Light Waves Interacting with Matter (Chapter 7)

Student Learning Objective Determine the behavior of electromagnetic waves as they interact with different types of matter.

How does “light” interact with matter? Light interacts differently with different types of matter.   Surface Material Angle Wavelength Absorption Some opaque materials absorb light.

Practice What is an opaque material? Do all opaque materials absorb light? If a material absorbs radiant energy, what is the result?

Light travels through transparent materials. Transmission Light travels through transparent materials. Absorbed & re-emitted Passes through Air Glass Air Light Ray

What is the law of reflection? Light may bounce from a surface back into the original medium. Parallel Reflection

The law of reflection applies to all reflective surfaces.  Incoming light rays and outgoing light rays have the same angle with respect to the normal line. I R Mirror The law of reflection applies to all reflective surfaces.

Does the law of reflection apply to diffuse reflection? Practice Does the law of reflection apply to diffuse reflection?

Virtual Image A flat mirror reflects all light rays in the same direction. Upright Same size Same distance Left-right reversed

Curved Mirrors Convex mirrors always produce a smaller image. Concave mirrors usually produce a larger image.

Practice Which mirror has the larger field of view, concave or convex? What kind of mirror is the side mirror on your car?

When and how is light refracted? Refraction Light will change speed and may change direction when it is transmitted from one material to another.   Wavelength Angle of incidence Material (n = c/v) Temperature

Moon Looks large Refraction affects where objects appear to be. Object sizes may be magnified. Moon Looks large

The Sunset

Practice 1) In which substance, would light travel faster, ethyl alcohol or air? 2) Calculate the speed of light in ethyl alcohol. 3) As the light goes from air into the ethyl alcohol, will it bend toward or away from the normal line?

When does total internal reflection occur? Light approaching a boundary at the critical angle is reflected rather than transmitted.

Fiber Optics & Diamonds

https://www.youtube.com/watch?v=EvHIedXw7Xo

How is a rainbow formed? Dispersion Visible light is divided into separate colors.

Each person sees their own set of colors (rainbow) from a particular set of raindrops. Sunlight http://www.physicsclassroom.com/class/refrn/Lesson-4/Rainbow-Formation

Practice Which color of light is refracted the most in a raindrop? How is a secondary rainbow formed?

What do lenses do to light? Convex Lens (true image) Concave Lens (virtual image)

What type of lens is the human eye? Question What type of lens is the human eye? http://www.mysciencesite.com/optics4.html

What is polarized light? Light is many transverse waves vibrating in many directions. Polarized light has only one direction of vibration.

Practice How could you determine whether sunglasses are truly polarized? Do polarized glasses make a difference? http://www.prescriptionglassesonline.net/cheapglasses/?help122.html http://www.prescriptionglassesonline.net/cheapglasses/?help122.html

What causes diffraction? Diffraction occurs when light waves bend around corners. The amount of diffraction depends on the how the size of the wavelength compares to the size of the barrier. Shadows

How are colors produced? Most of the colors you see in the world are due to selective reflection. Daytime Nighttime

Practice 1) What does selective reflection mean? 2) What is selective transmission? What is an example of this? 3) Why does the sky appear to be blue mid-day?

The sky on Earth appears blue because blue (and violet) photons are scattered as they collide with air particles. The sky on Earth appears red at sunset because the light must pass through a lot of atmosphere.

The Sun’s Rays in Space

Practice Both blue and violet photons are scattered by our atmosphere. Why does our sky appear to be mostly blue, and not violet, at mid-day? What color would our sky be if atmospheric particles were slightly larger?

More Practice Why is the sky black on the moon? 4) Why do stars twinkle?

https://www.youtube.com/watch?v=cjn0D-SoGlM

Wien’s Law T(Kelvin) = 3,000,000 lmax (nm) Some objects have a particular color because of temperature. Every object has a specific wavelength (color) at which it radiates most of its energy. T(Kelvin) = 3,000,000 lmax (nm)

Practice Our Sun has a surface temperature of 5,800 K. What is the wavelength of maximum emission for our Sun? What color is this? What is the surface temperature of a star for which the wavelength of maximum emission is 425 nm? What color would this star be?

More Practice 3) If the burner on your electric stove is 500 ºF (260 ºC), what is the wavelength of maximum emission? What “color” corresponds to this wavelength?