Physics 1230: Light and Color

Transcription

1 Physics 1230: Light and Color Exam 1 is tomorrow, Wed. June 9, in class. Covers material from Chapter 1, pgs 1-25, Lectures and Homework 1-3. HW4 will be up soon. Due Thursday, 5PM Lecture 5: Shadows, eclipses. Exam 1 Review. Reading: Chapter 1 for review. Chapter 2, pgs

2 Which level of physics is needed to explain what properties of light? Image formation - ray theory Wavelength color, polarization and diffraction - wave theory (electricity and magnetism) Interaction of light with atoms - quantum theory of photons Constant speed of light no matter how fast the source or observer is moving - special theory of relativity We have to stay focused to make progress! RAY THEORY for now.

3 Chapter 2 Geometrical Optics Geometrical optics is the theory of RAYS (straight lines) and how they reflect and refract (bend). Lots of similarity to GEOMETRY of lines and triangles. Main Topics We are here 1. Shadows 2. Reflection 3. Refraction 4. Dispersion 3

4 1. Shadows a. Point source or diffuse source? b. Umbra and penumbra c. Solar and lunar eclipses d. Pinhole cameras 4

5 A point light source emits rays in all directions radially outwards The rays from two point 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)

6 Shadows appear when rays are blocked Wall Wall Rays that ARE blocked by the book Point light source Rays that are NOT blocked by the book Book If we move the wall back from the book, the shadow gets A) Smaller B) Bigger C) no change A B 2 point light sources unblocked Book unblocked blocked umbra 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.

7

8 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

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

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

11 An "extended object" consists of many points. Each point on the object emits or reflects rays in all directions (unless the object is a mirror) MANY reflected rays come from each point on Alex. This is diffuse reflection Incident rays from a frosted light bulb

12 The more rays that reach a point the brighter the point This is why regions outside the penumbra and umbra are brighter These regions get light rays from both point light sources The more lights you turn on the brighter the reflected light from objects in the room See rays at right Light source 2 Light source 1 Reflected rays from light 1 Reflected rays from light 2 Your eye sees a brighter nose than with either light source alone

13 An extended light source such as the sun (or a large light bulb) also produces an umbra and penumbra in empty space behind the Earth (or another object) All rays coming from point A on the sun between the two dashed rays are blocked by Earth All rays coming from point B on the sun between the two dotted rays are blocked by Earth The umbra gets no light from any portion of the sun The umbra gets smaller not larger further behind Earth since the Sun is larger than Earth The penumbra gets light from part of the sun If you look back from the penumbra you can see part of the sun When the moon passes completely into the umbra there is a total eclipse of the moon. When the moon passes into the penumbra there is a partial eclipse of the moon Rays from this part of the sun DO reach the upper penumbra A Sun B Penumbra Umbra Penumbra Rays from this part of the sun DON'T reach the upper penumbra because they are blocked by Earth

14 Solar eclipse Geometry: moon NOTE: The umbra is usually about 200km wide

15 Total solar eclipse During a solar eclipse, the shadow of the Moon passes over the surface of the Earth. From the Earth, we can see the moon blocking the light of the Sun. Looking at the demonstration, you may think that solar eclipses happen very often. The Sun, Earth, and Moon must be lined up just right, in order for a solar eclipse to take place. This happens only two to five 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.

16 Map of the total solar eclipse, Aug 1,

17

18 Schedule:

19 Lunar eclipse (partial and total) During a lunar eclipse, the moon passes through the shadow of the Earth. As we look at the moon from the Earth, it looks to us as if the shadow of the Earth is slowly covering the moon. You may think that lunar eclipses happen very often. However, the Sun, Earth, and Moon must be lined up just right, in order for a lunar eclipse to take place. This happens very rarely. In most years there are only two lunar eclipses that can be seen only from certain places on Earth. In a partial lunar eclipse, the moon passes through the penumbra or part of the umbra. In a total lunar eclipse, the moon is completely within the umbra.

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

21 A pinhole camera works by blocking rays (demo) What is an image? A real image is formed on a screen when one or more rays from each point on the object reach the corresponding points on the screen and no other rays from other points on the object reach those points blocked rays Pinhole Camera Light bulb Image of light bulb Notice that this image is upside down and left-right reversed. Using shadows

22 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 Alex blocked rays Once again this image is upside down and leftright reversed. Early photographs (daguerreotypes) were always left-right reversed; Note the correspondence between the distances object-camera-screen and image vs. object sizes Reflected rays off the real Alex go through the hole and make the image Image of Alex

23 Finding an image by using rays is called ray tracing. Trace 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? a)smaller b)larger c)same size Is the image of Alex smaller or larger than the real Alex? a)smaller b)larger c)same size

24 Extra Credit Project (20 points): Construct and use a camera on your own (see textbook for details, pages 35 & 36)

25 Review (Chapter 1) Light is an electromagnetic wave EM waves have both traveling electric and magnetic fields EM waves are created by accelerating charge. EM waves can make other charges move. EM waves travel outward like waves in a pond, with electric and magnetic vectors perpendicular to the direction of motion. EM waves carry energy (and momentum). 25

26 Review (Chapter 1 too) c = 300,000 km/s =3 x 10 8 m/s c = fl, units are Hz for f and meters for l wavelength of light spans nm (blue to red) Atmosphere transmits light and radio, is opaque to UV, x-rays, some IR Amplitude of a wave is the half-height Wavelength is distance from crest to crest Period is the time for the wave to go up and down once at some location. 26

27 Review 27

28 Review The rotating mirror rotates 530 revolutions per second. Therefore, the frequency of revolution is: A) 530 seconds B) 84 Hz C) 530 Hz D) None of these. 28

29 Review The rotating mirror rotates 530 revolutions per second. Therefore, the period for a single revolution is: A) 1.9 seconds B) 530 seconds C) 1.9 milliseconds D) 1.9 nanoseconds E) Something else 29

30 Review The rotating mirror rotates with a period of 1.9 msec. Therefore, the time needed to rotate from one flat mirror to the next flat mirror is: A) 1.9 msec B) 238 msec C) 1.9 ksec D) 238 nsec E) Something else 30

31 Review The rotating mirror requires 238 msec to rotate from one flat mirror to the next. During that time, the reflected light ray travels out 22 miles are returns. The speed of light in miles per second is: A) 92,500 miles/sec B) 185,000 miles/sec C) 300,000 miles/sec E) None of these 31

32 That s IT! Good Luck on Exam 1 32