoes refraction. And then some of that light is transmitted again, which is the light you see. It's not an all-or-nothing option and the light doesn't have to choose to do one vs doing the other.

 

 

*unless it's undergoing total internal reflection at this point. I don't recall if this is always the case

 

edit: it can undergo TIR, but doesn't have to — it depends on the incident angle

 

You can get multiple reflections in the drop, which will give you a double rainbow

[Ronald Hyde]

If you are standing in a field of grass with the Sun behind you, you will notice that the grass in front of you,

opposite the Sun, will be brighter than the grass to the sides. That is a similar back scattering from the

cellulose fibers in the grass.