PH104 Lab 1 Light and Matter Pre-lab

Transcription

1 Name: Lab Time: PH04 Lab Light and Matter Pre-lab. Goals Since this is the first lab, we don t want to try to do things that are too complex. We would like to get used to the lab room and some of the steps that we will be using in all of our experiments. Our goals at the end of this week will be: To be able to make observations of a phenomenon, specifically light. To be able to create questions from observations. To understand the role of experiments in answering our questions. To be able to take and analyze data, specifically spectra. To be able to answer questions based on our experiments. Keep in mind that definitions and facts that you will be expected to know are contained in this lab and in the text.

2 2 PH04 LAB. LIGHT AND MATTER PRE-LAB.2 Pre-Lab Material in this section should be completed prior to attending lab. Material in the pre-lab will be covered in the lectures and the book..2. Introduction Astronomy is a science of distance. Unlike much of physics, nearly everything that we know about planets, stars, and galaxies is done with no human hands touching the objects being studied. How do we get the information about the universe? We use light. There are some things we need to get out of the way before we begin this pre-lab. This pre-lab is about light and matter. When we talk about light and matter, we use some standard units to describe light and some standard units to describe matter. Question#: Look in your text or online to find the following standard units: The standard unit of the wavelength of light The standard unit of the frequency of light The standard unit for the energy of light The standard unit for the mass of an atom The standard unit for the speed of light Question#2: Light does not move infinitely fast, it has a speed we normally write this as c. Write down the standard speed of light c in the space below remember units. c = (.).2.2 Observations In all of science observations play a vital role in our inquiry into the nature of the universe. Therefore, we should begin our labs with some observations. Observations

3 .2. PRE-LAB 3 are simple statements about a phenomenon that can lead to questions about that phenomenon. Example: White light(light with no obvious color) can strike an object, but the object will show a color - like the yellow of a sunflower. A question could be: What property of white light would make objects have color? Another question would be: What property of matter allows it to be seen as different colors? Question#3: What other everyday observations about light can we make that will lead to questions? These questions are open-ended, but they need to be thought out. You may need to go outside and think about these things..2.3 Questions After we have made our observations, we should be able to out together a list of questions based on those observations. Many times these are just repeats of the observations with a why or what attached to them. Question#4: What sorts of questions can we ask based on our observations?.2.4 The Effect of Distance on Light Now that we have practiced with observation and question, let s put this into use with some astronomical observations. We want to know what effect distance has on the brightness of an object. The brightness of an object can be thought of as the amount of light we receive from that object. Observations We will start with an analogy. Suppose you have two identical light bulbs.

4 4 PH04 LAB. LIGHT AND MATTER PRE-LAB Question#5: If you set both light bulbs at the same distance from you, what observations can yo make about them? Question#6: If you take one of the light bulbs put it twice as far from you as the other light bulb, what observations would you make about the light bulbs? Now let s make another observation. Below is a table of the planets with their distances from the Sun in astronomical units(au), how much sunlight they receive compared to the Earth, and the time (in minutes) it takes a photon of light to reach the planet from the Sun. An astronomical unit is defined as the average distance between the Earth and the Sun. This is 50 million km or 93 million miles Planet Distance from Amount of Light Travel Name the Sun (AU) Sunlight (Earth) Time (min.) Mercury Venus Earth 8.3 Mars Jupiter Saturn Uranus Neptune Pluto

5 .3. THE NATURE OF MATTER 5 Question#7: simple chart? What observations about light and distance can you make from this Instead of a question about light and distance, we have some definitions to make from these observations. The amount of light an object receives from any source decreases as one over the square of the distance to the source. Brightness = distance 2 (.2) Since the speed of light is a constant (300, 000 km sec ), the amount of time it takes for a photon from a source to reach an object increases with distance. This means that the further away we look from Earth, the further back in time we are seeing..3 The Nature of Matter.3. Observations Here I will make some observations, but you will be required to supply the observations. All matter is made up of the chemical elements. Most of us have heard of hydrogen, helium, oxygen, carbon, and all of the plentiful elements. How many of us know about Praseodymium or Ytterbium? Yet, there are traces of these two elements in and around us all of the time. All of the elements are made up of even smaller items called protons, neutrons, and electron. These three subatomic particles make up all of matter.

6 6 PH04 LAB. LIGHT AND MATTER PRE-LAB.3.2 Questions These two observations can lead to an interesting question. Question#8: What questions about protons, electrons, and neutrons can you come up with? It has been shown through time that the protons and neutrons collect together in the center of atoms. The electrons occupy orbits around the nucleus of the atom. The most simple model of an atom then is one electron in a circular orbit around one proton. This element is known as hydrogen. Hydrogen has an atomic number and an atomic mass number of as well. Task#: Draw a diagram that would best represent a hydrogen atom. Diagrams are labeled pictures. Make sure you label all of your diagrams with the appropriate parts. Below is a table with some of the common elements that we will discuss in astronomy. This list contains the elements name, the number of protons, electrons, and neutrons, the atomic number, and the atomic mass number. All of these atoms are considered to be in the neutral state. Question#9: What observations can you make from this table?

7 .4. CONCLUSION: CONNECTING LIGHT AND MATTER 7 Element Number of Number of Number of Atomic Atomic Mass Name protons neutrons electrons Number Number Hydrogen 0 Helium Carbon Nitrogen Oxygen Iron There are some rules that come out of this knowledge. In a neutral atom, the number of protons and electrons is always the same. In an atom, the atomic number is the number of protons. In an atom, the atomic mass number is the number of protons plus the number of neutrons. The different properties of the elements depends on the number of protons, electrons, and neutrons in the element..4 Conclusion: Connecting Light and Matter This is where we build a model of matter and its interaction with light..4. The Definition of Light When light propagates (moves) through space, it is acting like a wave. Just like a water wave light has a wavelength (λ), amplitude (A), and frequency (ν). There are some properties of waves in general and light waves specifically that you should know. The wavelength of a wave is measured from crest top to crest top. The Amplitude of the wave is half of the height between trough and crest. The frequency is a measure of the amount of wave crests passing a given point in one second. The speed of a wave s is calculated by multiplying the wavelength times the frequency (s = λ ν).

8 8 PH04 LAB. LIGHT AND MATTER PRE-LAB Since the speed of light is a constant (300,000 km sec ), all light waves travel at the speed of light. So as frequency increases, wavelength must decrease, and vice versa. Task#2: Draw a diagram of a wave and label it with the amplitude and wavelength. Below that diagram, give the definition of frequency as you understand it. Visible light (light that we see) is only a small part of the many wavelengths of electromagnetic radiation called the electromagnetic spectrum. Gamma rays, X- rays, UV radiation, visible light, Infrared radiation, microwaves, radio waves, and all wavelengths in between are all part of the same phenomenon. The only difference is in their respective wavelengths and frequencies. Thus the same phenomenon that makes visible light can create X-rays and gamma rays. When light interacts with matter, it acts more like a particle. That particle is called a photon. When we talk about photons, we sometimes use the Greek letter gamma (γ). A photon of light has a specific energy associated with it. That energy is proportional to the frequency and inversely proportional to the wavelength (E γ = h ν γ or E γ = h c λ γ ). Question#0: Two photons are recorded by your measurement instruments. Photon A has a frequency of 20 million Hz. Photon B has a frequency of 5 million Hz. Which Photon has the higher energy?.4.2 The Interaction of Light with Matter First... The Definition of Energy Levels Electrons in an atom can orbit the nucleus at different distances. These different distances require the electron to have different energies. Electrons in an atom can occupy different energy levels. Experiment has shown that these energy levels are quantized. This means that electrons in at atom are allowed to have some energies and not allowed to have other energies. Some of the allowed energies for hydrogen are shown in the table below. Note: The lowest energy level is called the

9 .4. CONCLUSION: CONNECTING LIGHT AND MATTER 9 Energy Energy Level (ev) Ground State Ionized 3.6 ground state of the atom. This is the preferred state for an atom. If possible an electron will always give off enough energy to go back into the ground state. Light and Matter Since the preferred state for the electron is the ground state, some energy must be introduced into the atom in order for the electron to jump energy levels. Photons of light offer this energy. Since light is composed of all wavelengths 0 2 m to 0 3 m, it is possible for some light striking an atom to have the right energy to help the electron jump up an energy level. Remember that this can t be just any energy. If a photon of 0.2 ev interacts with a hydrogen atom, the electron can absorb the photon and jump up an energy level. If a photon of 0 ev interacts with a hydrogen atom, the electron will ignore it, and the photon will go right by as though nothing was there. This means that if I shine a white light on a gas of hydrogen, some of the light will go right through while some of the light will be absorbed by the hydrogen. What would this look like? Think about this. When photon with 3.6 ev of energy interacts with a hydrogen atom the atom will ionize. The electron will have enough energy to no longer be bound to the proton. This is called ionization. Any atom that has more protons than electrons is called ionized. When there are no more electrons around a nucleus we call this fully ionized. When an electron is in an energy level higher than the ground state it will give off energy to jump back down to the ground state. That energy will come off as a photon of light, but not just any photon of light. Hydrogen will not give off 0 ev of light. An electron in the second energy level will give off exactly 0.2 ev of light in order to drop to the ground state. So, if we have an energized gas of hydrogen all of the electrons will be giving off light in order to drop to lower energy states. Since only specific wavelengths of light can come off, the light should not be white (white light is made up of all possible wavelengths in the given range of wavelengths). What would this look like? This is the end of the pre-lab. Bring this completed to your lab time.