Scholars wondered about the properties of light as early as 600 B.C.E. in Miletus, which was part of the Greek empire. We now know that light is a form of energy that travels through the universe in waves. All light energy exists in an electromagnetic spectrum. The visible spectrum, what we see as light, is part of the electromagnetic spectrum.
Experiments with a shutter
Isaac Newton (1642-1727), a brilliant English mathematician, had just received his bachelor's degree at the University of Cambridge when the bubonic plague hit Great Britain. Because the plague spread faster in cities, Newton continued his graduate studies for two years at his countryside home. During this time, he conducted many experiments. Early in 1666, Newton darkened his room and made a small hole in his shutters. After positioning a triangular glass prism in front of this small beam of sunlight, he noticed a band of colors called a spectrum. He concluded that when the light hit the prism, it was bent, or refracted, to form many colors. He demonstrated how the colors in sunlight could be separated, then joined again to form white light.
In his work, Newton proved three of the most important characteristics of light: that it travels in straight lines, that it can be reflected, and that it can be refracted, or bent. Newton also did an experiment showing sunlight's reflection and refraction inside raindrops. He discovered that raindrops formed tiny transparent prisms that reflected and refracted the Sun to produce colorful rainbows.
In 1801, Thomas Young, a London doctor, developed a theory that light traveled in waves and presented it to the Royal Society, a prestigious group of scientists. Christian Huygens of Holland had suggested the presence of light waves in his book published in 1690, but Young would go on to prove it with his experiments in 1803.
Young used a screen with one slit. In front of that, he placed another screen with two side-by-side slits, and watched how sunlight passed through. What he saw was bands of color fanning out and meeting each other on the other side. Young realized these bands of color called interference fringes could be made only by waves of light. Up to that time it was thought that there was no form to light and that it existed everywhere. Young's experiment also showed diffraction. Diffraction occurs when an uninterruped wave of light hits an obstacle. The obstacle bends the wave into a shadow zone. This results in light and dark fringes outside the shadow's edge.
It glows in the dark
Some substances produce visible light if excited by radiation, such as invisible ultraviolet light. Visible light that is produced only when the radiation source is present is called fluorescence. Certain chemicals in laundry soaps react with sunlight to produce a fluorescence that makes clothes look brighter. Visible light that is produced even after the radiation source is removed is called phosphorescence. Some plants and animals in the sea produce a phosphorescence.
Great scientists throughout history came to their conclusions about light by experimenting. Conducting some projects will enable you to become familiar with some of light's properties.
Words to Know
- The bending of light or another form of electromagnetic radiation as it passes through a tiny hole or around a sharp edge.
- Diffraction grating
- A device consisting of a surface into which are etched very fine, closely spaced grooves that cause different wavelengths of light to reflect or refract (bend) by different amounts.
- Electromagnetic spectrum
- The complete array of electromagnetic radiation, including radio waves (at the longest-wavelength end), microwaves, infrared radiation, visible light, ultraviolet radiation, X rays, and gamma rays (at the shortest-wavelength end).
- The emission of visible light from an object when the object is bombarded with electromagnetic radiation, such as ultraviolet rays. The emission of visible light stops after the radiation source has been removed.
- An idea phrased in the form of a statement that can be tested by observation and/or experiment.
- Interference fringes
- Bands of color that fan out around an object.
- A form of energy that travels in waves.
- The emission of visible light from an object when the object is bombarded with electromagnetic radiation, such as ultraviolet rays. The object stores part of the radiation energy and the emission of visible light continues for a period ranging from a fraction of a second to several days after the radiation source has been removed.
- Energy transmitted in the form of electromagnetic waves or subatomic particles.
- The bouncing of light rays in a regular pattern off the surface of an object.
- The bending of light rays as they pass at an angle from one transparent or clear medium into a second one of different density.
- Electromagnetic radiation (energy) of a wavelength just shorter than the violet (shortest wavelength) end of the visible light spectrum and thus with higher energy than the visible light.
- Something that can affect the results of an experiment.
- Visible spectrum
- The range of individual wavelengths of radiation visible to the human eye when white light is broken into its component colors as it passes through a prism or by some other means.
How to Experiment Safely
The chemicals in many detergents can irritate the skin, so avoid contact with the skin and eyes. Always use caution when handling household chemicals. Normally UV light is considered dangerous and harmful to the eyes. However, the fixture you are using emits very long wavelength UV, which is safe to use.
- Place the black light in a dark room and turn it on.
- Place a small amount of Wisk or Woolite on a piece of white paper. Let the detergent dry a little and place the paper so that the light shines on it. Notice the color of the chemical. Wisk is blue/green. Woolite is green/yellow.
- Place different objects in front of the black light, such as white socks, white or colored towels, or blue jeans. Record any color you notice. Test groups of objects such as rocks, minerals, household detergents, flowers, fabric dyes, and plastic objects.
- Repeat the test for each object. Record your observations.
Summary of Results
Keep a record or chart of the results of the project. It's fun to discover how many things glow under UV light.
Here is a problem that may arise during this project, a possible cause, and a way to remedy the problem.
Problem: None of the objects emits light.
Possible cause: The black bulb should glow a dark purple when on. If the bulb is not glowing, the light is not working. Turn the lights on in the room and unplug the black light from the wall outlet. Check to see if the lightbulb is firmly seated in its sockets on both ends. Repeat the project.
How to Experiment Safely
Do not stick your fingers into the light sockets. Make sure the fixture is unplugged before removing the bulb. Do not touch hot bulbs.
- Insert the white light bulb into the lamp. Plug the lamp in and turn it on.
- Turn off all other lights and darken the room as much as possible.
- Hold the diffraction grating approximately 0.5 inch (1.25 cm) away from your eye and look through it.
- Notice the colors of the visible spectrum. Use the colored markers to draw the spectrum on a piece of paper and label it.
- Turn the lights back on, shut off the lamp, and allow the bulb to cool.
- Unplug the lamp and remove the bulb.
- Repeat Steps 1 through 6 with each colored light.
Summary of Results
Make a chart displaying the spectrums made by the different colored bulbs. Compare your results. Write a summary of your findings.
What Are the Variables?
Variables are anything that might affect the results of an experiment. Here are the main variables in this experiment:
- the type of materials
- the distance from the material
- the light beam
- the distance from the ruler
In other words, the variables in this experiment are everything that might affect the passage of the light. If you change more than one variable, you will not be able to tell what had the most effect on how the material affected the light.
Level of Difficulty
- LED, available at hardware or electronic stores
- white paper
- aluminum foil
- wax paper
- plastic wrap
- glue stick
- a ruler with exact markings
- a dark room
- a helper
Approximately 45 minutes.
How to Experiment Safely
Avoid direct eye contact with the LED light.
- Turn off all lights and darken the room as much as possible.
- Observe light reflection: Place aluminum foil 6 inches (15 centimeters) in front of the LED light with white paper 6 inches behind the light. Have a helper turn on the LED and make a note of the location and intensity of the light.
- Repeat this process, replacing the aluminum foil with a piece of plastic wrap.
- Observe light transmission: Place a white piece of paper in front and in back of the aluminum foil. Record where you see the spot of light. Have a helper shine the LED toward the aluminum foil. (See illustration)
- Observe light refraction: Hold the LED against a ¼-inch (0.64-centimeters) piece of glue stick and turn on the LED. Note the intensity of the light. Now cover the LED with a piece of aluminum foil and again turn on the LED. Record your observations.
- Test light refraction; Hold the LED 3 inches (7.6 centimeters) above the ruler. It does not need to be exactly 3 inches (7.6 centimeters) above the ruler but you have to keep it the same distance for each material you test.
- Shine the LED on the ruler. Measure the diameter of the beam of light and note the light's intensity.
- Place a piece of wax paper against the LED and shine the light. Measure the diameter of the spot on the paper. Record the data.
- Repeat the process, replacing the wax paper one at a time with plastic wrap, ¼-inch (0.64-centimeter) piece of glue stick, and a ¼-piece (0.64-centimeters)of glue stick wrapped in foil. Each time, record the diameter of the spot and note the intensity of the light.
Summary of Results
Take a look at your data and notes. Was your hypothesis correct? When the light was directed at the glue stick, how did it differ with and without the aluminum foil? Was there one or more materials that caused the light to lose intensity? What material led to the beam of light having the largest diameter? Write a paragraph on your findings.
Experiments do not always work out as planned. Even so, figuring out what went wrong can be a learning experience. Here are some problems that may arise during this experiment, some possible causes, and ways to remedy the problems.
Problem: The light did not change in diameter as expected.
Possible cause: You may have moved the LED so that it was not the same from the ruler. Try having a friend place an object that is the same height as where you are holding the LED, and repeat the experiment.
Problem: The beam of light was not visible many times.
Possible cause: The room may not be dark enough. Try conducting the experiment in the evening, or block out more light from the windows.
Change the Variables
There are many variables you can change in this experiment. For example, you can try passing the light through a variety of materials that are only solids, such as different metals. Or you can turn the light on in front of various liquids. You can also dye the same liquid, such as water, to measure how color plays a factor in light transmission. You can also change the type of light you are using.