2. Chapter 26
Properties of Light

3. Visible light originates with accelerated motion of electrons.
It is an electromagnetic wave phenomenon.

4. 1. ELECTROMAGNETIC WAVES
Accelerating charges give off EM waves.
Demo - Tesla Coil, Radio, and Fluorescent Lamp
EM Wave – Next Slide

5. Electromagnetic Wave Velocity
A certain speed is required to satisfy conservation of energy requirements.
That speed is c.
c = 300 million m/s
Large Number of Dots – Next Slide
= 186,000 mi/s

6. 50, 000 Dots

7. Maxwell discovered the nature of light.

8. Hollywood’s View of Light Speed
Jump to Light Speed

9. Hollywood’s View of Light Speed
Star Trek IV

10. THE ELECTROMAGNETIC SPECTRUM

11. THE ELECTROMAGNETIC SPECTRUM
Different kinds of sources produce the different regions of the EM spectrum.

12. THE ELECTROMAGNETIC SPECTRUM
Sources
EM Waves
Radio Waves
Vibrating charges
Microwaves
Molecular rotations
Infrared
Molecular vibrations
Visible
Atomic vibrations
Ultraviolet
Atomic vibrations
X-rays
Atomic vibrations
Gamma rays
Nuclear vibrations

13. Video - Microwave
URL - X-ray of Hand
URL - X-ray of Lungs
Video - The Atomic Bomb
We live in an “ocean” of waves.
For more information visit URL1.

14. 2. TRANSPARENT MATERIALS
Atom Spring Model – Next Slide

15. For glass the natural frequency is in the UV.
Energy of EM wave in UV stays with atoms longer causing vibrations with neighboring atoms with loss of energy.
Thus UV is absorbed.

16. For visible light the electrons in the glass are forced into vibration, but at less amplitude.
The atom holds the energy for less time, with less chance of collision with neighboring atoms, and less energy transformed to heat.
All visible frequencies get through clear glass. (Not all their energies do.)

17. There is an apparent slow down in the speed of light.
Light in Transparent Medium – Next Slide

18. Besides the UV, heat waves (IR) don't get through either.

19. TRANSLUCENT MATERIALS
Frosted glasses are good examples of translucent materials.
They scatter light on transmission.
You can’t read or identify objects from light passing through translucent materials.
Demo – Wax Paper

20. 3. OPAQUE MATERIALS Most things around us are opaque.
Metals are shiny and reflect.
Because of free electrons - they don’t pass on energy to atoms.
Our atmosphere is transparent in visible and part of IR.
It's good that UV is absorbed in our atmosphere.
UV causes sunburns.

21. Demo - Radiometer and Filters

22. SHADOWS

23. Penumbra
Umbra

24. SHADOWS When light is blocked, shadows are created.
The kind of shadow (umbra or penumbra) depends on the source of light and the relative positions of the light source, object, and shadow of the object.
Extended light sources cause shadows of objects to be partial shadows (penumbras) that may or may not have umbras.
A point source of light will create a shadow that only has an umbra.
A penumbra will be small if the light source is far from the object and its shadow.
Demo – Shadows using Projector then Penlight

25. Priceless Solar Eclipse Sun Full Shadow Umbra Earth Moon
Partial Shadow
Penumbra
Solar Eclipse
Priceless
Looking at the sun while the
moon passes in front of it.

26. Annular Eclipse Again looking at the sun while the
moon passes in front of it.

27. Lunar Eclipse New Moon Phase Full Moon Phase Earth’s shadow
Looking at the moon while it
passes through the earth’s shadow.

28. 4. SEEING LIGHT - THE EYE Cornea - does most of the focusing Iris
- has the eye color and controls
light intensity
Pupil
- the hole in the eye
Lens
- does remainder of focusing
Retina
- location of light sensors, has rods and cones
Fovea
- center of vision, predominantly cones
Blind spot
- optic nerve exit, no light sensors

29. Demo - Blind Spot
Primates and a species of ground squirrel are the only mammals that experience full color vision.
At the periphery of our vision we see
Motions first as objects come from behind
Colors second

30. Rods are more sensitive to low light levels than cones are.
Examples: stars and twilight
Demo - Turn off Room Lights
Rods see blue better. Blue will appear much brighter than red in dim light, though the red might be much brighter than the blue in bright light.

31. Red Eye Photography

32. Lateral Inhibition
We don't perceive the actual differences in brightness in fields of light and dark.
We even out our visual field.
This allows us to discern detail in very bright areas and in dark areas at the same time.
We accentuate differences rather than similarities.
Lateral Inhibition – Next Slide

34. Chapter 26 Review Questions

35. The speed of light (a) has never been measured
(b) is about the same as that of sound
(c) is infinitely fast
(d) is very fast, but not infinite

36. You will observe a total eclipse of the sun when
(a) you stand in the penumbra of the moon’s shadow
(b) you stand in the umbra of the moon’s shadow
(c) sunlight diffracts around the moon
(d) sunlight reflects from the moon to the earth

37. Which of the following will cast a shadow that has an umbra but no penumbra?
(a) the sun
(b) an incandescent lamp
(c) a fluorescent lamp
(d) a point source of light

38. What is the purpose of the pupil in an eye?
(a) to adjust the focal length of the lens
(b) to adjust the amount of light passing through the lens
(c) to focus the image
(d) to change the width of the field of view