Blue Skies Blue Eyes Blue Butterflies

Transcription

1 Blue Skies Blue Eyes Blue Butterflies Friday, April 19 Homework #9 due in class Lecture: Blue Skies, Blue Eyes & Blue Butterflies: Interaction of electromagnetic waves with matter. Week of April 22 Lab: Diffraction of Light Monday, April 22 Reading Quiz #13 due (this is the original due date). This is based on Chapter 17. Lecture: Wave Optics. This will be a shorter treatment than originally anticipated. Tuesday, April 23 Recitation: Defining relationships exercise and quiz on wave optics. Wednesday, April 24 Lecture: Review & Synthesis: We'll treat a few capstone topics, and review for Exam #2. Friday, April 26 Exam #2. This will now cover Chapters 24, 25, 28, 30 and parts of Chapter 17. Don't worry about Chapters 26 and 29; we did small pieces of each, but not enough to worry about. The following week, we'll start out treatment of ray optics, which is the material of Chapters 18 and 19. This will follow the schedule as outlined in the syllabus. Homework #10 (new version!) will be due in class on Wednesday, May 1. Electromagnetic waves interact with matter This can absorb the wave, slow it, redirect it, reflect it.

2 True for any wave. v = f λ Light doesn t always travel at the speed of light. v = c n index of refraction Light of wavelength 500 nm in air enters a glass block with index of refraction n=1.5. When the light enters the block, which of the following properties of the light do not change? Think about what happens to the waves when they hit the block. speed wavelength frequency color

3 What Determines Color? The wavelength of a light wave is 700 nm in air; this light appears red. If this wave enters a pool of water, its wavelength becomes: λ = λ air 1.33 = 530 nm If you are swimming underwater, this light still appears red. Given this, what property of a light wave determines its color? Give a physical reason why you know this must be true. (Hint: think of light as a photon, and think of energy...) What does the existence of the tell you about the index of refraction of water and the index of refraction of air? Mirages There is no water in this picture. Total internal is the cause.

4 There is no lake here. Making a Rainbow Seeing a Rainbow

5 Scattering. Cloud droplets are much bigger than the wavelength of light, so scatter all wavelengths of light more or less equally. Scattering from Large Objects Skin is made of transparent components, with differing indices of refraction. The size of the elements is a bit larger than the wavelength of light. Net result: No pigment means white skin.

6 Match index of refraction: Transparent. Scattering from Small Objects Air molecules are very small compared to the wavelength of light. Short wavelengths scatter more. No Pigment, But Still Blue Scattering from melanocytes Scattering from particles in feathers Scattering from air molecules

7 Scattering from Small Objects Rayleigh scattering For small objects: Scattering 1 λ 4 Blue Skies Red Sunsets Longer wavelength: Less scattering After Sunset...

8 If you can see the sky, you are getting burned. Constructive & Destructive Interference Constructive: Displacements add Destructive: Displacements cancel Interference from two sources Constructive: Displacements add Destructive: Displacements cancel

9 Just checking. Two speakers are emitting identical sound waves with a wavelength of 4.0 m. The speakers are 8.0 m, apart, directed toward each other, as in the diagram below. At each of the noted points in the above diagram, say whether the interference is constructive, destructive, or something in between. Now, try it in 2D... Two speakers emit identical sinusoidal waves. The speakers are placed 4.0 m apart. A listener moving along a line in front of the two speakers finds loud and quiet spots as shown below. The grid lines are spaced at 1.0 m. What is the wavelength of the sound from the two speakers? Quiet Loud Quiet Speaker 1 Speaker 2 Thin Film Interference

10 Interference Colors How do you make a mirror with biological materials? Front Rear Thickness t Crystals n=1.83 Light of wavelength 600 nm in air passes into the layer of crystals with the noted index of refraction. What is the wavelength of the light in this layer?

11 Front Rear Thickness t Crystals n=1.83 λ = λ 0 n Front Rear Thickness t Crystals n=1.83 Given the indices of refraction of the cells and the crystals, there will be a phase shift for the front. As noted in the text, the effective path-length distance is thus 2t + ½λ. Front Rear Thickness t Crystals n=1.83 What is the path length difference for the two s, in terms of the thickness t?

12 Front Rear Thickness t Crystals n=1.83 If the layer of crystals is 80 nm thick, what is the longest wavelength for which there is constructive interference? What is this wavelength in air? Structural Color in the Blue Morpho Butterfly Structure of the Scales.

13 Changing Index, Changing Color. Four Ways To Be Blue