What is Light?

Introduction: (Initial Observation)

Without light we can see nothing. Sunlight is by far the most important source of energy on the earth and our entire solar system. Without sunlight no known type of life could exist on the earth. All plants and animals are depended on light.

Although we can see light, we still have many questions about that. What is it? Can you fill up a bottle with light and use it when you need some? What is light? Is it a substance? Matter? Energy? Wave?

In this project you will investigate the light in an attempt to learn about the nature of the light.

For those of you who want to study light as a part of an experimental science project, with hypothesis, results table, graph and the whole nine years, the following are some specific questions that you can study.

1. Does the light energy change by distance from the light source?
2. What color lights have the most energy? Does invisible light also have any energy? (I am not suggesting any experiment for this question. You will need plastic or glass color filters for this experiment. Procedures re very similar to the experiment 3 described below. The only difference is that instead of changing the distance of the light source to the radiometer, you will change color filters so each time a different color light will get to the radiometer.)

Information Gathering:

Find out about light. Read books, magazines or ask professionals who might know in order to learn about the nature of the light and how it may get affected by different factors. Keep track of where you got your information from.

Following are samples of information that you may find:

Experiments on light indicate that light is in wave form. At the same time some other experiments shows that light is moving particles. The dual nature of light cannot be explained with what we have learned in regular physics. A different branch of physics known as quantum physics is used to study the physics of very small particles like electrons.

While regular/Newtonian physics can suitably describe the orbit of the planets or the energy transformations during a game of pool, quantum physics describes how electrons surround the nucleus of the atom and other subatomic actions.

After gathering information I have prepared the following project plan or task list:

1. Setup an experiment to test or demonstrate the wave nature of the light.
2. Setup an experiment to test or demonstrate the particle nature of light.
3. Setup an experiment to see if light energy changes by changing the distance from the light source?

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 project is to learn about the nature of light. I will perform experiments to show if light is a wave, particles, or both.

My specific question about light is:

1. Does light energy change according to distance from the light source?

Select only one of the above questions for your project.

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.

Depending on the question, there are different variables that can be defined.

For my specific question:

Independent variable is the distance from a light source.

Dependent variable is relative amount of energy.

Constants are the light source, exposure time, method and instruments.

Controlled variable is environmental lights (visible and invisible) that may have unexpected effect on our results.

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.

Following is a sample hypothesis:

My hypothesis is that light is formed of particles traveling in the form of a wave. So light is both wave and particles. For my specific question I think:

• The light energy is reduced by increasing the distance to the light source.

My hypothesis is based on my gathered information and personal observation.

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 1: Wave nature of the light (Young’s experiment).

Introduction:

Waves have certain properties that can easily be observed by looking at water waves.

When two waves cross, they will amplify or cancel each other at different points.

When a wave hits a slit that is smaller than its wave length, it diffracts through the slit and propagates on the opposite side in a perfect semicircular pattern.

If light is in wave form, we must be able to observe the same behavior with light.

In this experiment we test a red laser beam to determine if it demonstrates the wave properties.

The reason that we use a laser beam is that laser is a pure single frequency (single color) light that does not fade easily. All other lights are a mix of different wave lengths. Other lights can also produce a similar result. If you are able to transmit a beam of sunlight into a dark room, you can use sunlight instead of laser light.

Material:

Small white projection screen, photocopier to copy and reduce the templates provided, photocopier transparencies, rigid cardboard holders for transparencies, laser pen (available at warehouse discount centers and large national retail chains for around $20), ruler, white paper

Procedure:

Make a single slit and a double slit for your experiment. You can make your slits in two different ways.

• Method 1
  To make a single slit, remove the black plastic disks inside a floppy diskette, cut it in a straight line, place the pieces about 1/2 mm apart. Attach them in that position using a few pieces of tape. This will be a single slit for your experiment.
  To make a double slit, remove the black plastic disks inside a floppy diskette, cut it in a straight line, place the pieces about 2 mm apart. Attach them in that position using a few pieces of tape. Then cut another strip about 1 mm wide and mount it in the middle of the slit. This strip will divide the single slit to two slits or a double slit.
• Method 2
  In this method you will print the slits on transparencies using a laser printer.

You can print the two images that you see in the right or make your own drawings in windows paint program. The width of each slit is only one pixel.

When your slits are ready, first try the single slit.

Turn on your laser light and hold it about one foot away from the slit. Let the light pass trough the slit and get to the screen or wall about 4 feet away. What do you see? Does light spread? (This is called diffraction)

Now try double slit.

Turn on your laser light and hold it about one foot away from the double slit. Let the light pass trough the slit and get to the screen or wall about 4 feet away. What do you see? Do you see the locations where light waves cancel or amplify each other?

Did your experiment show the wave nature of light?

You may repeat the same experiment with a beam of sunlight entering a dark room and get a similar result. However, you cannot use a flashlight for this experiment unless your flashlight is able to produce a narrow beam similar to sunlight or similar to laser light.

Following are some links related to this experiment.

http://school.discovery.com/lessonplans/activities/demonstratingthewave/index.html

http://www.bu.edu/core/cc105/lectures/L7-Waves/Ripple.html

http://www.physics.fsu.edu/courses/fall98/phy2049c/labs/L10/lab10.pdf

http://www.bottomlayer.com/bottom/reality/Chap1.html

http://micro.magnet.fsu.edu/primer/java/doubleslitwavefronts/

http://micro.magnet.fsu.edu/primer/lightandcolor/particleorwave.html

Experiment 2: Setup an experiment to test or demonstrate the particle nature of light.

Introduction: Moving particles must have an energy stored in them that makes them move. Such energy is called kinetic energy. For example, a moving ball may have enough energy to break a window.

Kinetic Energy is a function of mass and velocity. KE can be calculated using the formula in the right.

The kinetic energy of the wind is used to spin wind turbines and that is our way of getting energy from the wind.

If light is made of moving particles, the kinetic energy of these particles must be able to spin a very sensitive turbine.

Since light particles are very small, the sensitive turbine that we use must be in vacuum or air molecules stop it from spinning. Such a turbine is called a radiometer.

You can make your own radiometer or purchase a radiometer for your experiment.

A radiometer has four vanes suspended inside a glass bulb. Inside the bulb, there is a good vacuum. When you shine a light on the vanes in the radiometer, the particles (photons) bouncing off the silver side of the vanes push the vanes, causing them to rotate. However, this force is exceedingly small.

How to make a Radiometer (without vacuum)?

A radiometer is an instrument which uses reflection and absorption to
measure solar energy.

Material:

1. black felt tip marker & pencil
2. scissors
3. strong glue
4. thread
5. Aluminum foil or chewing gum wrapper (long – flat type)
6. ruler
7. match stick
8. jar

*Note: Chewing gum wrapper is far better as it’s much easier to color the
back. It is, however, possible to color the dull side of the tin aluminum foil.

Procedure:

1. Color the paper side of your wrappers with the black pen/ marker.

2. Cut the wrappers into 4 pieces measuring 2 cm by 2.5 cm.

3. Make a small fold at one end of each piece, about 1/2 cm from the end.

4. Stick these pieces on one end of the matchstick with the shiny surfaces all facing the same way.

5. Glue a length of thread to the matchstick.

6. Wrap the loose end of the thread around a pencil and secure it.

7. Now hang the radiometer inside your jar, making sure it hangs freely
and does not touch the sides or bottom of the jar.

8. Place the jar in a very sunny position and watch the radiometer turn
as the solar energy is absorbed by the black surfaces and reflected by the
shiny surfaces. See what direction the vanes are moving in. Do photons (light particles) push the silver side?

Partial Results: You may notice that the vanes are turning in an opposite direction than what you expect. This radiometer and many other inexpensive radiometers in the market are not vacuumed or are partially vacuumed. Such radiometers spin in light because dark colors absorb the sun’s rays and so get hotter more easily whereas light colors reflect more light and so reflect more of the sun’s heat rays away. The turning action of these radiometers has to do with heat, not particles.

This experiment will not provide a conclusive result unless you have access to a fully vacuumed radiometer and be able to see that light beams are pushing the reflective side of the vanes.

Draw a conclusion:

Do your results support the particle nature of the light? Why?

(Note that you can have a totally different conclusion. Don’t think that other scientists are always right.)

Experiment 3: Does the light energy change by distance from the light source?

Introduction: Light energy can be measured in different ways. Since here we discussed radiometers and we know that radiometers are easily available in the market, we will use a radiometer to measure the light energy.
Exposing the radiometer to the light for a certain amount of time, will make it spin and speed up. When we stop the light, the vanes continue to spin for a certain period of time depending on the strength of the light source and its exposure time. We measure this time and use it as an indication for the light energy.

Procedure:

1. In a low light or dark room, place the radiometer on a desk so it will not vibrate or move.
2. Surround the back, two sides and top of radiometer with black cardboards so it will get light only from one direction.
3. Place a desk lamp or portable light at a distance of 4 feet to the radiometer. If your light has a reflector, make sure it is faced to the radiometer.
4. Make sure that the radiometer is not spinning; turn on the light and keep it on for exactly 5 seconds.
5. As soon as you turn off the light, use a stopwatch and see how many seconds it takes for the vanes inside the radiometer to come to a full stop.
6. Repeat steps 4 and 5 two more times and take the average of the number of seconds it takes for vanes to stop. Record this number in your results table.
7. Place your light source at a distance of 3 feet from the radiometer and repeat steps 4 and 5 three times. Take the average of the number of seconds it takes for vanes to stop. Record this number in your results table.
8. Place your light source at a distance of 2 feet from the radiometer and repeat steps 4 and 5 three times. Take the average of the number of seconds it takes for vanes to stop. Record this number in your results table.
9. Place your light source at a distance of 1 foot from the radiometer and repeat steps 4 and 5 three times. Take the average of the number of seconds it takes for vanes to stop. Record this number in your results table.

This is how your results table may look like.

You can also use the above table to make a bar graph or line graph.

* You can optionally try other distances such as 5′ and 6′.

Materials and Equipment:

This is the list of material for experiment number 3.

1. A light source with a 65 to 100 watts light bulb. Incandescent light or condensed fluorescent lights are preferred.
2. Radiometer
3. Ruler or meter stick
4. Stopwatch or any watch that shows seconds

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:

The only calculations that you need is calculating the averages in experiment number 3.

Summary 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:

List of References

http://www.nobel.se/physics/articles/ekspong/

http://www.colorado.edu/physics/2000/schroedinger/

http://www.astro.ufl.edu/~gomez/astrolab/Light_Wave_files/frame.htm

Quantum Physics