1. The agenda includes physics presentations on light, an extra credit opportunity to build a multi-level mobile involving torque, and information about the physics olympics presentations. 2. Light demonstrates both wave and particle properties. It can reflect, refract, diffract, interfere and polarize which supports the wave model, but it also explains the photoelectric effect as particles. 3. Visible light is a small portion of the electromagnetic spectrum, and its wavelength corresponds to color from red to violet with shorter wavelengths having higher frequencies and more energy.

1. agenda
1. Physics presentations
2. What you should know about light
Updates:
- EXTRA CREDIT OPPORTUNITY: construct a mobile that is at least three levels
- What physics concept does this involve?

2. Mobiles use torque!
• Be creative – make the mobile reflect YOU and your interests, or have a theme to the mobile!
• The more levels, the more extra credit

3. Physics in the summer olympics!
• Order will be based on volunteer, and then based on names picked from popsicle sticks
• YOU WILL LOSE POINTS if you interrupt another group’s presentation
• There will be time for questions at the end
• I will be timing your presentations

4. What you should know about light
• How light supports the wave model of light:
• It reflects, refracts, diffracts, and can be polarized.
• Recognize that light has a finite speed
• Know the order of visible light on the electromagnetic spectrum and how the color of light tells
us about the wavelength, frequency, and energy contained in light.

5. Electromagnetic spectrum
• Visible light is just a small portion of the EMS
• We see that from ROY G. to BIV
• Wavelength gets shorter
• Frequency gets higher
• Energy gets higher

7. Wave-particle duality theory
• Light is a more complex phenomenon than just a simple wave or a simple
beam of particles!
Phenomenon Explained in terms of Explained in terms of
waves? particles?
Reflection ✔ ✔
Refraction ✔ ✔
Interference ✔ ✗
Diffraction ✔ ✗
Polarization ✔ ✗
Photoelectric Effect ✗ ✔

8. reflection
• It allows us to see objects as different colors
• All other colors are absorbed
White Only red light
light is reflected

9. Reflection & direction
• The direction of light reflected depends on the direction that light is aimed at a reflective object
Normal
Incident ray Reflected ray
Angle of Angle of
incidence reflection
Mirror

10. Using mirrors
• Two examples:
2) A car headlight
1) A periscope

11. refraction
• As light goes from one medium to another, the path of light bends
• Velocity changes, so its path changes.
• Analogy is a troop of soldiers going from firm ground to mud

12. refraction
• As light goes from one medium to another, its path changes.
• Some light is reflected, while some is refracted!

13. Refraction – Snell’s Law
• A scientist by the name of Snell discovered that the ratios of the index’s and the ratio of the
sine of the angles are the same value!
• n = index of refraction
• Index of refraction is a function of wavelength – the longer the wavelength, the less light
will bend
n1 sin θ 2
=
n2 sin θ 1
Snell ' s Law
n1 sin θ1 = n2 Sinθ 2

14. refraction
• Index of refraction is also the ratio of the speed of light
in a vacuum to the speed of light in that substance
n=c
v

15. diffraction
• Bending of waves around the edge of a barrier. New waves are formed from the original.

16. Light interferences
• Constructive & destructive interferences can cause light and dark patterns of light

17. dispersion
• Each color has a different wavelength, so they bend differently when going into a new
medium since index of refraction depends on the wavelength for that media.
• It is greater for shorter wavelength – which color on the spectrum has the
smallest wavelength?

18. Reflection & refraction allow us to see rainbows!
• Sun rays are hit by spherical water droplets and are reflected off the back surface.
• Red is bent the least (since it has the lowest wavelength) so it reaches the observer’s eye
from higher in the sky!

19. Polarized waves
P
• The alignment of electromagnetic waves in such a
way that the vibrations of the electric fields in each
of the waves are parallel to each other.
• Bottom line…polarizing light shields some
light by block the light at right angles from
the polarizer.

20. polarization
• Its practical uses:

21. Facts about Light
The speed of light, c, is constant in a vacuum.
Light can be:
•REFLECTED
•ABSORBED
•REFRACTED
Light is an electromagnetic wave in that it has wave like properties which can be
influenced by electric and magnetic fields.

22. sources
• http://www.bowlesphysics.com