1. Light in a Medium

2. What makes some objects transparent and others opaque?
How a receiving material responds to an interaction with light depends on 2 things:
Frequency of the incoming light
Natural frequency of the object being hit
If incoming frequency matches natural frequency, resonance occurs!

3. The resonance that occurs can cause light to be re-emitted!
What does light hit?
Individual atoms!
Atoms of different elements hold onto their electrons with different amounts of strength.
We will represent this as “spring strength.”
The resonance that occurs can cause light to be re-emitted!

4. No match, little vibration…
Different frequencies of light hit identical atoms
No match, little vibration…
Good match, large vibration!

5. No match, little vibration…
Light of same frequency hits different atoms
No match, little vibration…
Good match, large vibration!

6. What makes some objects transparent and others opaque?
Light can interact with matter in several ways.
Can you think of them and give an example of each???
Will the interaction result in transparency or opacity?
Absorption and re-emission resulting in delayed transmission (Transparent)
Reflection off of the surface (Opaque)
Absorption resulting in energy lost to heat (Opaque)

7. Excited atom vibrates and immediately re-emits light
Reflection off of the surface
Reflected Light
Shiny Surface
Atoms
Incoming light
Excited atom vibrates and immediately re-emits light

8. Reflection off of the surface
Metals are shiny because the light that shines on them forces free electrons into vibration. These electrons emit their own light as a reflection.

9. Absorption and re-emission resulting in delayed transmission (Transparent)
Transparent material (glass)
Transmitted light
Incoming light
Delay from repeated absorption and re-emission makes light slower in medium
Atom absorbs light, vibrates, and re-emits

10. Light hits atom at matching frequency and vibrates a lot!
Absorption resulting in energy lost to heat (Opaque)
Incoming light
Energized atom begins bumping into other atoms and causes temperature of material to rise.
Light hits atom at matching frequency and vibrates a lot!

11. Absorption resulting in energy lost to heat (Opaque)
Absorption and re-emission resulting in delayed transmission (Transparent)
Visible light can get through glass windows
Glass blocks infrared and UV waves causing the glass to warm up.
Absorption resulting in energy lost to heat (Opaque)

12. At the beach…
Luckily our atmosphere is mostly opaque to UV light due to scattering.
Clouds still transmit some UV light, hence sunburn on a cloudy day.
Sand and water can reflect UV light, so you can get a tan even when under your umbrella!

13. The speed of light in media
Light is slower than when in a vacuum in all other media!
In water- 75% c
In Glass- 67% c
In Diamond- 40% c
Depends on the index of refraction (n)

14. Why is glass transparent to visible light but not IR or UV?
What would it be like to be invisible?
Why is wetter, darker?