Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Two Questions About Light

  1. Apr 14, 2004 #1
    First Question:

    I think I have the question worded correctly. How many seconds would it take for the luminosity of an area to double if the amount of input light was unchanged, but the output light was forced to remain in the same place?

    Second Question:

    There are 2 rooms. One room has lights, the room is well lit. The 2nd room has no lights, and is pitch black. Neither rooms have windows. There is a door between the two rooms,
    which is open. I am standing in the pitch black room, looking through the door, into the well lit room. Objects can only be seen when light reflects off of them, other wise they are pitch black. I can see clearly what's in the well lit room. Light must be reflecting off the objects towards my direction for me to see them.

    How come if light is being reflected in my direction, I am not visable?
  2. jcsd
  3. Apr 14, 2004 #2
    Sorry, I didn't understand your first question; could you clarify somehow?

    And to your second question: You are visible, but it takes eyes like these :eek: to see you. If there is an open door between the two rooms, the unlightened one will never be completely dark. As you say, the light that comes from the lamp is (partially) reflected by the bodies in the first room and part of it reaches the dark room and enlightens it very little.
    Last edited: Apr 14, 2004
  4. Apr 14, 2004 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    If light is being reflected in our direction you are visible to another observer. it just so happens the intensity of reflected light from a room with no lights is very low as it varies with the inverse separation squared. As for your first question I do not fully understand it. Coul you reword it somewhat?
  5. Apr 15, 2004 #4
    if you had a container which let light in, but not let it out, how many seconds would it take for the "brightness" to double?
  6. Jul 22, 2004 #5
    This is a thought provoking question! I believe that you are asking how many seconds it would take for the brightness INSIDE the container to double! If so, it's kind of abstract because you cannot say what the brightness is inside the container while being outside it because the container will not emit any of the light, since it does not let any light out.

    But the closest thing I think one could have experienced similar to this is being in a car that has tinted windows, but even then, the light will escape through the front windscreen.

    And also, the material the container is made out of might also absorb the light and not emit any out, in which case, the brightness will never double. But if it is totally reflective, i.e. with mirrors as inner surfaces, the brightness might increase. And it probably will take very very little time to double as light travels very fast.

    That is all I make of it but being no Light expert, I might be wrong.

    PS: What triggered such a thought, if you don't mind me asking?
  7. Jul 22, 2004 #6
    Wow, old post :)

    and yes, i mean the light inside the container, and yes, im asuming it's made out of 100% reflective material.

    as for what made me think of this question, just one of those random "wouldnt it be cool if we could make/open a box of infinate light" type moments.
  8. Jul 22, 2004 #7
    Err, Yeah, old post. I'm kinda new on PF.

    I just thought that it is quite similar to the inside of a refrigerator, which is usually white and therefore absorbs very little light. So, maybe if you leave a torch on inside your refrigerator for a while and then opened it, you'd find out! But I doubt it would be anything exciting. But then again that light energy has GOT to go somewhere!

    BTW, might be a good idea to tape the refrigerator light switch so that it's off even when you open the door.
  9. Jul 23, 2004 #8
    I think it would be like an easy bake oven, only with a colder oven :)
  10. Jul 26, 2004 #9
    Gara, had you had this idea at an earlier point in time, you could have contributed to the invention of the LASER at BELL Labs!!

    I have just found out that the concept of trapping light in a box already exists. It's the way the LASER works! Inside a LASER, you have light bouncing between a mirror and a partially silvered (reflective) mirror. http://science.howstuffworks.com/laser.htm

    I believe if you had two 100% reflective mirrors, you could store the light and release it at a later point!
  11. Jul 26, 2004 #10


    User Avatar

    But that would break the second law, I think. So no such thing as 100% reflective mirrors.

    You know, that's what I thought, some years ago.

    The truth is, it is a little more complicated than that.

    LASER = Light Amplification by Stimulated Emmission of Radiation, or something like that.

    But goes between your mirror and your partially reflective one is very important. The way it works is to utilise a prediction of QM where photons of a certain energy can stimulate the emission of more photons of a certain energy. So, in a simple laser, you have something like a ruby, which has been pumped to excite the electrons within it. Then, you trigger the laser, and the pulse travels up the laser, causing more photons of the same energy to be released, all at the same time.
  12. Jul 27, 2004 #11
    Where as my idea would be storing normal natural sun light. think of a sphere more than a box, where light enters from all angles and after a few hours (or minutes, still dont know the rate at which lumin doubles, if input is unchanged, but output is stopped, or "stored") you just throw the glass mirror ball and it would shatter letting all the light it's "saved up" out in one go/pulse.

    it would be intresting to know just how much light you could store.

    enough to heat water to boiling point?
    enough to heat paper to flash point?
    enough to make an entire house burst into flames?

    since light can pass through its self, would it be true in saying IF we did have 100% reflextive mirrors, we could store an unlimited amount of light? and since light = heat, unlimited amount of light = unlimited amount of heat?
    Last edited: Jul 27, 2004
  13. Jul 27, 2004 #12


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    1. I posted in another string about the Lena Hau's experiment that "stored" light in an atomic gas. So far, this is the ONLY demonstration to date on the complete storage of light in a medium. Maybe you need to check that out to see just how difficult that is to do.

    2. There are no 100% reflective surface. One needs to understand that photons do not simply bounce off a surface. The conduction electrons and even the phonon modes in the conductor plays a direct role in optical conductivity/reflectivity of a material. In addition, anyone who has done quantum statistics would know that photon number is typically NOT conserved in many processes (unlike lepton number, baryonic number, etc., that are typically conserved in many situations).

    3. The "light bouncing in a laser" is nothing more than a waveguide which allows for self-amplification. It is why this is sometime called a "pump". One really needs to talk to experts in laser generation (not just reading off websites) to figure out the physics (and mechanics) of such a process. It isn't as trivial as it appears.

  14. Jul 27, 2004 #13


    User Avatar
    Science Advisor
    Homework Helper

    Enough to heat water to the boiling point? Puh-lease! I hope you could store more energy than that!

    After the '99 Womens World Cup, there was an interesting discussion about what would have happened if the referee had 'properly' called Scurry for coming off her line early to save one of the shots during the shoot-out at the end of the game.

    If 100,000 fans all had a perfect reflecting mirror (I know, it doesn't exist) that was .25 square meters and held it to reflect the sunlight (1000 W/m^2) at a 45 degree angle onto the poor referee, she would have been radiated with 18,000,000 Watts of solar energy.

    Assuming for an extremely rough estimate that a human body composed mostly of water would have roughly the same specific heat as water (about 4.2 kJ/kg-degree Celsius), and the referee weighed roughly 50 kg, the fans would have had to keep their mirrors focused on her for about .7 seconds in order to bring her blood to boiling.

    Fans trying to sneak mirrors into games is obviously what security is really looking for when they screen fans at the gate, right?

    Okay, this idea is really taken from Arthur Clarke's "A Slight Case of Sunstroke", not the Womens World Cup - and the physics is definitely rough.

    But, a solar oven can boil water by just focusing reflected light. Storing the light should provide even more energy, especially if it's focused on the output side.
  15. Jul 28, 2004 #14
    ". I posted in another string about the Lena Hau's experiment that "stored" light in an atomic gas"

    i meant to say the inside would be a vacuum.

    what is the best reflextive mirror we have? even if it's ~99.9 i asume it would heat up and shatter, releasing the light in what ever direction the gaps in the sphere are.

    as for how MUCH light, surely there could be some type of (number of photons)cm2

    let's asume normal day light is 1 million Pcm2 (P for photon, ya see?) and a really bright day would be 1.5 million Pcm2.

    if, on a normal day, even if half of the light was reflexted back in the sphere, and that light moves at... lets say 180,000 mph, just a guess. so if we removed a square centermeter block of light, it would take only 0.1~ seconds for fresh light to refill that.

    by 10 seconds, you've got 500,000 x 100 = 50,000,000 Pcm2.

    random numbers where used, but is there some way of finding out how to calculate it with real numbers, proving the thoery is sound? (though if it takes that lenth of time to make such bright light, you proberly only have 1 second to uncover the sphere, and leg it, before the glass melts and you die of radiation o_o)

    oh, and BobG, i once saw some one cover a 5 foot parabolic bowl. (old satalight dish, the kind that has the collection mirror cone that bounces what ever enters into the center, with the hole right in the center of the dish) they covered it with glue, and layed highly reflextive tin foil onto it. remember, this is just a 5 foot in diamiter dish. light was hitting the dish all over, going to the focal point, bouncing off the center cone and put 5 feet diamiter's worth of light, through a hole only 3 inches wide. it was enough to make wood logs burst into flames. whee?
    Last edited: Jul 28, 2004
  16. Jul 28, 2004 #15


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    I am aware of what you meant to say. I am just pointing out that the ONLY experimentally-demonstrated means of "storing" light is NOT in the way you are thinking of, that it involves a lot more than that.

    And you should ask people who have to deal with light with sufficient intensity that the mirrors, filters, and other optical equipment have to be cooled to prevent them from being destroyed. In case the point of this is completely missed, let me re-emphasize that photons being absorbed and retransmitted in the apparent form of "reflection" can, with sufficient intensity, generate HEAT in the material itself. Therefore, there is an INTRINSIC loss of energy as the intensity of light in any cavity increases! Your 99.9% reflectivity (if there is such a thing within the visible range) would not stay at that efficiency as you increase the intensity - it will get worse!

  17. Jul 30, 2004 #16
    if only we could control the direction the 2nd photon is emitted somehow. then 100% of photons would be emitted back into the sphere. as for cooling, nm that for now.

    can i get an asnwer on my other question about how long would it take for the area to double in brightness please?
  18. Jul 30, 2004 #17


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    I give up. It's like talking to a wall....

  19. Jul 31, 2004 #18
    lasers and boxes

    one of the ways that scientists make this box you describe is really cool. they take a ruby and cut it in such a way as to make it absorb light from 99 percent of the surface area. the other 1 percent releases all the light in one concentrated beam in a certain direction. laser.
  20. Jul 31, 2004 #19
    revision of boxes and lasers

    just realized something... the laser i described would be red. why? light color depends on wavelength so... hmm. why would it be red?
  21. Aug 1, 2004 #20
    Red LASERS are ruby lasers and are of 694nm. The colour of a LASER as you said, depends on the wavelength. And the wavelength depends on the type of material you use in the laser. It's all to do with transition of electrons in the atoms from one level to another.

    Check out this for a very clear and basic explanation.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Two Questions About Light
  1. 1 question about light (Replies: 22)