• [ended]Physics Tutoring Session #11 Light Wave on Fri. 4th March., 2016 @ 9:00 - 10:00PM

    Physics

    Chima Sunday (Tutor)
    04-03-2016 17:53:00 +0000

    This session has ended for today

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    Light is part of the electromagnetic spectrum, the spectrum is the collection of all waves, which include visible light, Microwaves, radio waves ( AM, FM, SW ), X-Rays, and Gamma Rays.

    In the late 1600s, important questions were raised, asking if light is made up of particles, or is it waves?

    Sir Isaac Newton, said yes that light was made up of tiny particles. In 1678, Dutch physicist, Christian Huygens, said Newton you are wrong, light is made up of waves vibrating up and down perpendicular to the direction of the light travels, and therefore formulated a way of visualizing wave propagation. This became known as 'Huygens' Principle'
    But today, we now know that the both were actually right. That light is dual in nature i.e it is wave like and also particle like. So today our concern is on the study of the wave nature of light.

    YOU ARE WELCOME!

    LIGHT
    Light is a visible form of energy which is radiated outward from a source. Light energy produce a sensation of vision. We see object either by the light they produce(luminous) e.g sun, stars, fire flies etc. or from the light they reflect from other object(non-luminous)
    The phenomenon of light traveling in a straight-line is called rectilinear propagation of light. Consequence of light traveling in a straight line results to Shadow and Eclipse formation.

    Magnification M in a pin hole camera of an image is defined as the ratio of the height of the image to the height of the object.
    Thus M = image height/object height or image distance/object distance or length of camera/object distance

    Reflection
    Reflection of light at plane surface is the throw back of light beam into the same medium when it is incident on a surface. We have Regular and Diffuse reflections
    Laws of Reflection
    (1) The incident ray, the reflected ray and the normal at the point of incidental lie in same plane.

    (2) The angle of incidence is equal to the angle of reflection

    Nature of Image Form by a Plane Mirror
    (i) Same size as the object (ii) Is same distance from the mirror as the object is from the mirror
    (iii) It is laterally inverted
    (iv) It is virtual
    (v) It is upright
    When two mirrors are inclined at angle, the number of image formed in such configuration is given as 360/θ - 1, Where θ is the angle of inclination of the two mirror.
    A practical application of plane mirror reflection is seen in periscope. A simple periscope contain two parallel plane mirrors fixed facing one another and inclined at angle 45 to the line joining them.
    Rotation of Mirror
    When a mirror is turned through an angle θ, its reflected ray is turned turn two times the of rotation(2θ). This implies that when a mirror is rotated through any angle say θ the reflected ray turns through twice the angle of rotation of the mirror. This result is used in some scientific instruments like navigator, sextant or mirror galvanometer.

    Solved Examples
    (1) An object 50cm tall is placed 1m in front of a pin hole camera with length 20cm. What is the height of the image and the magnification produced? If the object distance is doubled, what is the size of the image produce? and how is the magnification affected?

    Solution

    parameters given are object height = 50cm, object distance = 1m = 100cm, length of camera = 20cm.
    recall that image height/object height = length of camera/object distance
    by subject formula, image height = (object height * lenght of camera)/object distance = (50 * 20)/100 = 10cm
    So magnification M = image height/object height OR length of camera/object distance = 10/50 = 0.5
    Observe M has no unit ok.
    Now if the the object is distance is doubled i.e 2 * 100 = 200cm, size of the image produced is
    image height = (object height * length of camera)/object distance) = (50 * 20)/200 = 10/2 = 5cm, so if the object distance is double by say n, then height of the image formed is divided by that factor n.
    So M = image height/object height = 5/50 = 0.1. This implies that M is reduced.

    (2) The length of a pin-hole camera is 10cm. It is used to photograph an object 80cm away from the hole, and 10cm high. Find the height of the image and the magnification produced.
    Solution

    Recall that image height/object height = length of camera/object distance,
    so parameters given are, length of camera = 10cm, object distance = 80cm, object height = 10cm, image height = ?
    by subject formula, image height = (object height * length of camera)/object distance) = (10 * 10)/80 = 100/80 = 1.25cm
    M = image height/object height = 1.25/10 = 0.125

    (3) If two images are inclined at an angle of 30. How many images are obtainable?
    Solution

    recall that number of image is given as 360/θ - 1,
    θ = 30
    so number of image is given = 360/30 - 1 = 12 - 1 = 11 images

    Now Try These

    1) Can you state the laws of reflection?

    2) How many images are form if two mirrors are parallel to each other?

    3) A square of area 16cm2 was placed in front of a pinhole camera. If the image Magnification is 4. Calculate the area covered by the image.

    4) A man 2m tall stands 3m from a pin hole camera. How is the image in the camera affected when he now moves 100cm closer to the camera?

    5) A ray of light is incident at angle 40 to a given plane mirror. What is the angle between the incident and the reflected ray.

    ** This session will begin from 9:00pm - 10:00pm,. Before the session begins, you can put down your questions on paper and then ask them when the session officially begins by 9:00pm.

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