How can we see visible light since electric and magnetic fields are invisible?

  1. what do we mean exactly by visible light?
    Since light is oscillating electric and magnetic fields, and both electric and magnetic fields are invisible, what do we actually see? is color the wave itself?when we see colors do we see the waves themselves and if their wavelength were bigger would we be able to see crests and troughs or is color a construction of our brain? when we see interference fringes do we see the waves themselves?
  2. jcsd
  3. Ryan_m_b

    Staff: Mentor

    Photons are absorbed by different materials according to their wavelength. When light enters the eye it is focused on photoreceptive rod and cone cells (the former detects light intensity and the latter colour). As a photon travels through the cell it is absorbed by a rhodopsin molecule causing a conformational change. This actives a cell signalling pathway resulting in a signal being sent down the optic nerve to the brain. Different colours can be seen because the three types of cone cells (one to detect green, one red, one blue) have slightly different forms of rhodopsin that are activated by different wavelengths of light. The ratio of different cones cells activating creates different colours.
    Last edited: Feb 6, 2012
    mishal.jumma likes this.
  4. Pengwuino

    Pengwuino 6,942
    Gold Member

    To answer part of your question (I'm limited on time this morning), the very definition of "seeing" and "visible" is intimately related to the presence of electromagnetic fields and waves. "Visible" to humans means electromagnetic waves within a very very narrow frequency range. Our eyes have adapted to see/detect electromagnetic waves with frequencies between 400 THz and 750THz (THz = 1,000,000,000,000 Hz). A vast vast majority of light (read: electromagnetic waves) all around us does not fall within these wavelengths so our eyes are incapable of detecting or "seeing" most of the electromagnetic waves around us. However it DOES see that small frequency range and that is what we call "seeing" or "visible".
  5. thank both of your for your time but what i don't understand is, since electric and magnetic fields are invisible, how is it possible for oscillating and magnetic fields to be visible. Also what we see (the color) is the wave itself, as it happens when we look at water waves?
  6. Ryan_m_b

    Staff: Mentor

    Whether or not a photon is "visible" or "invisible" tends to refer to whether or not we can detect them with our natural sight. When a photon enters the eye the process I described above^ occurs. Different wavelengths of life (i.e. different colours) are absorbed by different materials. We have separate cells in our eye that detect different wavelengths and our brain processes that as different colours. Wavelengths that are invisible do not interact with the molecules in our eye and thus we cannot see them.

    I have no idea what you mean by bringing water waves into it.
  7. do we actually see the wavelengths themselves that interact with the molecules with our eye, or is color something different? when I see green do i see just green or a wave of green, a wave with crests and troughs? ok a particular wavelength activates a cell, that makes me see green, but does this green oscillate, like the electromagnetic oscillation that made me see it ?
  8. You need to think about what you mean by 'visible'. Light is visible because it is absorbed by the cells of your retina which send signals via the optic nerve to neurons in your brain which are programmed to react to the signal to produce the sensation of sight. The cells of your retina are unaffected by static electric or magnetic fields, or by oscillating fields (waves) of all but a tiny range of frequencies, so you cannot 'see' them (which is just as well otherwise we would be blinded by the Earth's magnetic field).

    Different cells absorb different frequencies of electromagnetic wave, so your brian is sent a different pattern of signals depending on the frequencies it is receiving: we experience this as colour.

    I am not sure the analogy with water waves helps much here, except perhaps to think about a rowing boat and a cork floating on a pond. If you throw a small rock into the water, the cork bobs up and down: it 'sees' the waves, but the boat is unaffected.
  9. You do not see the oscillations: these are absorbed by electrons inside atoms within the retina. Think of it like a single water wave overflowing a bowl: you get water on the floor which stays there, it does not oscillate like the wave which made it get there. The brain sees the static pool of water on the floor, not the oscillating wave.

    It is the same with light, the brain 'sees' the electrochemical signals from the retina which like the pool of water do not oscillate at the frequency of the waves that produced them.
  10. thank you so much. you made it crystal clear
  11. You can answer that question yourself can you not - have you ever seen a crest or trough of light? Or the wavelength of light? when you look at something green does the color oscillate?

    My guess is your answer will be no.

    The oscillation of the light wave is what gives light its 'color' - it is an all or nothing. I put 'color' in quotes because light itself has no color. What you perceive as color ( in the visible spectrum ) is a certain frequency or wavelength of light impinging upon you eye. The eye is sensitive or if you prefer responds to those frequencies/wavelengths only within the visible spectrum. The eye responds to the frequency/wavelength of light and not to the crests or troughs as you call them.
    So red has a certain frequency/wavelength, which is different from green, yellow, blue ...

    You should note that a wave has certain characteristics such as frequency, wavelength, amplitude, and if you talk about only the crests or troughs then you are not talking about the wave but a certain feature that describes the wave.
  13. when i see interference fringes eg. do i not see minima and maxima which means crests that meet and add together?
  14. Drakkith

    Staff: Mentor

    Visual perception is based on the rod and cone cells in your eyes absorbing energy to excite a molecule inside them which starts a chain of events that eventually results in you "seeing" something. This energy is delivered via an electromagnetic wave, aka light. The properties of the wave, such as wavelength, determine the amount of energy it has and whether it can get through to your retina and activate your rod and cone cells or not. Too long of a wavelength means that the light, the "photon", doesn't have enough energy to excite that molecule, while too short is blocked by the various parts of your eye.

    In short, your eye's don't "see" the fields. The properties of the fields simply determine how the light interacts with your eye. If they are the right kinds of photons they will be absorbed by the right molecule in the right way and that will result in you seeing something.
  15. I think I got it. It seems so obvious. Vision is the result of the interaction of the field with the eye .
    The field is the source of my vision not its object
    I am grateful to you for your clear explanation.
  16. Ryan_m_b

    Staff: Mentor

    Another thing to think about is that this applies to all of your senses. Everything you perceive about your environment is a model built by your brain on the basis of the sensory input but it isn't really what the world is like. This can be seen with any optical illusion such as this blind spot test. Simply close one eye, focus on the cross and move your head forward and back.


    Eventually the spot will disappear but not only will it disappear but your brain will fill in the missing space with the background.
  17. thanks for your reply. i love these physics forums. And i love this deceptive and mysterious world we live in
  18. Ryan_m_b

    Staff: Mentor

    You're not the only one :wink:
  19. When I look at the left cross with my right eye, the dot on the right disappears.

    When I look at the right dot with my left eye, the cross on the left disappears.

    But it doesn't work in reverse - I can't change the combination.

    In both cases, I don't have to move my head, I just have to view at 30 cms and the phenomenon is immediate.

    Is this brainwork, or is it something to do with the positioning of the cones? Are the three types of cones evenly distributed at the back of the retina (except where the optic nerve goes out) or not? I don't understand these 2 different results I am getting depending on which eye I use, and I don't understand why I don't have to move my head backwards and forwards like you said. Can you explain more what is going on?
  20. Ok, I think I have got it. The place where there are no photocells i.e. where the optic nerve goes out, is on the 'nose' side in both eyes. I was assuming that the eyes are identically built, which is apparently not the case.
  21. Drakkith

    Staff: Mentor

    Your body is symmetrical for the most part, or bilateral. Both sides are, to a certain extent, mirror images of each other.
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