Chapter 12: Light

1. Chapter 12: Light Section 1: The Behavior of Light Section 2: Light and Color Section 3: Producing Light Section 4: Using Light

2. Section 1: The Behavior of Light The Nature of Light A basic question: Is light a wave or a particle? • General properties of waves: • Propagation within a uniform medium is along straight lines • Reflection occurs at a surface or boundary(known for at least 2,000 years) • Refraction occurs where a change in the speed of the wave occurs (studied and observed since 2nd century A.D.) Light has these properties, so light must be a wave In 1670, Isaac Newton demonstrated that particles travelled in straight paths and would reflect and refract, so light could be consist of particles. The question on the nature of light was a bit of a controversy. Since both waves and streams of particles behaved similarly, what exactly was light? • The key to answering the question was how the speed of light would change as it was refracted. • The wave guys said the speed would decrease, while the particle guys said the speed would increase • Once scientists were able to measure the speed of light it was determined that light’s speed decreased when it was refracted. It was concluded that light behaved as a wave.

3. Law of Reflection: = i i r r Incident Wave Reflecting Surface Section 1: The Behavior of Light Normal Line Reflected Wave The Nature of Light In the late 19th century, experiments started showing problems with the wave model, and Max Planck was able to prove that light was “bundled”, and he called these bundles of light quanta • Max Planck demonstrated the particle nature of light by the year 1901 • In 1905, Albert Einstein was the particle nature of light to explain the photoelectric effect and further confirmed the particle nature of light Today, scientist say that light is both a wave and a particle simultaneously depending on how it is observed and measured. This is called the wave/particle duality of light. Reflection Reflection can be defined as the bouncing back of a wave or particle when it strikes a surface or boundary. • Law of Reflection – the angle the incident ray makes to the normal is equal to the angle the reflected ray maketo the normal line

4. i r r r i i r i >  <  Incident ray Incident ray Section 1: The Behavior of Light medium 1 medium 1 medium 2 medium 2 Refracted ray Refracted ray Refraction Refraction – the bending of the path of a wave or particle as it travels from one medium to another As light travels from a less dense medium to a moredense medium, the wave bends toward the normal As light travels from a more dense medium to a less dense medium, the wave bends away from the normal normal line normal line

5. Section 1: The Behavior of Light Refraction Refraction occurs because the speed of light changes when the light goes from one medium to another • We know that the speed of light in a vacuum is 3.0 x 108 m/s. However, the speed of light in any other medium is always less than that value • The ration of the speed of light in a vacuum and the speed of light in a medium is called the Index of Refraction. That is: Example 1: The speed of light in a certain substance is 2.21 x 108 m/s. What is the IR of that substance? Solution Notice that IR has no units; it is just a number

6. Section 1: The Behavior of Light Refraction The IR for many transparent materials is known. If you know the IR of a medium you can determine the speed of light in that medium Example 2: A transparent solid has an Index of Refraction (IR) of 2.4195. What is the speed of light in the solid? Solution Snell’s Law Suppose we have a light traveling from one medium to another. We know that the speed of the light will changes, and because of the change in speed, the light will be refracted. Two questions: • At what angle will the light be refracted? • Does the angle at which a light travels through one medium influence the angle at which it refracts upon entering the second medium?

7. Section 1: The Behavior of Light Θ1 medium 1 N1 medium 2 N2 Θ2 Snell’s Law This diagram illustrates the variables we need to consider when trying to answer those questions: A Dutch scientist named Snell determined the relationship to be: • We can use this equation to determine the angle at which the light is traveling through either medium, or what of material either of the medium is by solving the equation for either N1 or N2 Where: N1 = the IR of medium 1 N2 = the IR medium 2 Θ1 = angle the incident ray makes to the normal Θ2 = angle the refracted ray makes to the normal

8. Section 1: The Behavior of Light Example: A light wave is traveling through crown glass at 45o, when passes into a mass of sapphire. What is the angle of refraction? Solution When solving a Snell’s Law problem your solution must include a drawing similar to the one in the example labeling the variables you know and the variable you are solving for.

9. Section 2: Light and Color Color and Wavelength • The colors of the visible spectrum differ from one another in terms of wavelength • Each combination of wavelengths has its own special sense of color • When white light strikes most objects certain colors are absorbed and certain colors are reflected, the reflected light is the color you see • The human eye is capable of distinguishing among 17,000 colors How You See Color • In the retina of the eye there two types of cells that respond to light • Rods – are light sensitive and allow you to see in dim light • Cones – detect color and control the sharpness of the image you see. They do not work in dim light. • There are three types of cone cells, each type is sensitive to a different set of wavelengths • Red receptors receive only the longer wavelengths • Blue receptors respond only to the shorter wavelengths • Green receptors react to light in the middle of the visible spectrum • Colorblind – the inability to distinguish certain colors

10. Section 2: Light and Color A Model for Color • Primary colors – red, green, and blue—when combined these colors produce any other color • Secondary colors – any color formed by a combination of two primary colors • Pigments – colors used to tint other materials • pigments are used to give paints their colors

11. Section 3: Producing Light • Incandescent Light - light produced by heating a piece of metal until it glows • Fluorescent Light - uses phosphors to convert ultraviolet radiation to visible light • Lasers - produce light waves that have the same wavelength • To create a laser a number of identical atoms are given a certain amount of energy. Once each atom has received that energy the energy is released as a light wave. Each light wave is identical to the other light waves, that is, they have the same frequency and wavelength • Coherent light - light of only one wavelength that travels with its crests and troughs aligned • Lasers produce coherent light • The beam from a laser is a constant diameter, the light does not spread out • Incoherent light - can contain more than one wavelength, and the electromagnetic waves are not aligned • Light from ordinary light bulbs is incoherent

12. Section 4: Using Light • Linearly polarized light - light with a magnetic field that vibrates in only one direction • A polarizing filter reflects all the waves except those vibrating in a certain direction • Sunglasses have a vertical polarizing filter, only light waves vibrating straight up and down pass through the lens. This vertical polarizer blocks the glare from the sun • Holography - a technique that produces a hologram, a completer three-dimensional photographic image of an object • Fiber Optics - transparent glass fibers that transmit light from one place to another • Total internal reflection - light traveling from one medium to another is completely reflected at the boundary of the two mediums • Optical scanner – a device that reads intensities of reflected light and converts the information to digital signals