Lesson 1: Reflection and Its Importance - Mr. Mackenzie's ...



LIGHT LESSON 1 Reflection and Its Importance

The Line of Sight

It was stated that "without light, there would be no sight."

Everything that can be seen is seen only when light from that object travels to our eyes.

Whether it be a luminous object (which generates light of its own) or an illuminated object (which reflects the light which is incident upon it), you can only view the object when light from that object travels to your eye.

In order to view an object, you must sight along a line at that object; and when you do light will come from that object to your eye along the line of sight.

Although this light diverges from the object in a variety of directions, your eye only sees the very small diverging cone of rays that is coming towards it.

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DEMO TRIAL: Investigated the manner in which light travels from an object to your eye as you view the image of the object in a mirror.

First, you use the method of parallax to locate the image of the object. Two pencils are inserted into rubber stoppers. One pencil is placed in front of a plane mirror; this was called the object pencil.. You then sight at the image of the object pencil in the mirror. As you sighted along the line of sight at the image of the pencil, you placed the second pencil behind the mirror along the same line of sight; this is called the image pencil (or virtual image). You then reposition your eye location to the other side of the pencil and repeated the process. The precise image location of the object is the location where all lines of sight intersect regardless of where the eye is located. Two important ideas can be gleaned from the Lab: one pertains to how light travels from the object to the eye and one pertains to the location of the image of an object.

As you sighted at the image of the object in the mirror, light travels along your line of sight towards you eye. In fact, that is why you must sight along a line at an object in order to see it - because light always travels along this line to your eye. The object is being illuminated by light in the room; a countless number of rays of light are reflecting off the object in a variety of directions.

When viewing the image of the object in a plane mirror, one of these ray of light originates at the object location and first moves along a line towards the mirror. This ray of light is known as the incident ray - the light ray approaching the mirror. The incident ray intersects the mirror at the same location where your line of sight intersects the mirror.

The light ray then reflects off the mirror and travels to your eye (as represented by the red ray in the diagram below); this ray of light is known as the reflected ray.

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The second important idea which can be gleaned from the Lab pertains to the location of the image. Observe in the diagram above that the image is positioned directly across the mirror along a line which runs perpendicular to the mirror. The distance from the mirror to the object (known as the object distance) is equal to the distance from the mirror to the image (known as the image distance). For all plane mirrors, this equality holds true:

Object distance = Image distance

 

 The Law of Reflection

If a ray of light could be observed approaching and reflecting off of a flat mirror, then the behavior of the light as it reflects would follow a predictable law known as the law of reflection:

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In the diagram, the ray of light approaching the mirror is known as the incident ray and the ray of light which leaves the mirror is known as the reflected ray.

At the point of incidence where the ray strikes the mirror, a line can be drawn perpendicular to the surface of the mirror; this line is known as a normal line (labeled N in the diagram). The normal line divides the angle between the incident ray and the reflected ray into two equal angles, the angle of incidence and the angle of reflection.

To view an image of a pencil in a mirror, you must sight along a line at the image location. As you sight at the image, light travels to your eye along that path. The light reflects off the mirror in such a manner that the angle of incidence is equal to the angle of reflection.

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It just so happens that the light which travels along the line of sight to your eye follows the law of reflection.

If you were to sight along a line at a different location than the image location, it would be impossible for a ray of light to come from the object, reflect off the mirror according to the law of reflection, and subsequently travel to your eye. Only when you sight at the image, does light from the object reflect off the mirror in accordance to the law of reflection and travel to your eye.

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Specular vs. Diffuse Reflection

Relection off of smooth surfaces such as mirrors or a calm body of water leads to a type of reflection known as specular reflection.

Reflection off of rough surfaces such as clothing, paper, and the asphalt roadway leads to a type of reflection known as diffuse reflection.

Whether the surface is microscopically rough or smooth has a tremendous impact upon the subsequent reflection of a beam of light.

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A beam can be thought of as a bundle of individual light rays which are traveling parallel to each other. Each individual light ray of the bundle follows the law of reflection.

If the bundle of light rays is incident upon a smooth surface, then the light rays reflect and remain concentrated in a bundle upon leaving the surface. On the other hand, if the surface is microscopically rough, the light rays will reflect and diffuse in many different directions.

The roughness of the material means that each individual ray meets a surface which has a different orientation. The normal line at the point of incidence is different for different rays. Subsequently, when the individual rays reflect according to the law of reflection, they scatter in different directions. The result is that the rays of light are incident upon the surface in a concentrated bundle and are diffused upon reflection.

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One application of this distinction between the two reflections pertains to the relative difficulty of night driving on a dry asphalt roadway compared to a wet asphalt roadway. Most drivers are aware of the fact that driving at night on a wet roadway results in an annoying glare from oncoming headlights. The glare is the result of the specular reflection of the beam of light from an oncoming car. Normally a roadway would cause diffuse reflection due to its rough surface. But if the surface is rough, water can fill in the crevices and smooth out the surface. Rays of light from the beam of an oncoming car hit this smooth surface, undergo specular reflection and remain concentrated in a beam. The driver perceives an annoying glare caused by this concentrated beam of reflected light.

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REFLECTION LESSON 1 Homework

The following diagrams depict some ideas about how light might travel from an object location to an eye location when viewed the image of the object is viewed in a mirror. Comment on the incorrectness of the following diagrams. Discuss what makes them incorrect.

1.

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2.

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3.

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4.

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5. Consider the diagram at the right. Which one of the angles (A, B, C, or D) is the angle of incidence? Which one of the angles is the angle of reflection?

6. A ray of light is incident towards a plane mirror at an angle of 30-degrees with the mirror surface. What will be the angle of reflection?

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7. Why do windows of distant houses appear to reflect the sun only when rising or setting? Use the diagram below to explain, drawing appropriate light rays on the diagram.

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8. A ray of light is approaching a set of three mirrors as shown in the diagram. The light ray is approaching the first mirror at an angle of 45-degrees with the mirror surface. Trace the path of the light ray as it bounces off the mirror; continue tracing the ray until it finally exits from the mirrror system. How many times will the ray reflect before it finally exits?

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9. If a bundle of parallel incident rays undergoing diffuse reflection follow the law of reflection, then why do they scatter in many different directions after reflecting off a surface?

10. Perhaps you have observed magazines which have glossy pages. The usual microscopically rough surface of paper has been filled in with a glossy substance to give the pages of the magazine a smooth surface. Do you suppose that it would be easier to read from rough pages or glossy pages? Explain your answer.

REFLECTION LESSON 1 HOMEWORK ANSWER KEY

1. This diagram depicts light moving in curved lines. The light by which we view objects on Earth moves in straight lines.

2. This diagram depicts the eye looking at a location that does not correspond to the image location. The eye must sight along a line at the image location.

3. This diagram depicts light passing through the mirror on the way to the mirror and on the way to the eye. Light always reflects off the mirror; and never passes through it.

4. This diagram depicts the eye looking at a location that does not correspond to the image location. The eye must sight along a line at the image location.

5. Angle B is the angle of incidence (angle between the incident ray and the normal). Angle C is the angle of reflection (angle between the reflected ray and the normal).

6. The angle of reflection is 60 degrees. (Note that the angle of incidence is not 30 degrees; it is 60 degrees since the angle of incidence is measured between the incident ray and the normal.)

7. A ray of light drawn from the sun's position at 7 pm to the distant window reflects off the window and travel to the observer's eye. On the other hand, a ray of light drawn from the 1 pm sun position to the window will reflect and travel to the ground, never making it to the distant observer's eye.

8. The light reflects twice before it finally exits the system. Draw a normal at the point of incidence to the first mirror; measure the angle of incidence (45 degrees); then draw a reflected ray at 45 degrees from the normal. Repeat the process for the second mirror.

9. Each individual ray strikes a surface which has a different orientation. Since the normal is different for each ray of light, the direction of the reflected ray will also be different.

10. It is much easier to read from rough pages which provide for diffuse reflection. Glossy pages result in specular reflection and cause a glare. The reader typically sees an image of the light bulb which illuminates the page. If you think about, most magazines which use glossy pages are usually the type which people spend more time viewing pictures than they do reading articles.

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