Lesson Objectives:- What is light
- Light as a wave
- Light as a particle
- Forms of light
- Understanding matter and atoms
- Interaction between light and matter
Prior to Newton's experiments with light in the 1660s, it was already known that passing white light through a prism created a rainbow of color called a spectrum. The visible light seen in a spectrum, however, is only a tiny part of the complete spectrum of light, which is usually called the electromagnetic spectrum. The image in the figure shows the electromagnetic spectrum, which is the complete spectrum of light. Another name for light is electromagnetic radiation.
Light is a wave, which means it transmits energy without carrying material along with it. A wave moves up through peaks, and down through troughs.
Wavelength is the distance between adjacent peaks. Frequency is the number of times the wave moves up and down each second. It is measured in hertz, or the number of cycles per second.
Wavelengths increase as we go from gamma rays to radio waves, while frequency and energy are the opposite. The longer the wavelength, the lower the frequency, and vice-versa.
Light can interact with both electrically charged particles and magnetic fields, meaning it is an electromagnetic wave. That is why it is called electromagnetic radiation and the full spectrum of light is called the electromagnetic spectrum.
Light can behave both as a wave and as a particle. Particles of light are called "photons," and are characterized by a wavelength and frequency. The higher the frequency of the photon, the more energy the light carries.
All light travels at the same speed, which is about 300,000 kilometers per second. So a radio wave moves just as quickly as visible light, which moves at the same speed as a gamma ray. The difference in light is based on the frequency and wavelength - the speed stays the same.
There are many forms of light - radio waves, infrared, ultraviolet, x-rays, microwaves, and gamma rays, with radio waves being on the left end of the electromagnetic spectrum and having the longest wavelength, to gamma rays on the right extreme with the shortest wavelength.
Visible light has a wavelength of 400 nanometers, or 400 billionths of a meter, at the blue or violet end of the spectrum and 700 nanometers at the red end. If the light has a wavelength a bit longer than red light, then it is infrared. If it has a wavelength a little shorter than blue or violet light, then it is ultraviolet.
Around the border between infrared and radio waves is often called microwaves, since the wavelengths range from micrometers to centimeters.
The light our eyes can see is inside of an extremely limited segment of the electromagnetic spectrum. The reddest light we can see has a wavelength only twice as long as the bluest blue, while the radio waves from a radio station are a billion times longer than the X rays used in a doctor's office.
Ancient Greeks believed all matter came from fire, water, earth and air. Some Greeks starting with Democritus believed that these four elements were made up of tiny particles called atoms, which comes from a Greek word meaning indivisible.
Over time, we have learned that there are actually more than 100 chemical elements, each composed of a different type of atom, and that atoms themselves are made up of particles called protons, neutrons, and electrons.
Electrical charge is the physical property that describes how an object will interact with electromagnetic fields. Protons have a positive electrical charge (written as "plus 1") and electrons have a negative charge (minus 1). Neutrons are neutral with no charge.
At the center of the atom is the nucleus, which is made up of the protons and neutrons. The electrons surround the nucleus. The atom is held together because oppositely charged particles attract, so the negatively charged electrons are attracted to the positive charge of the protons inside of the nucleus.
What makes each element unique is the number of protons in each of its atoms. The number of protons in an element is called its atomic number. For example, Carbon always has six protons, so its atomic number is six.
When you combine the number of neutrons and protons in the nucleus of an atom, you get that element's atomic mass number. Carbon usually has an atomic mass number of twelve, since a typical atom has six protons and six neutrons, but the number of neutrons can vary. Isotopes are versions of an element with a different number of neutrons, so the most common isotope of carbon is carbon-12, with six protons and six neutrons, but there are also carbon-13 and carbon-14, with seven and eight neutrons, respectively.
Although there are only 118 chemical elements that have been identified so far, the number of material substances that exist is far greater because atoms from different elements combine to form molecules. For example, water exists because two hydrogen atoms bond with one oxygen atom.
For the study of astronomy, understanding how light and matter interact is very important. Light and matter interact in four ways.
The first way is emission. Matter can emit light; for example, we see a light bulb emit visible light.
Matter can absorb light. An example of this is when you place your hand near a light bulb. Your hand absorbs some of the light and this absorbed energy warms your hand.
Some forms of matter transmit light, or allow it to pass through. Examples are glass and air.
Finally, matter can reflect or scatter light when it causes light to bounce off. If the light bounces all in the same general direction, it is called reflection, and when it bounces off more randomly, we say that it scatters.
When a material transmits light, it looks transparent, whereas when it absorbs light, it is opaque. Many materials are partially transparent or partially opaque.
The color of an object is based on what light it transmits or reflects. For example, a leaf reflects green light but absorbs all other colors.