1. Topic 5-3
Light

2. History Early Concept:
Greek philosophers thought light was made of tiny particles
Since Newton:
Light was thought of as a wave.
Einstein:
Said both concepts are correct in the theory known as the “photoelectric effect”

3. History cont.
Photoelectric effect states that light consists of particles called photons
Photons – massless bundles of concentrated electromagnetic energy
Scientists now agree that light has a dual nature: part particle, part wave

4. Speed of Light When measured in a vacuum is equal to 3x108 m/s
A value you can relate to:
187,500 mi/s
Astronauts traveling in the space shuttle are at about 5 mi/s
Beam of light could travel around the Earth 7.5 times in one second
Light Year:
Distance light travels in 1 year
Speed of light is abbreviated with a lower case “c”
Mathematically:
c = λ f Or
d = c t

5. Sample Questions
How long does it take for light from the sun to reach Earth if the sun is 1.5x1011 m away?
Microwave ovens emit waves of 2450 MHz. What is the wavelength of this light?
How many Km will light travel in one year?

6. Electromagnetic Waves
Electromagnetic waves are partly electric and partly magnetic
Electromagnetic Spectrum is used to depict the range of electromagnetic waves

7. Electromagnetic Spectrum

8. Outside the Visible Spectrum
Infrared:
Frequency lower than visible light (safe to be exposed to)
Example: Heat lamp
Ultraviolet:
Frequency higher than visible light (Hazardous)
Example: UV waves responsible for sun burns

9. Light and Material Transparent: Allows light through Example:
Windows, clear plastics
Opaque:
Does not allow light through
Example:
Your desk, a brick wall

10. Polarization Is due to the fact that light waves are transverse
Polarization will eliminate all waves not traveling in a given direction using a filter

11. Polarization Applications
Polarized glasses:
Only allows waves that align with its axis through.
Why:
Reduces light getting to the eyes and cuts down on glare as long as the axis runs vertically

12. Polarization and 3-D Viewing
Vision in three dimensions depends on both eyes giving impressions simultaneously from slightly different angles.
Movies can project a pair of views through polarization filters onto a screen with their polarization axes at right angles to each other
The overlapping pictures look blurry to the naked eye.

13. To see in 3-D, the viewer wears polarizing eyeglasses with the lens axes also at right angles.
Each eye sees a separate picture. The brain interprets the two pictures as a single picture with a feeling of depth.

14. Novel Applications

15. Right-eyed or Left-eyed?
Hold your finger out…..
Look through a hole….

16. END OF Light

17. Shadows Are formed where light rays can not be reach A ray:
A thin beam of light
Sharp Shadow:
Shadow is same darkness from inner circle to outer circle (small source close or large source far)
Blurry Shadow:
Center is darker than outer edge. These are most shadows.

18. Eclipse Is a shadow cast on a heavenly body Solar:
Moon’s shadow is cast on the earth
Lunar:
Earth’s shadow is cast on te moon

19. Solar Eclipse

20. Lunar Eclipses