Light Rays, Geometrical Optics, physics 10

Physics 10.VI Geometrical Optics T.Trường 9/6/17 1,009 0
  1. Light Rays, Geometrical Optics, physics 10
    1/ Light Rays

    In physics we use the idea of a light ray to indicate the direction that light travels. Light rays are lines with arrows and are used to show the path that light travels. In Figure 7.1, the light rays from the object enters the eye and the eye sees the object.

    The most important thing to remember is that we can only see an object when light from the object enters our eyes. The object must be a source of light (for example a light bulb) or else it must reflect light from a source (for example the moon), and the reflected light enters our eyes.
    Important: We cannot see an object unless light from that object enters our eyes.
    Definition: Light ray
    Light rays are straight lines with arrows to show the path of light.
    Important: Light rays are not real. They are merely used to show the path that light travels
    [​IMG]
    Figure 7.1: We can only see an object when light from that object enters our eyes. We draw light as lines with arrows to show the direction the light travels. When the light travels from the object to the eye, the eye can see the object.
    Activity :: Investigation : Light travels in straight lines
    Apparatus:

    You will need a candle, matches and three sheets of paper.
    Method:
    1. Make a small hole in the middle of each of the three sheets of paper.
    2. Light the candle.
    3. Look at the burning candle through the hole in the first sheet of paper.
    4. Place the second sheet of paper between you and the candle so that you can still see the candle through the holes.
    5. Now do the same with the third sheet so that you can still see the candle. The sheets of paper must not touch each other.
    [​IMG]
    1. What do you notice about the holes in the paper?
    Conclusions:
    In the investigation you will notice that the holes in the paper need to be in a straight line. This shows that light travels in a straight line. We cannot see around corners. This also proves that light does not bend around a corner, but travels straight.
    Activity :: Investigation : Light travels in straight lines
    On a sunny day, stand outside and look at something in the distance, for example a tree, a flower or a car. From what we have learnt, we can see the tree, flower or car because light from the object is entering our eye. Now take a sheet of paper and hold it about 20 cm in front of your face. Can you still see the tree, flower or car? Why not?
    Figure 7.3 shows that a sheet of paper in front of your eye prevents light rays from reaching your eye.
    [​IMG]
    Figure 7.3: The sheet of paper prevents the light rays from reaching the eye, and the eye cannot see the object.

    2/ Shadows
    Objects cast shadows when light shines on them. This is more evidence that light travels in straight lines. The picture below shows how shadows are formed.
    [​IMG]

    3/Ray Diagrams
    A ray diagram is a drawing that shows the path of light rays. Light rays are drawn using straight lines and arrow heads. The figure below shows some examples of ray diagrams
    [​IMG]
    Exercise: Light Rays
    E - 1.
    Are light rays real? Explain.

    E - 2. Give evidence to support the statement: “Light travels in straight lines”. Draw a ray diagram to prove this.

    E - 3. You are looking at a burning candle. Draw the path of light that enables you to see that candle.

    High School Students Studying the Sciences Physics
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