PHYSICS help pleaseee.ASAP thanks?

Question

how do you figure out what the speed of light is in a substance? i need the speed of light in a substance to use in the equation n=c/v to figure out n (index of refraction) cause all i have is c. idk how to figure out the speed of light in the substance...im guessing i have to use another eqation? does it have to do with Snells law? i just dont know, please enaswer and explain if you can.

Here is another question:
a 60 watt light bulb is located 10m away from a 2m by 3m wall. the light intensity on the wall is 1900 lux use the inverse suare law to find the intensities if the distance is change to 20m 30m and 40m.
-i kinda get the inverse square law liek if it is double the distance away you take the inverse fo 2 ( which whould be 1/2) and square it but...i had a practice problem JUST like this with different numbers and i got it wrong ann i know im missing somethingg?? idk what though. please help and explain<333 thank you

Answer 1

There is no general equation for the speed of light in a material. It depends on the specific material and the specific frequency of the light. That means that the index of refraction also depends on the material and frequency

Here are two lists of the index of refractions for some common materials at a specific frequency (yellow sodium D):

http://hyperphysics.phy-astr.gsu.edu/hbase/tables/indrf.html#c1
http://en.wikipedia.org/wiki/List_of_refractive_indices

If you really need to find the velocities of light in various materials, you can work back from these numbers. :-)

The reason that we see rainbows from prisms and sunshowers is that the index of refraction changes with frequency. This is called dispersion. Here is a nice article on that:
http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/dispersion.html#c1

As for the second question, twice the distance should be 1/4 the intensity. If this didn't work for you with the practice problem, why not post the practice problem, how you did it, and what you were told the right answer was?

Answer 2

Ok, you've got several things listed there, I'll try to clarify the areas for you. a. That would be electromagnetism, since lightning is a mass transfer of electrons from the clouds to the ground. Electromagnetism deals with anything that's basically electronic or magnetic. Such as magnetic fields, circuits, lightning, static electricity, etc. b. How energy is produced by the sun is a basically relativity and to a degree nuclear physics, the primary equation for mass to energy conversion is E = mc^2. The following is a more detailed explanation for how energy is produced by the sun. Stars do rely on the interactions of nuclei to produce energy. The type of nuclear interaction that stars generate is a fusion reaction. Stars are able to compress deuterium atoms together and create helium via a fusion reaction. The energy released is enormous, it can be calculated by e=mc^2. The mass that's converted to energy is somewhere along the lines of .07% of the total mass is converted to energy. Thus making it a very large source of energy. It couldn't be a complete conversion otherwise we'd have no elements and the sun would not exist. A common misconception is that fusion takes hydrogen atoms and turns them into helium. The fusion reactions uses an isotope (element with a greater or lesser number of neutrons than the standard element on the periodic table) called deuterium or "heavy hydrogen" and slams those atoms together to create helium. The reason it has to be deuterium is that normal hydrogen consists of 1 proton and 1 electron. Helium consists of 2 protons, 2 neutrons and 2 electrons. Deutrium which is an isotope of hydrogen contains 1 proton, 1 neutron, and 1 electron. Thus making it the only real candidate for fusing together to create helium. I realize the answer is rather long, but it's the best one I could come up with.