Physics 1230: Light and Color Ivan I. Smalyukh, Instructor

1. Office: Gamow Tower, F-521 • Email: • ivan.smalyukh@colorado.edu • Phone: 303-492-7277 • Lectures: • Tuesdays & Thursdays, • 3:30 PM - 4:45 PM • Office hours: • Mondays & Fridays, • 3:30 PM – 4:30 PM • TA: Jhih-An Yang • jhihan.yang@colorado.edu Physics 1230: LightandColorIvan I. Smalyukh, Instructor Class # 5

2. Physics 1230: Light and Color Lecture 5: Shadows, eclipses. Reading: Chapter 2, pgs. 29-68 HW due on Thursday IR camera demo/video

3. Chapter 2 – Geometrical Optics Geometrical optics is the theory of RAYS (straight lines) and how they reflect and refract (bend). • Shadows • Reflection • Refraction • Dispersion Main Topics We are here

4. 1. Shadows • Point source or diffuse source? • Umbra and penumbra • Solar and lunar eclipses • Pinhole cameras

5. Point source or diffuse source? Sun looks smaller • Point-like source of light is small in size. • Diffuse source of light has a large area. • Size of a source is measured by angle to decide. than a television

6. A point light source emits rays in all directions radially outwards The rays from twopoint light sources look like this The rays only tell us which direction the light goes in. We know that the light gets dimmer as you move further away from the light source. (Think of the sun. It would be blinding if we were closer to the sun)

7. Each point on the object emits or reflects rays in all directions (unless the object is a mirror) MANY reflected rays comefrom each point on Alex. This is diffuse reflection Incident rays from a frosted light bulb Many rays from different points on a large frosted lightbulb hit Alex's nose

8. Book Shadows appear when rays are blocked In your notebook, draw the rays that are and are not blocked by the book. Where’s the shadow? Wall Rays that are NOT blocked by the book Rays that ARE blocked by the book Shadow Point light source • If we move the wall back from the book, the shadow gets… • Smaller B) Bigger C) no change

9. Height of Gamow Tower Height of you length of your shadow -------------------- = ------------------------------ Height of tower length of tower shadow Real height: 41.5 m Height of tower Height of you Length of shadow Length of shadow

10. Notice that a shadow on the wall is the same situation, flipped Wall Height of your shadow Height of you flashlight

11. unblocked bright penumbra blocked penumbra Book bright unblocked Shadows appear when rays are blocked What else did you notice about the shadows outside? Wall The two parts of the penumbra each get light from only one of the two bulbs. The umbra gets no light from either of the two bulbs. The bright region gets light from both of the bulbs. A umbra B 2 point light sources

12. Wall We can extend the definition of the umbra and penumbra to exist in space even without a wall or screen! The light from B doesn't reach this penumbra bright penumbra A Book umbra B penumbra bright The light from A doesn't reach this penumbra

13. Concept question Shadows tell us: A) What direction the light is shining from B) That something is blocking the light C) That light travels in straight lines D) A, B, & C E) A & C

14. We can think about large light sources as being composed of many small light sources

15. Schedule: • Sun, earth and moon aligned • Moon entirely within umbra = total eclipse • Viewed anywhere on night side of earth • Why does the moon look red during an eclipse? http://antwrp.gsfc.nasa.gov/apod/image/0311/112003lunareclipse_koehn.gif

16. Based on what we know about eclipses, how do we prove this?

17. Umbra and penumbra • In the umbra, all light from the source is blocked • In the penumbra, some light is blocked. Penumbra Umbra Moon Sun What you see at the different positions:

18. Solar eclipse Geometry: moon NOTE: The umbra is usually about 200km wide http://www.mreclipse.com/Special/SEprimer.html

21. Solar eclipse • Solar eclipse - shadow of the Moon passes over the surface of the Earth. • From the Earth, we see the moon blocking the light of the Sun. • The Sun, Earth, and Moon must be lined up just right • This happens 2-5 times a year. • Since the Moon's shadow is so small, compared to the size of the Earth, a solar eclipse can be seen from only small portions of the Earth. http://micro.magnet.fsu.edu/primer/java/scienceopticsu/solar/index.html

22. Map of the total solar eclipse, Aug 1, 2008 http://en.wikipedia.org/wiki/Image:Solar_eclipse_animate_(2008-Aug-01).gif

23. blocked rays A pinhole camera works by blocking rays In your notebook, draw the rays coming from the lightbulb. What do you think will happen? Pinhole Camera Image oflight bulb Light bulb • What will the image look like? • This image is upside down and left-right reversed. Pinhole film cans Using shadows…

24. Another prediction • What do you think you will see? Pinhole Camera Image of two bulbs But let’s go outside and see stuff for real

25. blocked rays The object photographed with a pinhole camera does not have to be self-luminous! One of many rays of light shining on Alex Pinhole Camera Image ofAlex Reflected raysoff the real Alex go through the hole and make the image Alex • Once again this image is upside down and left-right reversed. • Note the correspondence between the distances object-camera-screen and image vs. object sizes

26. Finding an image by using rays is called ray tracingTrace rays from the object through the pinhole in the camera to find the image rather than trusting your intuition! Is the image of Alex smaller or larger than the real Alex? Smaller Larger Same size Is the image of Alex smaller or larger than the real Alex? Smaller Larger Same size

27. Construct and use a camera on your own (see textbook for details, pages 35 & 36)