Reflection, Geometrical Optics, physics 10

Physics 10.VI Geometrical Optics T.Trường 9/6/17 1,054 0
  1. Reflection, Geometrical Optics, physics 10
    1/ Reflection

    When you smile into a mirror, you see your own face smiling back at you. This is caused by the reflection of light rays on the mirror. Reflection occurs when a light ray bounces off a surface.
    2/ Terminology
    In Chapters 4 and 5 we saw that when a pulse or wave strikes a surface it is reflected. This means that waves bounce off things. Sound waves bounce off walls, light waves bounce off mirrors, radar waves bounce off aeroplanes and it can explain how bats can fly at night and avoid things as thin as telephone wires. The phenomenon of reflection is a very important and useful one.

    We will use the following terminology. The incoming light ray is called the incident ray. The light ray moving away from the surface is the reflected ray. The most important characteristic of these rays is their angles in relation to the reflecting surface. These angles are measured with respect to the normal of the surface. The normal is an imaginary line perpendicular to the surface. The angle of incidence, θ$_{i}$ is measured between the incident ray and the surface normal. The angle of reflection, θ$_{r}$ is measured between the reflected ray and the surface normal. This is shown in Figure 7.4.

    When a ray of light is reflected, the reflected ray lies in the same plane as the incident ray and the normal. This plane is called the plane of incidence and is shown in Figure 7.5.
    [​IMG]
    Figure 7.4: The angles of incidence and reflection are measured with respect to the surface normal.
    [​IMG]
    Figure 7.5: The plane of incidence is the plane including the incident ray, reflected ray, and the surface normal.

    3/ Law of Reflection
    The Law of Reflection states that the angles of incidence and reflection are always equal and that the reflected ray always lies in the plane of incidence.
    Definition: Law of Reflection
    The Law of Reflection states that the angle of incidence is equal to the angle of reflection.
    θ$_{i}$ = θ$_{r}$
    The simplest example of the law of incidence is if the angle of incidence is 0◦. In this case, the
    angle of reflection is also 0◦. You see this when you look straight into a mirror.
    [​IMG]
    Figure 7.6: When a wave strikes a surface at right angles to the surface, then the wave is reflected directly back. If the angle of incidence is not 0◦, then the angle of reflection is also not 0◦. For example, if a light strikes a surface at 60◦ to the surface normal, then the angle that the reflected ray makes with the surface normal is also 60◦ as shown in Figure 7.7.
    [​IMG]
    Figure 7.7: Ray diagram showing angle of incidence and angle of reflection. The Law of Reflection states that when a light ray reflects off a surface, the angle of reflection θ$_{r}$ is the same as the angle of incidence θ$_{i}$.

    Worked Example : Law of Reflection
    Question:
    An incident ray strikes a smooth reflective surface at an angle of 33◦ to the surface normal. Calculate the angle of reflection.
    Answer
    Step 1 : Determine what is given and what is required

    We are given the angle between the incident ray and the surface normal. This is the
    angle of incidence. We are required to calculate the angle of reflection.
    Step 2 : Determine how to approach the problem
    We can use the Law of Reflection, which states that the angle of incidence is equal to the angle of reflection.
    Step 3 : Calculate the angle of reflection
    We are given the angle of incidence to be 33◦. Therefore, the angle of reflection is also 33◦.

    4/ Types of Reflection
    The Law of Reflection is true for any surface. Does this mean that when parallel rays approach
    a surface, the reflected rays will also be parallel? This depends on the texture of the reflecting
    surface.
    [​IMG]
    Figure 7.8: Specular and diffuse reflection.
    Specular Reflection
    Figure 7.8(a) shows a surface that is flat and even. Parallel incident light rays hit the smooth surface and parallel reflected light rays leave the surface. This type of reflection is called specular reflection. Specular reflection occurs when rays are reflected from a smooth, shiny surface. The normal to the surface is the same at every point on the surface. Parallel incident rays become parallel reflected rays. When you look in a mirror, the image you see is formed by specular reflection.
    Diffuse Reflection
    Figure 7.8(b) shows a surface with bumps and curves. When multiple rays hit this uneven surface, diffuse reflection occurs. The incident rays are parallel but the reflected rays are not. Each point on the surface has a different normal. This means the angle of incidence is different at each point. Then according to the Law of Reflection, each angle of reflection is different. Diffuse reflection occurs when light rays are reflected from bumpy surfaces. You can still see a reflection as long as the surface is not too bumpy. Diffuse reflection enables us to see all objects that are not sources of light.

    Activity :: Experiment : Specular and Diffuse Reflection
    A bouncing ball can be used to demonstrate the basic difference between specular
    and diffuse reflection.
    Aim:
    To demonstrate and compare specular and diffuse reflection.
    Apparatus:
    You will need:
    1. a small ball (a tennis ball or a table tennis ball is perfect)
    2. a smooth surface, like the floor inside the classroom
    3. a very rough surface, like a rocky piece of ground
    Method:
    1. Bounce the ball on the smooth floor and observe what happens.
    2. Bounce the ball on the rough ground floor and observe what happens.
    3. What do you observe?
    4. What is the difference between the two surfaces?
    Conclusions:
    You should have seen that the ball bounces (is reflected off the floor) in a predictable manner off the smooth floor, but bounces unpredictably on the rough ground. The ball can be seen to be a ray of light and the floor or ground is the reflecting surface. For specular reflection (smooth surface), the ball bounces predictably. For diffuse reflection (rough surface), the ball bounces unpredictably.

    High School Students Studying the Sciences Physics
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