Physics 1230: Light and Color Ivan I. Smalyukh, Instructor Office: Gamow Tower, F Phone: Lectures: Tuesdays & Thursdays, 3:30 PM - 4:45 PM Office hours: Mondays & Fridays, 3:30 PM – 4:30 PM TA: Jhih-An Yang Class # 9

HW #1 (out of 10 points maximum)

Exam #1: September 29 or October 4?

Clicker question: which of the following statements is true A. We can see two rainbows after the rain; B. The observation of primary rainbow involves total internal reflection, refraction at 2 droplet-air interfaces, and dispersion; C. The observation of the secondary rainbow involves two total internal reflections, refraction at 2 droplet-air interfaces, and dispersion; D. All of the above; E. None of the above. 4

5 How we see two rainbows sun total internal reflection two total internal reflections

6

7 Total internal reflection makes fiber optic communication possible The ray bends from the low n material toward the high n material. Demo: glass tube, laser pointer

Time for a demo! 8 If we pull the cork, and water starts to pour out of the tank, the laser light will… A)Shine across the room to the wall. B)Stay entirely inside the tank C)Stay entirely inside the water stream D)Something else happens.

9 This illustration appears in "La Nature" magazine in Demo: tank

10 Lec. 6: Ch. 2 - Geometrical Optics 1.Shadows 2.Reflection 3.Refraction 4.Dispersion Move to Chap. 3 1.Virtual images We are here

11 How does the eye make sense of all these light rays? You are not aware of all those rays, but rather of 3 points of light A)Uniform background B)Several points of light C)This is not an image D)No idea.

12 Chapter. 3 – Spherical Mirrors and Lenses 1.Virtual images (review) 2.Spherical mirrors 3.Spherical lenses 4.Aberrations of lenses We are here Skip 3.3c anamorphic art. We are here

13 Review: plane mirrors, specular reflection Equal angle rule Similar triangles are useful Ray tracing a mirror X object = X image Image point is on the normal (mirror might need an extension)  r ii Normal Mirror X object X image Extension

14 Review: What is an object? What is an image? X object X image Extension = We will often replace a real object (say a bottle) with an arrow, to simplify the drawing.

15 Review: What is an object? What is an image? X object X image Extension = In this context, an object is a point, or set of many points, that emits light rays in a range of directions:

16 What is an image? X object X image Extension =

17 Curved Mirrors First, a little geometry review: This line segment (from center of circle)......is perpendicular (or normal) to this tangent.

18 Ray aimed toward center of sphere comes straight back (specular reflection with normal incidence) What about other rays? All rays aimed at the center C come straight back out. C Rays reflecting from a convex (spherical) mirror

19  r ii Specular reflection Find where incoming ray hits mirror surface Find surface normal at that point (along line from center--remember geometry review?) Angle of incidence = angle of reflection Reflection (of parallel ray) looks like it’s coming from F -- turns out this is true for all parallel rays! The focus is halfway to the center C F What happens to all rays that come in parallel to the OPTICAL AXIS (the line running through C)?

20 Easy rule for parallel incoming rays (parallel to the line through F and C): they are reflected as if they came from F. Focal point = focus is behind the surface C F What happens to all rays that come in parallel? The focus is halfway to the center

21 Easy rule for rays aimed at focus: An incoming ray aimed at F gets reflected back parallel (to the C-F axis). Focal point = focus is behind the surface C F What about rays aimed at the focus? (This is the previous rule, backwards) The focus is halfway to the center

22 1.All rays incident parallel to the C- F axis are reflected so that they appear to be coming from the focal point F 2.All rays that (when extended) pass through the center C are reflected back on themselves. 3.All rays that (when extended) pass through the focal point F are reflected back parallel to the axis 2 C F 1 3 Three easy rules for convex, spherical mirrors

23 Ray tracing: convex mirror C F What does the observer see in the mirror?

24 Ray tracing: convex mirror C F What does the observer see in the mirror? Draw in the rays and extrapolate back

25 Ray tracing: convex mirror C F (A) Or (B) ?? Is the image real or virtual? Is the image larger or smaller than the object? Is the image right-side-up or upside-down? How could a mirror be useful when used like this?

26 Ray through the center reflects straight back at its source C Rays reflecting from concave (cavity) mirrors

27 All (incoming parallel) rays reflect and go through the focus half way from center to mirror C F Incoming parallel rays reflect through focus As usual, this rule works backwards: incoming rays that go through the focus reflect back parallel (to the C-F axis).

28 C F Rays through focus reflect back parallel to C-F axis.

29 C F Concave mirrors are very useful light beam emitter (flashlight) solar light collector or oven

30 1.All rays incident parallel to the C-F axis are reflected through the focal point F 2.All rays that pass through the center C are reflected back on themselves. 3.All rays that pass through the focal point F are reflected back parallel to the axis C F Three easy rules for concave, spherical mirrors

31 Ray tracing: concave mirror object outside center CF

32 Ray tracing: concave mirror object outside center CF Questions: (A) OR (B) Is the image real or virtual? Is the image larger or smaller than the object? Is the image right-side-up or upside-down? How could a mirror be useful when used like this?

33 Ray tracing: concave mirror object between center and focus CF Questions: Is the image real or virtual? Is the image larger or smaller than the object? Is the image right-side-up or upside-down? How could a mirror be useful when used like this?