1. Homework Set 2 P2, P5, P12, P14, P18, P21, P23, PM2
From “Seeing the Light” Chapter 2:
(starting page 68)
P2, P5, P12, P14,
P18, P21, P23, PM2
Due: Monday, Feb 16

2. Geometric optics
Geometric Optics: Study of light-rays based on the assumption that
light travels on a straight line
Useful when the wavelength of light is much smaller than the physical sizes.
Can explain most phenomena in daily life!

3. Light Sources
Point source: size small compared with any other sizes in the problem
Small light bulbs, candles, or sometimes even the sun
Light rays travel from the source in all directions in straight lines
Extended source:
Two bulbs, long fluorescent tubes…
Light rays travel from all points of the source in all possible directions.

4. Shadows
When the light-rays are blocked by an obstacles in the way, shadows are formed on a screen:
Did You see your shadow from the sun light? X-ray pictures
We can figure out the location and shape of shadows by drawing the light-rays.
At night, we are in the shadow of the Earth!.

5. Shadow and art Silhouette portraits
Using shadow to show the object in the air.
More examples?
Drop shadow
Shadow play
Shadow stamping
Shadow box….

6. Umbra and Penumbra
Two light-sources will create two shadow, and extended light-sources create infinite many shadows.
Umbra: a region where all shadows overlap, completely dark
Penumbra: the region it is partially dark.
When you are in penumbra, you see a part of the light-source.

8. Example: sun and earth

9. Solar Eclipse
Eclipses can occur sometime when there is a New Moon. This is when the Moon passes between the Earth and Sun. Then, the Moon's shadow sweeps across the Earth's surface and we may see some part, or all, of the Sun covered or 'eclipsed' by the Moon.
Light-source: sun
Obstacle: moon
Screen: earth

10. Total Solar Eclipse

12. Total Solar Eclipse
When the moon is closer to Earth, the size of the Moon's disc
is the same as that of the Sun's, and the Sun is completely blotted
out, resulting in a Total Solar Eclipse at some point on Earth, and
a partial solar eclipse in places close to the zone of totality.

13. Annular Solar Eclipse
When the moon is farthest from Earth, the size of the Moon's disc
is slighltly smaller than that of the Sun's, and does not completely
cover the Sun in an eclipse. When this happens , a ring of Sunlight
surrounds the Moon's disc during the eclipse giving rise to what is
known as an Annular Eclipse

14. Lunar Eclipse:
A lunar eclipse takes place when the Earth passes between the Sun and the Moon. The Earth's shadow falls on the full moon, making it appear like the it's going through a cycle of phases--but in fast motion! A lunar eclipse, from start to finish, takes only a matter of hours--as long as it takes for Earth to "get in" and then "get out" of the way of the light hitting the Moon.
Light-source: sun
Obstacle: earth
Screen: moon

15. Three types of Lunar Eclipses

16. Pinhole Camera
A small, light-tight can or box with a black interior and a tiny hole in the center of one end. You can design it to accept roll or sheet film. The two ends of the camera are parallel. The end opposite the pinhole is flat so that the film is held in a flat plane. The pinhole has a cover to prevent light from entering the camera.

20. Pinhole Camera
The light intensity is small. It takes a long time to take a picture
The hole cannot be too big. Or the picture will be blurred.
The hole cannot be too small. Or the wave property of the light will blurred the picture.

21. Inverted Images