01 – Introduction to Light & its Speed PW2 - Reflection 01 – Introduction to Light & its Speed

Plan of Attack Light is a very vast and diverse concept. Therefore it will be split up into two (2) smaller, more manageable units Each of these smaller units will have their own unit exam Light Part 1: Reflection Topics include: basic light facts, speed of light, plane mirror reflection, curved mirror reflection Light Part 2: Refraction refraction of light, lenses, vision problems & correction

Introduction to light – some fun facts Wave-particle Duality: Idea that everything exhibits both wave and particle behavior There is much debate over what light is: particle or wave Particle Model of light: endorsed by Issac Newton Believed light was a stream of fast moving particles – much like the flow of electrons Wave Model of light: Light is in fact a wave, since it displays a large number of wave-like properties (i.e. – shape, wavelength, refraction, reflection etc.) For the purposes of Physical Science 20, we will study light in terms of it being a wave & exhibiting wave behavior – however, it is important to keep an open mind and think critically about the concept of light both in and after you finish this course

Introduction to light – some fun facts Light behaves like a transverse wave. A “ray” is a straight line representing the path of light. Light refers to the small range of frequencies of electromagnetic radiation that the eye can detect. The visible spectrum of light has an ordered arrangement of 7 colors. Each of the 7 colors has its own wavelength and frequency. An object will reflect all wavelengths of light that correspond to its color.

Introduction to light – some fun facts Light can travel in a vacuum. The speed of light, c= 3 x 108 m/s in a vacuum Light travels in straight lines. Light will refract when it crosses, on an angle, a boundary between 2 mediums. Light will diffract when it encounters an obstacle Light can be bent by gravity. Light is not bent by magnetic or electric fields.

What is Light? Light: electromagnetic wave light naturally travels in straight lines. This is called “rectilinear propagation”. the only time it does not travel in a straight line when it passes through a strong gravitational field. produced by vibrating electrons is a transverse wave but vibrates in many directions “planes” unlike our slinky which was vibrating in one plane (horizontal) A light wave which is vibrating in more than one plane is referred to as unpolarized light Polarizing Sunglasses – they block light waves in the horizontal plane with vertically oriented polarizers. This greatly reduces glare. Good for driving, biking, sports etc.

Electromagnetic Spectrum The electromagnetic spectrum is the range of wavelengths that light can exist in. An electromagnetic wave is simply a light wave. However, we reserve the term light for the part of the spectrum we can see. This is called the visible part of the spectrum. Other parts of the spectrum have different names. It is important to note that each type of wave is a light wave, but just has a different wavelength, energy, and behaviors/purposes.

Electromagnetic Spectrum

Speed of Light - History Although the speed of light is well known and commonly used today, the origins of how its value was determined are highlighted by three influential scientists 1) Galileo 17th century, was the first person to try to measure the speed of light. He and his assistant stood on hilltops 1 mile apart with lanterns. Galileo would flash his light at his assistant, who opened a shutter on his lantern as soon as he saw Galileo’s light. Galileo timed how long it took to see his assistant’s light. However, he found that the time of the event was too small to measure.

Speed of Light - History 2) Ole Roemer 1600’s; first to determine that light had a definite, measureable speed made careful observations of Jupiter’s moon “Io”. Since Io has a very stable orbit, Romer concluded he should be able to predict where Io is in its orbit. However, he noticed that sometimes the moon was behind schedule and sometimes it was ahead, depending on the time of the year. Romer discovered that Io was ahead of schedule when Earth was closest to Jupiter and behind when they were farthest apart. This is due to the fact that light has to travel farther when Earth and Jupiter are far apart. Based on his observations & data, he concluded that light took 22 minutes to cross the diameter of Earth’s orbit However, further research has proven that light actually takes 16 minutes to cross Earth’s orbit, not 22

Speed of Light - History 3) Albert Michelson In 1926, conducted an experiment in which light was send through a simulated vacuum Constructed a 35km long pipe and removed all of the air Measured the time it took for the light to travel the distance of the pipe Came up with a value of 2.997996 x 108 m/s Won a Nobel Prize for his discovery Value of 3.00 x 108 m/s has been used for the speed of light ever since

Speed of Light – Values & Applications Speed of light that we will be using in this class will be: 3.00 x 108 m/s This value has its own symbol: “c” For calculations, use the same formulas that you are comfortable with, however, plug in “c” for velocity For example: v = d/t now becomes c = d/t v = fλ now becomes c = fλ

Speed of Light – Values & Applications Example: A planet is 1.486 x 109 m away from the earth. How long would it take a beam of light to reach the planet from earth? c = d/t  manipulate equation to get t = d/c d = 1.486 x 109m c = 3.00 x 108m/s t = ? Plug in your numbers & go t = 1.486 x 109m 3.00 x 108m/s t = 4.953 s

Speed of Light – Values & Applications Example: What is the frequency of yellow light, λ = 556nm? c = fλ  rearrange equation to get f = c/λ f =? c = 3.00 x 108m/s λ = 556nm x 1 m = 5.56 x 10-7m 109nm Plug in your numbers and go f = 3.00 x 108m/s 5.56 x 10-7m f = 5.40 x 1014Hz

Speed of Light – Long Distances Light Year the distance light travels in 1 year. Note that it is not a measure of time! 1 light year = 9.46 x 1015m Astronomical Unit (AU) the distance between the sun and the earth 1 AU = 1.50 x 1011m Parasec Definition & derivative beyond the scope of the course 1 parasec = 3.26 light years or 3.09 x 1016m