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This lab will explore several related aspects of light refraction.
Everyone knows that a strong magnifying glass can concentrate rays of the sun onto a point and potentially start a fire or at least scorch wood or paper.  But some people have asked Internet help sites if they need to worry about where they hang a wind chime or ornament with a sphere of glass that dangles in the sun.  Could sunlight shining through the glass sphere concentrate rays of light and set curtains on fire?
The answer is possible yes if you consider the following news story from Washington State back in 2009.

Unit 5 Optics Lab Theory

This lab will explore several related aspects of light refraction.

Everyone knows that a strong magnifying glass can concentrate rays of sun onto a point and potentially start a fire or at least scorch wood or paper. But some people have asked Internet help sites if they need to worry about where they hang a wind chime or ornament with a sphere of glass that dangles in the sun. Could sunlight shining through the glass sphere concentrate rays of light and set curtains on fire?

The answer is possibly yes if you consider the following news story from Washington State back in 2009.

Dogs’ water bowl may be cause of fire in Bellevue

Let’s investigate using physics. First light carries energy. Fires start when a flammable substance has its temperature raised above a certain temperature, called the ignition temperature. The ignition temperature of paper is 451ºF (see Ray Bradbury’s book named after this temperature). When energy is concentrated on a point, the point’s temperature rises.

But why do the rays of light bend when they pass through transparent glass or water? To get a better understanding of how light passes through transparent objects, open the following simulation: Bending Light. If there is not link for this simulation on Canvas, use the search term “PHET Physics Bending Light” (make sure the simulation uses HTML 5. Or type in this URL: https://phet.colorado.edu/en/simulation/bending-light.

After it opens, click on the center option, Prisms. Once it is selected, click on it again.

The simulation will look like this:

Click and drag.

Click on the light source that shows 4 red lines extending from it. Then drag a spherical shape to the work area. Turn on the light by clicking on the red dot.

Hover the mouse near the light and Click on the laser when you see green arrows extending up, down, left and right. Click and drag the light up and down and observe where the light converges, called the focal point. This shows the paths rays of light take passing through the sphere, which is made of glass (using the default object material).

Answer the following questions based on your observations.

As you move the light up and down where do you observe the rays converging to a point after it passes through a glass spehre?
(Put an X in the yellow box next to your choice.)

a

All the rays converge to the same point just to the right of the sphere.

b

The rays that pass through the center of the sphere converge to a point just to the right of the sphere, but the rays near the top and bottom pass closer to the sphere.

c

The rays diverge, spreading out and never crossing paths.

Change the material of the sphere to water and do the same test.

How are the results different from the glass sphere?

a

The rays converge farther from the sphere for water than for glass.

b

The rays converge at the same point for water and for glass.

c

The rays converge closer to the sphere for water than for glass.

Now change the light source to produce a single beam and have it pass through the top part of the sphere. Change the material back to glass.

Change the color from red to violet and back to red while looking at the path of the light after emerging from the sphere.

Which of the following statements is true?

a

Red light bends more than violet light (emerges at a steeper angle).

b

Red light bends less than violet light (emerges at a less steep angle).

c

All colors of light bend the same amount.

You can test this idea by changing the light source to white light and looking at the light that emerges down at the bottom right. Do you see the colors begin to separate?

Answer the question on the next page.

Which do you conclude is the most accurate summary of this investigation?

a

All colors and all paths of light through glass or water converge to the same point, making it an excellent focusing tool for sunlight.

b

Both water and glass cause light to converge on the same point for all paths of light, but different colors focus at slightly different points, slightly diminishing the focusing power of the sphere.

c

Both water and glass spheres concentrate light, but all the light does not converge to the same point and all the colors do not follow the same path. Furthermore, the focus of a water sphere is farther from the edge of the sphere, making it a better focusing tool than glass, although both could concentrate sunlight somewhat.

So a transparent sphere can act like a magnifying lens and focus sunlight. But could it light something on fire? You’ll have a chance to investigate this in the lab activity that follows this assignment.

Cylinder as Magnifier or Lens

A lens bends light in a manner similar to a sphere or a cylinder. A cylinder of transparent material, such as water, can either transmit words on a piece of paper on the other side of the cylinder such that they look normal or it can flip the image sideways. You will use the simulation to explore how light moves through a cylinder to create these two effects. When you do the lab activity, you can see the effect for yourself.

Set up the simulation using the five parallel laser beams (any color) and two water circles to recreate the following arrangement.

When rays spread out from a point, it simulates the way light moves out from a point, diverging or spreading out.

Now place a second circle of water in the path of the diverging rays just to the right of the point from which the rays diverge.

What happens to the rays after passing through the second circle of water?

a

They spread out less wide than the rays spread out without the 2nd circle, but they do not converge.

b

They come out parallel to each other.

c

They converge toward a point.

Take a screen shot of your simulation showing what you observed.

***Place screen shot here.***

For instructions on how to take a screen shot, see the end of this assignment.

Now move the second circle to the right of the point from which the rays diverge so that the rays pass through the second sphere moving roughly parallel to each other.

Now what happens to the rays after passing through the second circle of water?

a

They spread out wider than the rays spread out without the 2nd circle.

b

They come out parallel to each other.

c

They converge toward a point.

Take a screen shot of your simulation showing what you observed.

***Place screen shot here.***

During the lab activity, you’ll set up a physical example of this and observe how things look through the cylinder when placed at various distances behind the cylinder.

Using Refraction to Look around Corners

For this part, set up a single laser so that the light will emerge inside a square block (material = water). The laser ray should move up toward the horizontal surface of the square, like this:

Answer the following question.

What happens to the ray as it comes out of the water at the top

a

It comes out and bends more toward the vertical.

b

It comes out and does not bend at all

c

It comes out and bends toward the horizontal.

Take a screen shot of your simulation showing what you observed.

***Place screen shot here.***

In the lab activity, you’ll demonstrate this by observing a coin in the bottom of a mug without and with water in the mug.

The Reason Light Bends

So we have seen in simulation that light bends when it passes into or out of transparent materials. Why does this happen.

Look in the textbook for what this phenomenon is called (Chapter 9). When light bends on passing into or out of transparent materials, it is called

a

Reflection

b

Refraction

c

Diffraction

d

Dispersion

To learn more about why this happens, return to the simulation and click on More Tools tab (see the black area at the bottom of the simulation).

Turn on the laser and drag a tool called “Speed” from the toolbox on the left. Change the material to water. To measure the speed of the light, place the triangle on the left side of the tool in the path of the light. The unit is “c” which stands for the speed of light in a vacuum (no air). For red light slide the slider to the extreme right; for violet, the exteme left.

Record the speed for the following situation:

Material

Speed of Red Light

Speed of Violet light

Air

Water

Glass

With the top material set to Air and the bottom set to Water, answer the question about which way the light bends.The dotted verticle line is called the “Normal”. It is perpendicular to the surface at the point the ray enters the material

If the ray bends this way, it is called bending “away from perpendicular” or “away from the normal.”

If the ray bends this way, it is called bending “toward perpendicular” or “toward the normal.”

When light passes from air into water, the light

a

slows down

b

does not change speed

c

speeds up

When this happens, the light

a

bends away from the perpendicular

b

does not change path

c

bends toward the perpendicular

Now change the upper material to water and the bottom material to air. Now the light is moving from inside the material out into the air. For this case

When light passes from water into air, the light

a

slows down

b

does not change speed

c

speeds up

When this happens, the light

a

bends away from the perpendicular

b

does not change path

c

bends toward the perpendicular

How to make a screen shot:
1. First, make the simulation fill the screen.
2. Press the keyboard key labeled “Print Screen” or “Prnt Scn” or something similar. It usually is to the right of the function keyes along the top row of keys on the keyboard and above the Insert/Home/Page Up keys.
3. There now should be an image copy of the screen on the clipboard.
4. Navigate in Word to the place you wish to insert the screen shot.
5. Paste. (The easiest way to paste is to hold down the CTL key and press “v”.)
6. The image should appear, sized to fill the page.
7. Crop the image if you want to eliminate extraneous elements but this is optional. (To crop, right-click on the image and look for the “Crop” option.)

Alternate way to make a screen shot: If you computer has a snipping tool, open that and after selecting a section of your screen, copy and paste it into the document.
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