Basic Physics-Light Nature

Chapter 3
Basic Physics-Light Nature

PART I: Nature of Light, Polarization, Color

The Big Idea
Light is a wave of changing electric and magnetic fields. Light waves are caused by disturbances in the electromagnetic field that permeates the universe, for example, the acceleration of charged particles (such as electrons). Light has a dual nature: at times, it acts like waves; at other times it acts like particles, called photons. Light travels through space at the maximum speed allowed by the laws of physics, called the speed of light. Light has no mass, but it carries energy and momentum. Of all possible paths light rays will always take the path that takes the least amount of time (not distance). This is known as Fermat’s Principle.
Light, more generally known as Electromagnetic Waves (EM Waves), can be produced in many different wavelengths that can be very large to extremely small. EM waves can be polarized when produced or after going through a filter (natural or man-made). Polarization of light, means that the light wave oscillates in only one direction rather than unpolarized light that oscillates in two directions as it moves forward.
The visible range of light (i.e. the range of wavelengths that our eyes can detect) is a very narrow piece of the full EM spectrum. In the visible range our eyes differentiate between the different wavelengths by producing ‘color’ for them. When we observe something that is green, it is green to us, because the wavelength of the light hitting our eyes is around 500 nm. If the wavelength of light is slightly smaller than this it starts to look red, if it is slightly larger it looks blue. White light is the combination of all then colors. Black light is the absence of EM waves in the visible spectrum for human beings.


Key Concepts
• Light is produced when charged particles accelerate. As a result changing electric and magnetic fields radiate outward. The traveling electric and magnetic fields of an accelerating (often oscillating) charged particle are known as electromagnetic radiation or light.
• The color of light that we observe is nothing more than the wavelength of the light: the longer the wavelength, the redder the light.
• Light can have any wavelength at all. Our vision is restricted to a very narrow range of colors between red and violet.
• The spectrum of electromagnetic radiation can be roughly broken into the following ranges:

• The brightness, or intensity, of light is inversely proportional to the square of the distance between the light source and the observer. This is just one of many inverse square laws in physics; others include gravitational and electrical forces.
• Fermat’s Principle makes the angle of incident light equal to the angle of reflected light. This is the law of reflection.
• When light travels from one type of material (like air) into another (like glass), the speed changes due to interactions between photons and electrons. Transparent materials transmit the electromagnetic energy at a speed slower than c; opaque materials absorb that EM energy and convert it into heat.
A material may be transparent to some wavelengths of light but opaque to others.
Polarized light is made of waves oscillating in only one direction: horizontal or vertical. The
direction of the oscillation of the light waves is the same as the direction of oscillation of the electron creating the light. Unpolarized light can be polarized selectively by reflections from surfaces (glare); the orientation of the reflected light polarization is the same as that of the surface.
• White light consists of a mixture of all the visible colors: red, orange, yellow, green, blue, indigo, and violet (ROYGBIV). Our perception of the color black is tied to the absence of light.
• Our eyes include color-sensitive and brightness-sensitive cells. The three different color-sensitive cells (cones) can have sensitivity in three colors: red, blue, and green. Our perception of other colors is made from the relative amounts of each color that the cones register from light reflected from the object we are looking at. Our brightness-sensitive cells (rods) work well in low light. This is why things look ‘black and white’ at night.
• The chemical bonds in pigments and dyes - like those in a colorful shirt - absorb light at frequencies that correspond to certain colors. When you shine white light on these pigments and dyes, some colors are absorbed and some colors are reflected. We only see the colors that objects reflect.







Key Applications
Rayleigh scattering occurs when light interacts with our atmosphere. The shorter the wavelength of light, the more strongly it is disturbed by collisions with atmospheric molecules. So blue light from the Sun is preferentially scattered by these collisions into our line of sight. This is why the sky appears blue.
Beautiful sunsets . The blue light and some green is scattered away, leaving only red and a little green, making the sun appear red.
Key Equations
c = λ f ; The product of the wavelength λ of the light (in meters) and the frequency f of the light (in Hz, or 1/sec) is always equal to a constant, namely the speed of light c = 300, 000, 000 m/s.
Light Problem Set
1. Which corresponds to light of longer wavelength, UV rays or IR rays?
2. Which corresponds to light of lower frequency, xrays or millimeter-wavelength light?
3. Approximately how many blue wavelengths would fit end-to-end within a space of one millimeter?
4. Approximately how many short (“hard”) xrays would fit end-to-end within the space of a single red wavelength?
5. Calculate the frequency in Hz of a typical green photon emitted by the Sun. What is the  physicalinterpretation of this (very high) frequency? (That is, what is oscillating?
6. FM radio stations list the frequency of the light they are emitting in MHz, or millions of cycles per second. For instance, 90.3 FM would operate at a frequency of 90.3×106 Hz. What is the wavelength of the radio-frequency light emitted by this radio station? Compare this length to the size of your car’s antenna, and make an argument as to why the length of a car’s antenna should be about the wavelength of the light you are receiving.
7. Here on Earth you can run a solar calculator using a solar panel about 1 cm × 2 cm. Suppose you take your calculator to Neptune, 30 AU from the sun. (The earth-sun distance is defined as 1 AU, or astronomical unit).
a. What fraction of sunlight intensity hits your solar panel at Neptune compared with at Earth?
b. Design a solar panel (size) that would allow you to collect the same amount of solar power (same amount of light energy in same time) on Neptune as on Earth.
8. Our sun is 8 “light minutes” from earth.
a. Using the speed of light as c calculate the earth-sun distance in m, then convert it to miles ( one mile = 1609 m)
b. The nearest galaxy to our Milky Way is the Andromeda Galaxy, 2 million light years away. How far away is the Andromeda Galaxy in miles?
9. Consult the color table for human perception under the ‘Key Concepts’ section and answer the questions which follow.
a. Your coat looks magenta in white light. What color does it appear in blue light? In green light?
b. Which secondary color would look black under a blue light bulb?
c. You look at a cyan-colored ribbon under white light. Which of the three primary colors is your   eye not detecting?
10. Why is the sky blue? Find a family member who doesn’t know why the sky is blue and explain it to them. Ask them to write a short paragraph explaining the situation and include a sketch.
11. Describe the function of the dye in blue jeans. What does the dye do to each of the various colors of visible light?
12. Explain why polarized sunglasses are often used by skiers, boaters, drivers, etc. Which way would the lenses be polarized? Draw a diagram to illustrate your answer.
Answers:
1. IR
2. millimeter-wavelength light
3. 2200 blue wavelengths
4. 65000 xrays
5. 6 × 1014 Hz
6. 3.32 m
7. a. 1/900b. 9 m × 18 m
8. a. 8.9 × 107 mb. 1.9 × 1022
9. a. blue b. yellow
c. red
d. red
10. .
11. .
12. .

Authors : James Dann, James H. Dann
@CK-12 Foundation

CK-12 Foundation is a non-profit organization with a mission to reduce the cost of textbook materials for the K-12 market both in the U.S. and worldwide. Using an open-content, web-based collaborative model termed the “FlexBook,” CK-12 intends to pioneer the generation and distribution of high-quality educational content that will serve both as core text as well as provide an adaptive environment for learning,
powered through the FlexBook Platform™.

Printed: March 23, 2011


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