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Focal Points in Optics

  1. Oct 26, 2012 #1
    Hello everyone,

    I'm an optometry student who is currently doing Optics 1, and I have a general question about focal points. I can do the math regarding F1, F2, and the nodal ray, and I am decent at drawing the ray traces, but I would like to know more about the actual real world applications and the big picture regarding these points. Can someone possibly tell me what the focal points are in real life? For example, when one sits with their face inside of the focal point of a concave mirror, one gets a magnified image of the face. This describes how a make-up mirror works. However, what I want to know is what does that actually mean? I feel like it has never been explained, and unlike many in my school, I didn't take any basic optics before coming into the program. Does this mean that for every lens or mirror, there is one spot where the image will be the best? When I take a pair of plus lens glasses and move them in front of my face, there is one spot where the glasses will be most clear, which is the focal point, but I just want to have a more clear understanding of what that actually means. Thanks for your time.
  2. jcsd
  3. Oct 27, 2012 #2


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    Dearly Missed

    Yes, that would be true.
  4. Oct 27, 2012 #3

    Simon Bridge

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    Welcome to PF;

    The focal point of a lens is the place the hot-spot is when you let sunlight through it ... it is where things burn. The name "focus" means something like "fireplace".

    (The virtual focus you get from a diverging lens is because parallel light is bent so that it appears to be coming from a fireplace at that spot.)

    It means exactly what it says: your face looks bigger.

    Light is reflected in such a way that the image appears different from a plane mirror.

    It is common to refer to a clear picture as one that is "focussed" and the process of adjusting screens and lenses to get a clear picture is called "focussing". This is sloppy jargon. If you put a screen at the focus of a converging lens, you will not usually see a clear image (unless the object is a very long way away).

    If you have a real image, you will be able to see it in any position where your eye is further from the lens than the image. Try it next time you have access to some lenses. If you have a virtual image, you can see it in any position that puts the lens between it and your eye.

    If you want to see a real image on a screen, however, there will be one screen-lens distance that will make the image clear. This is because the screen intercepts light from the lens and scatters it to many angles. The screen has to be at the position of the image to scatter a clear picture of the image (the picture on the screen is not always the image). You should be able to see why this is with a ray diagram that includes the screen.
  5. Oct 27, 2012 #4
    Optics is a very simple subject which completely relies on the point that light travels in a straight line. Regarding image formation in mirrors and lenses you should know few basic things.
    You know a virtual image of same size forms in a mirror.Now, some ball came and hit the mirror and mirror had a crack. How many images do you see now. Only one, right? But actually there will be two images forming.
    See you face in a small mirror and large mirror. Your face looks brighter in a large mirror than a small mirror, why?
    If you know the basics properly in optics you can solve most of the questions based on optics.
    Regarding focal point of lenses.
    When we are kids, we used to use a lens to burn a paper under sun. While doing this you will move the paper or the lens to get a bright spot. What is this bright spot? Why are we increasing or decreasing the distance between lens and paper to get this bright spot?
    As the sun is very far away from earth, we get parallel light light rays from sun. The lens we used to use when we are kids is a converging or convex lens. The light rays coming from the sun will be converged by the lens to a single point, making a bright spot on the paper(Parallel rays when projected on to a convex lens will converge at the focal point). As all the light rays fall at a single point, the heat energy at that point is more and paper starts burning. This bright spot is the focal point of the lens we used.
    We can apply physics even without knowing what exactly is happening. That's the beauty of physics.
  6. Oct 27, 2012 #5

    Simon Bridge

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    Niggle: but isn't the Sun quite wide?
    What happens is that you are projecting a small image of the Sun onto some thing as on a screen ... the farther the object, the closer that image is to the focus, and the smaller it is. Ergo the sunlight that hits the lens is concentrated in a small space... the rest follows.

    There are a lot of common misconceptions in optics - one of the main ones is characterizing lenses by their shape rather than by how they bend light. I notice you didn't fall for that one ;)

    Characterizing the focus as the place where you can burn things is useful for avoiding the kinds of misunderstandings shown in the first post though. The actual geometric focus used for ray optics is slightly closer than that.
  7. Oct 28, 2012 #6
    Thanks for the replies, everyone! They were very helpful:smile:
  8. Oct 29, 2012 #7
    There isn't just "one" focal point, there are oo2 (focal plane). Are you referring to the one in the optical axis?
    Anyway, a focal point is the point were all rays parallel to a given direction will converge; change this direction (the angle that these rays form with the optical axis) and the focal point will be different.
    The consequence in the "real word" is, for example, that a point object very far from the lens (e.g. a star in the sky) will produce rays which are all parallel near the lens, so the object will be focalized in just one point of the focal plane. Its position will depend on the angle between the light rays and the lens' optical axis, that is, on the position of the star. For this reason you can see every star in a specific postion, when you look through a telescope, and you can project this image in a plane after the lens at the focal distance.
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