1. Light Waves Interacting with Matter
(Chapter 7)
Student Learning Objective
Determine the behavior of electromagnetic waves as they interact with different types of matter.

2. What determines how light will interact with matter?
Light interacts differently with different types of matter.
Surface
Material
Angle of incidence
Wavelength
Opaque materials absorb (Blacktop) or reflect light.
Practice: If a material absorbs radiant energy, what is the result?

3. Light is able to travel through transparent materials. (Transmission)
Light may be absorbed and re-emitted.
Light may pass through the material.
Speed changes
Air & Glass

4. What is the law of reflection?
Reflection occurs when light bounces from a surface back into the original medium.
Law of Reflection:
Incoming light rays and outgoing light rays have the same angle with respect to the normal line.
The law of reflection applies to all reflective surfaces. I R
Mirror

5. Diffuse Reflection Practice: Does the law of reflection apply to diffuse reflection?
A flat mirror reflects all light rays in the same direction.
Upright
Same size
Same distance
Left-right reversed

6. Photo Source: Copyright © Bobby H. Bammel. All rights reserved.
Reflected Light
It is the reflection of light that allows us to see things and in places where direct light does not hit – in the shadows!
Photo Source: Copyright © Bobby H. Bammel. All rights reserved.
Section 7.1

7. For a person to see her/his complete figure in a plane mirror, the height of the mirror must be at least one half the person’s height.
Section 7.1

8. Curved Mirrors Convex mirrors always produce a smaller image.
Curved Mirrors
Convex mirrors always produce a smaller image.
Concave mirrors usually produce a larger image.

9. Practice Why do we “see” an image behind a mirror?
Which mirror has the larger field of view, concave or convex?
What kind of mirror is
the side mirror on your
car?

10. When and how is light refracted?
Light will change speed and may change direction when it is transmitted from one material to another. (Refraction)
 Properties Affect Refraction
Wavelength
Angle of incidence
Material (n = c/v)
Temperature
Why is glass transparent? - Mark Miodownik

11. Refraction affects where objects appear to be.
The change in index of refraction determines the direction light “bends”.
Low to high n  light bends toward
normal line
High to low n  light bends away
from normal line
Refraction affects where objects appear to be.
(1:14)

12. Indexes of Refraction for Common Substances
The higher the Index of Refraction (n) the more the light slows.
Section 7.2

13. Practice
1) In which substance, would light travel faster, ethyl alcohol or air? 2) Calculate the speed of light in ethyl alcohol. (n=1.361 for ethyl alcohol) 3) As the light goes from air into the ethyl alcohol, will it bend toward or away from the normal line?
𝑛= 𝑐 𝑣

14. When does total internal reflection occur?
Total internal reflection occurs when light approaches a boundary at the critical angle.
The critical angle is the angle at which the material reflects the light ray and does not allow transmission.

15. Internal Reflection Exceeding the Critical Angle (qc)
Diamonds and other gemstones are facetted in such a way to enhance internal reflection.
Fiber-optics – used to “pipe” light down and back along glass/plastic fibers
Section 7.2

16. How is a rainbow formed?
Dispersion occurs when visible light is divided into separate colors.
Each person sees their own set of colors (rainbow) from a particular set of raindrops.

17. Practice
1) What three processes happen to light to form a rainbow? 2) Which color of light is refracted the most in a raindrop?

18. Line Spectra Each line spectrum is unique to a specific element.
Section 7.2

19. What do lenses do to light?
Refraction
Convex Lens
(true image)
Concave Lens
(virtual image)
Question: What type of lens is the human eye?

20. What is polarized light?
Light is many transverse waves vibrating in many directions.
Polarized light has only one direction of vibration.

21. What causes diffraction?
Practice: How could you determine whether sunglasses are truly polarized?
What causes diffraction?
Diffraction occurs when light waves bend around corners.
The amount of diffraction depends on the how the size of the wavelength compares to the size of the barrier.
Shadows

22. How are colors produced?
Most of the colors you see in the world are due to selective reflection.
Practice
1) What does selective reflection mean?
2) What do you think selective transmission means? What is an example of this?
3) Why does the sky appear to be blue mid-day?

23. The Earth appears blue because blue (and violet) photons are scattered as they collide with atoms and molecules within the air.
The sky on Earth appears red at sunset because the light must pass through a lot of atmosphere.

24. Practice
1) Both blue and violet photons are scattered by our atmosphere. Why does our sky appear to be mostly blue, and not violet, at mid-day? 2) What color would our sky be if atmospheric particles were slightly larger? 3) Why is the sky black on the moon? 4) Would the stars appear to twinkle on the Moon?

25. Some objects have a particular color because of temperature.
Wavelength of Maximum Emission:
Every object has a specific wavelength (color) at which it radiates most of its energy.
T(Kelvin) = 3,000,000
lmax (nm)

26. Practice
1) Our Sun has a surface temperature of 5,800 K. What is the wavelength of maximum emission for our Sun? What color is this? 2) If the burner on your electric stove is 500 ºF (260 ºC), what is the wavelength of maximum emission? What “color” corresponds to this wavelength? 3) What is the surface temperature of a star for which the wavelength of maximum emission is 425 nm? What color would this star be?