Introduction: (Initial Observation)
While walking toward the pool, I noticed that the pool seems deeper as I get closer. I tried that a few times and it seems the bottom of the pool is raising as I walk away. I was wondering why this happens so I decided to do some experiments of my own. I filled up a clear glass cup with water and looked at that from the top and from the sides. It happened again. Just to make sure, I dropped a coin in the cup and I was able to see 2 coins while looking from the sides.
These observations gave me the idea of using this subject as my science project. I want to know why the bottom of a cup or pool seems to be raising when we look at the water surface from an angle? What happens to the light when it passes through water?
Information Gathering:
Find out about light and it’s properties. Read books, magazines or ask professionals who might know in order to learn about the effect of water and other liquids on light. Keep track of where you got your information from.
Observe reflection and refraction at a boundary layer.
Mirages occur when two air masses of different temperatures are up against each other. The different temperatures will mean the air masses will have different densities (one is heavier per cubic inch than the other.) Light when it passes from a medium with one density to a different density can be refracted (bent) or reflexed (bounced).
While it’s difficult to observe reflection or refraction at the boundary layer between densities of air, we can easily see it between air and the heavier medium of water.
STEP 1: Find a clear container like a glass or bottle.
STEP 2: Fill it with tap water to about the 3/4 .
STEP 3: Look down at the boundary layer between the water and the air. Notice that some of the light is reflected back so that the surface looks like a mirror. This is especially noticeable if the light source you are using is above the container.
STEP 4: Now hold the container up so that it is above you and the light source. Notice that the boundary layer from below also reflects light.
STEP 5: Take a pencil or straw and put it into the container at an angle. Observe it through the top of the boundary layer. Notice how it appears to be crooked where it passes through the boundary. The pencil isn’t bent, but the light rays passing through the boundary layer are bent. Their refraction gives the appearance that the object is bent.
STEP 6: If you have some cooking oil pour it into the top of the container until you have a layer of oil floating on top about a half inch thick. The oil should float on top of the water since it has a lighter density and does not easily mix with water. Pour slowly and try to avoid getting water bubbles in the oil. It may be necessary to gently stir the oil layer until bubbles of water caught in it fall to the bottom of the oil layer.
STEP 7: Notice there are two boundary layers now. One between the air and the oil, the other between the oil and the water. Both will show some reflection and refraction. Reflection at the oil/water boundary will be most noticeable when you look from the side of the container into the oil and down to the boundary with the water.
Bending light – refraction producing an optical illusion
Ever reached down into the bath tub water to grab a toy and found that it was not in the position it appeared to be? What you are experiencing is the effect called refraction . When light enters the water (or any transparent material) it slows down slightly. If the light enters the water at an angle then this change in speed causes the light beam to bend away from its original path. This is called refraction.
Now let’s conduct an experiment that allows us to see the effect of refraction.
- Fill the glass 2/3 full of water.
- Place the pencil in the glass holding is straight up and down (i.e. not at an angle). Notice that the pencil still appears straight when viewed through the side of the glass.
- Now take the pencil and let it lean against the side of the glass. Now look through the glass at the pencil. Notice that it appears bent. This is the effect of refraction or bending of light.
So if light is bent when passing through a transparent material does this mean that everything we see through our clear glass windows is actually distorted and not in the position that it appears to be. Yes and no. Refraction does occur but the effect of the bent light actually cancels itself out. Remember that a clear glass has 2 surfaces that the light passes through – the inside surface of the glass and the outside surface of the glass. When the light passes through the inside surface of the glass the path is bent in one direction and when the light passes back out through the outside surface of the glass it is bent in the other direction, hence canceling out the effect of refraction. Everything is pretty much located exactly where it appears to be.
What causes Rainbows?
Introduction
Everyone has seen a rainbow. They are arguably one of the most beautiful displays of nature, and they seem to come in many different sizes, situations, and settings. But what is a rainbow? If you ask, most people will say that a rainbow is “light going through raindrops or something”. This is correct, but it’s not the complete answer. In this lesson we will learn how and why rainbows form and how you can make one yourself in the classroom.
Procedure
- Gather these items from your home or workplace:
- a water glass
- a flashlight, or other directed beam of light
- a round-bottomed flask, or some spherical glass container (a round fishbowl would work)
- a small piece of cardboard or posterboard that fits over the head of the flashlight
- a 10″x14″ piece of white posterboard
- Place the glass filled with water near the edge of a table that is bathed in sunlight. As in the drawing below, a “rainbow”, or spectrum, should appear on the floor near the glass. If the sun is low on the horizon, look for the spectrum on the wall!
- What is the glass and water doing to the light? Visit the Rainbow Readings to find out…Return here when you are finished.
- Let’s repeat our experiment with the water and the light. This time we need to use the round container, the flashlight, and the small and large pieces of cardboard.
- Cut a narrow slit in the small piece of card/poster board and tape the piece to the head of the flashlight, so that only a small beam of light emerges
- Now cut a fist-sized hole in the middle of the large piece of posterboard. Also fill the spherical container with water.
- Shine the narrow beam of light from the flashlight through the hole in the posterboard and onto the container. You may need an extra person to hold the container while you shine the light and hold the white posterboard.
- What do you see on the container-side of the posterboard as you shine the narrow beam of light onto the water-filled flask? Can you position the flashlight and container so that you see a spectrum on the white paper?
- What is the shape of the spectrum on the piece of posterboard? How is this similar to a rainbow in the outdoors? How is it different?
- If you have access to a garden hose, a sprinkler, and a water spout, go outside and try to make your own rainbow!
-
- Get a fine misting sprinkler and set the water pressure high enough to get a large volume of water droplets in the air.
- Based on what we’ve learned up to now, where should you stand with respect to the water and the sun?
- Find the shadow of yourself on the ground. Try to measure the angle from the shadow of your head to the rainbow arc. Use the “hand and fist” method to measure this angle: if you stretch out your arm and make a fist, the width of your fist is about ten degrees; the width of a finger at arm’s length is about two degrees. Try it!
Continue Reading to learn more about rainbows and how they are formed.
The Pot of Gold
Think back for a moment to the last time you saw a rainbow. Where were you standing relative to the sun? Were you looking toward the sun, or away from it? When you did the activity with the flashlight, the flask of water, and the posterboard, where did you see the spectrum? How does this relate to the position of the sun when you see a rainbow?
As you stood looking at the rainbow, the sun was probably above and behind you. The rain was probably somewhere in front of you, if not landing on you at the time! Since the sun was behind and the rain in front of you, the sunlight must have been bouncing off the raindrops and reflecting into your eyes.
If the sunlight is primarily white light and is reflecting off the raindrops, why do we see colors in a rainbow? The colors are present because the sunlight is not only reflecting off the raindrops, it is also refracting and dispersing in the raindrops.
Notice in the drawing how the raindrop acts like a prism, splitting the white light from the sun (in the upper left) into its component colors. For simplification, I have only shown the red and blue beams.
But notice also how the raindrop acts like a mirror in that it reflects (some of) the refracted light back toward the sun. These refracted rays are the ones that you see as a rainbow.
When the white light from the sun hits the raindrop, the light is dispersed as it enters the raindrop, much like light is split as it passes through a prism. The separate colors are then reflected from the backside of the raindrop and exit, where they refract once again, due to the change of index of refraction between the water and the air.
The angles at which each color emerges is different (or else you wouldn’t see different colors!): red light emerges at 42° and blue light emerges at 40.6° relative to the incoming ray of sunlight.
Each raindrop contributes only one color to the raindbow that you see.
…but we saw in the rainbow activity that one raindrop can show the whole spectrum! How can a raindrop contribute only one color to the real rainbow?
Somewhere…Over the Rainbow
How much light does each raindrop contribute to our rainbow? Recall from our experiment what the spectrum from our “mock raindrop” (the round flask) looked like. Why was it a complete circle around the flask? The light wasn’t only shining on one little area of the raindrop: the whole front side was illuminated by the sun. We had white light shining on the left side and colored light emerging on the right side.
Similarly, we had white light shining on the right side and exiting as colored light on the left side of the raindrop. Actually, this was happening at every point on the sunward side of the raindrop. The result? We see a circle of light created by the incoming parallel light beams reflecting and refracting through a spherical surface.
How does this apply to a big rainbow with many raindrops?
Each raindrop is emitting the spectrum of colors, but surely a normal rainbow is far too large to be created by just one raindrop! We find that we are seeing only one color (indeed, only one tiny beam of light!) from each raindrop. In looking at a rainbow, we must be seeing the refracted light contributed from many raindrops. As it turns out, the raindrops lower in the sky contribute blue and green light; the drops higher in the sky contribute red and yellow light.
So if a single raindrop doesn’t make a rainbow, how do we get the “bow” from many raindrops?
The Antisolar Point
If we look at the ground on sunny day, the shadow of our head marks the point called the antisolar point, 180° away from the sun. If the sun is in the sky, the antisolar point is below the horizon. If the sun has set, the antisolar point is above the horizon.
What does this have to do with rainbows? The antisolar point tells us where we can expect a rainbow to form, since the colored light from the raindrops exit those raindrops at specific angles that we can measure with respect to the antisolar point.
In other words, any rainbow we see in the sky is due to the raindrops 40.6° – 42° from the antisolar point reflecting colored light into our eyes.
Gravity’s Rainbow
Here’s the scenario: we have a block of raindrops in the sky being illuminated by the sun, refracting and dispersing the light into its various colors, and reflecting those colors back. An important thing to notice is that the red light gets reflected back at 42° from our antisolar line, and the blue light gets reflected back at about 40° from our antisolar line.
That is, every raindrop 42° of the antisolar line is reflecting red light back to our eyes. If we look at every raindrop that is 42° from this line, we would see a circle of raindrops centered on the line.
Unfortunately, the horizon gets in the way of most of the rain, so we only see an arc or a bow; however, from the vantage point of a mountain-top or an airplane, full rainbows can be seen on cloud layers below.
Rainbows in 3D
Remember that rain is not falling in a flat sheet, but in varying distances from you. This causes the “rainbow circle” to be formed at varying distances. What shape is made by circles at varying distances from an apex (in this case, your eye)?
We find that the raindrops that contribute to your rainbow all lie on a cone with its apex at your eye. So when you are looking at a rainbow, you are looking at the collected light from many many raindrops which, for a fleeting moment, collectively produce a “cone of color” with its apex at your eye. If you move to the left or the right, your are looking at new raindrops, and hence, a new rainbow!
If you are admiring a rainbow with a friend, you are both seeing different rainbows.
Each rainbow is your own.
Another Rainbow Experiment:
Introduction: The Sun is the beginning source of rainbows. Because of the light of the Sun and the properties of refraction, reflection, and absorption we have rainbows.
Using thinking and observation skills this experiment will help the student learn what a rainbow is, the colors of a rainbow, how to make a rainbow, and what causes a rainbow.
Materials:
- Bright sunshine
- Hand mirror – any size
- Tin foil pie plate
- Water – to fill the pie plate
- One piece of 8-1/2″ X 11″ unlined construction or regular paper.
Procedure:
- Fill pie plate with water
- Put pie plate on a table in a very bright, sunny room.
- Place the mirror in the pan so that the mirror is underwater and leaning against the side of the pan.
- Place the pan so that the Sun is shining directly on the mirror.
- Slowly move the mirror in different directions, until you see a row of colors on the wall or the ceiling.
- Take the materials outside.
- Again set up the pie plate on a table or other flat area in full sunshine.
- Refill the pie plate and reposition the mirror so that it is leaning against the side.
- Take the piece of paper and put it in front of the pie plate and mirror and catch the colors on the paper.
- Try different angles of the pie plate and mirror to get the Sun shining on the paper correctly.
- Count the colors that you can see.
Results:
As the light strikes a solid object, it is absorbed or reflected. When it strikes a transparent object, such as a pan of water, the light rays are bent or refracted. The different colors are bent at different angles, and when the light emerges, the colors will break apart into a rainbow.
As the sunlight hits the water in the pie plate it is slowed down. This causes the colors to separate. When the light is reflected back from the mirror back through the water and into the air the colors are separated further. This is why you see the colors so clearly.
This is how a rainbow is formed. Sunlight appears white to us, but it is really made up of red, orange, yellow, green, blue, indigo, and violet. All these colors together are called the colors of the spectrum.
Question/ Purpose:
What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.
The purpose of this investigation is to find out why the objects in water seem out of place when looking at them at an angle. How does the view angle affect the angle of image?
Identify Variables:
When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.
The independent variable is the angle of view in relation to the surface of water.
The dependent variable is the displacement of image or the angle of image.
Hypothesis:
Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.
I think when the light from an object in the water is coming out of the water it will bend toward the ground or toward the surface. I also think that the amount that light bends depends on the angle that it hits the surface. Smaller angles result more bending of light.
Experiment Design:
Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”
Experiment: How does the view angle (light angle) affect the angle of image?
In this experiment I will use a laser light to observe the effect of water on light beam. Laser light is a single color light that provides me with a narrow light beam. To make the laser beam visible while it is passing through water, I will make the water cloudy by adding some milk or water color to the water.
- The ray of light which travels through the incident, or first, medium and strikes the boundary, or interface, is called the incident ray.
- The ray of light which travels into the refracted, or second, medium and leaves the interface is called the reflected ray.
- A line perpendicular to the surface is imagined at the point of refraction. This line is called a normal. In this context the word normal means perpendicular. In the above diagram the normal is colored blue.
- The angle between the incident ray and the normal is called the angle of incidence, or the incident angle.
- The angle between the refracted ray and the normal is called the angle of refraction, or the refracted angle.
Procedure:
Make a small container in the shape of a rectangular prism using small pieces of glass such as microscope slide. You can use silicon glue to connect these pieces together. Instead of glass, you may use Plexiglas or any similar clear plastic. Your prism can have 3, 4 or 5 sides. One base of the prism can be a larger sheet of hard plastic and the other base can remain open. That opening will be the top of your container.
Place two piece of wood on two sides of the prism so you can align the light beam on their surface. Fill half of the clear prism with cloudy water. One drop of milk can make your water cloudy.
Pass the light beam of laser pointer through the prism from different angles and observe the light beam that exists the prism.
Take some pictures from the top so you can later use a protractor to measure and compare the angles of light beam in water and outside water.
Can you find a relation between the angle of light beam that enters and the angle of light beam that exits the prism?
Try different light angles including vertical.
Does the light bend when the light beam hits the container in a 90º angle?
Do you see any reflections of light beam? What is the angle of reflection in relation to the angle of light beam?
Does your experiment show that light bends while passing from one medium to the other?
Use a protractor to measure the angle that the light beam makes with a vertical line at the point of entering to the water. This angle in the air will be different from the angle in the water.
Divide the angle of light beam in the air by the angle if light beam in the water and record the ratio.
Use your experiment setup as a part of your display.
Change the angle of laser light beam (entering water) to 30, 45, 60, 75 and 90. For each of these angles measure the refraction angle inside the water. Record your results in a table like this:
| Angle of the light beam entering water | Bending angle |
| 30 | |
| 45 | |
| 60 | |
| 75 | |
| 90 |
You will need a protractor to measure angles.
Make a graph:
Make one vertical bar for each angle of beam entering water (This angle is known as the angle of incidence). The height of each bar will have a numeric value equal to the the bending angle (refraction) for that specific angle of incidence. For example if the light bends 20 degrees, you make a bar that is 20 inches tall.
Bars will make a bar graph; however, you can also make a line graph by connecting the top of the bars to each other.
Materials and Equipment:
List of material can be extracted from the experiment section.
Results of Experiment (Observation):
Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.
Calculations:
You may calculate the rate of light refraction.
Summery of Results:
Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.
It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.
Conclusion:
Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.
Related Questions & Answers:
What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.
Possible Errors:
If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.
If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.
References:
Visit your local library and review a book about physics of light.
angle of incidence: The angle between an incident ray (light beam) and the normal to a reflecting or refracting surface.
