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L1

Page history last edited by gerryc 12 years, 2 months ago

L1 - Light travels in straight lines called rays. These rays can be reflected from non-luminous objects or refracted through transparent objects.

 

Nice introduction to all these concepts - by Bill Nye the Science Guy. This is just part 1 (includes how to make a periscope).

 

Click on these links for Part 2 (has stuff on lenses) and Part 3 (stuff on lasers including a nice experiment).

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Student Outcome L1.1 Understand that when light hits a mirror or highly reflective surface the angle of incidence is equal to the angle of reflection.

 

The Line of Sight

"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.

 

As you look at Mary in class, you are able to see Mary because she is illuminated with light and that light reflects off of her and travels to your eye. In the process of viewing Mary, you are directing your sight along a line in the direction of Mary. If you wish to view the top of Mary's head, then you direct your sight along a line towards the top of her head. If you wish to view Mary's feet, then you direct your sight along a line towards Mary's feet. And if you wish to view the image of Mary in a mirror, then you must direct your sight along a line towards the location of Mary's image. This directing of our sight in a specific direction is sometimes referred to as the line of sight.

It is a rather simple principle:

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.

A luminous objects emits light in a variety of directions; and an illuminated object reflects light in a variety of directions. 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. If your eye is located at a different location, then you would see a different cone of rays. Regardless of the eye location, you will still need to sight along a line in a specific direction in order to view the object.

 

When a ray of light strikes a plane mirror, the light ray reflects off the mirror. Reflection involves a change in direction of the light ray. The convention used to express the direction of a light ray is to indicate the angle which the light ray makes with a normal line drawn to the surface of the mirror. The angle of incidence is the angle between this normal line and the incident ray; the angle of reflection is the angle between this normal line and the reflected ray. According to the law of reflection, the angle of incidence equals the angle of reflection. These concepts are illustrated in the picture below.

  

What is true of light is also true in eight ball!

 

A fairly clear if unexciting video demonstrating this law.

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Open and print this file to get two copies of paper protractors. It is not the same as the one's above but makes it easier measure angles.

 Paper protractors v2003.doc

 

Student Outcome L1.2 Explain that when light travels through transparent substances it bends and changes direction.

 

The Angle of Refraction

Refraction is the bending of the path of a light wave as it passes across the boundary separating two media. Refraction is caused by the change in speed experienced by a wave when it changes medium.

 

If a light wave passes from a medium in which it travels slow (relatively speaking) into a medium in which it travels fast, then the light wave will refract away from the normal. In such a case, the refracted ray will be farther from the normal line than the incident ray.

 

On the other hand, if a light wave passes from a medium in which it travels fast (relatively speaking) into a medium in which it travels slow, then the light wave will refract towards the normal. In such a case, the refracted ray will be closer to the normal line than the incident ray is.

 

The question is: "By how much does light refract when it crosses a boundary?" Perhaps there are numerous answers to such a question. (For example, " a lot," "a little," "like wow! quite a bit dude," etc.) The incident ray is a ray (drawn perpendicular to the wavefronts) which shows the direction which light travels as it approaches the boundary. Similarly, the refracted ray

 

is a ray (drawn perpendicular to the wavefronts) which shows the direction which light travels after it has crossed over the boundary

.

 

In the diagram, a normal line is drawn to the surface at the point of incidence, This line is always drawn perpendicular to the boundary. The angle which the incident ray makes with the normal line is referred to as the angle of incidence. Similarly, the angle which the refracted ray makes with the normal line is referred to as the angle of refraction.

 

 

The amount of bending which a light ray experiences can be expressed in terms of the angle of refraction (more accurately, by the difference between the angle of refraction and the angle of incidence). A ray of light may approach the boundary at an angle of incidence of 45-degrees and bend towards the normal. If the medium into which it enters causes a small amount of refraction, then the angle of refraction might be a value of about 42-degrees. On the other hand if the medium into which the light enters causes a large amount of refraction, the angle of refraction might be 22-degrees. (These values are merely arbitrarily chosen values to illustrate a point.) The diagram below depicts a ray of light approaching three different boundaries at an angle of incidence of 45-degrees. The refractive medium is different in each case, causing different amounts of refraction. The angles of refraction are shown on the diagram.

Of the three boundaries in the diagram, the light ray refracts the most at the air-diamond boundary. This is evident by the fact that the difference between the angle of incidence and the angle of refraction is greatest for the air-diamond boundary. 

 

Here is an experiment showing this law. It is a bit complex for Year Nine so you will need to modify. It does have the good idea of using a diagram of a protractor on the white piece of paper that you do the experiment on.

 

This might be better. A simple applet that show not only what happens when you change the angle of the incident light but also the density of the medium. Go here. (From a website called "Flash Animations for Physics")

 

Here is another one (at a site called Molecular Expressions) showing the same thing...but differently?!?

 

Here is my Geogebra Interactive on Refraction

 

This photo shows how refraction can fool you if you are not on top of the idea.

 

Nifty little video showing some other refraction tricks.

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Student Outcome L1.3 Show that total internal reflection occurs when the angle between the light ray and the normal is greater than the critical angle.

To understand total internal reflection, we will begin with a thought experiment. Suppose that a laser beam is submerged in a tank of water (don't do this at home) and pointed upwards towards water-air boundary. Then suppose that the angle at which the beam is directed upwards is slowly altered, beginning with small angles of incidence and proceeding towards larger and larger angles of incidence. What would be observed in such an experiment? If we understand the principles of boundary behavior, we would expect that we would observe both reflection and refraction. And indeed, that is what is observed (mostly). But that's not the only observation which we could make. We would also observe that the intensity of the reflected and refracted rays do not remain constant. At angle of incidence close to 0 degrees, most of the light energy is transmitted across the boundary and very little of it is reflected. As the angle is increased to greater and greater angles, we would begin to observe less refraction and more reflection. That is, as the angle of incidence is increased, the brightness of the refracted ray decreases and the brightness of the reflected ray increases. Finally, we would observe that the angles of the reflection and refraction are not equal. Since the light waves would refract away from the normal, the angle of refraction would be greater than the angle of incidence. And if this is the case, the angle of refraction would also be greater than the angle of reflection (since the angles of reflection and incidence are the same). As the angle of incidence is increased, the angle of refraction would eventually reach a 90-degree angle. These principles are depicted in the diagram below.

The maximum possible angle of refraction is 90-degrees. If you think about it (a practice which always helps), you recognize that if the angle of refraction were greater than 90 degrees, then the refracted ray would lie on the incident side of the medium - that's just not possible. So in the case of the laser beam in the water, there is some specific value for the angle of incidence (we'll call it the critical angle) which yields an angle of refraction of 90-degrees.

 

This photo has not been touched up in any way. The red beam is clearly a laser pointing to the surface of the water and being fully refracted back into the water. (Source: http://sciencedemonstrations.fas.harvard.edu/icb/icb.do)

 

 

This is how total internal reflection can be used on bikes and cars:

 

 

You can actually lose coins because of this and beakers as well!

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Note: if you want to demo this, put the coin under the beaker!                 For this video, you need to add oil.

 

 

 

Source for most information on this page: Source: http://www.glenbrook.k12.il.us/gbssci/phys/class/refln/u13l1c.html - an excellent site by the way if you want a deeper understanding of light.

 

 

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