1. Geometrical Optics (Lecture III)
Chapter 26
Geometrical Optics (Lecture III)
Dr. Jie Zou
PHY 1161

2. Outline Features of refraction Optical illusions caused by refraction
Apparent depth
Total internal reflection
Real world application: Optical fibers
Dispersion and the rainbow
Dr. Jie Zou
PHY 1161

3. Features of Refraction
When a ray of light enters a medium where its speed decreases, it is bent toward the normal.
When a ray of light enters a medium where its speed increases, it is bent away from the normal.
There is no change in direction of propagation if there is no change in index of refraction. The greater the change in index of refraction, the greater the change in propagation direction.
If a ray of light goes from one medium to another along the normal, it is undeflected, regardless of the index of refraction.
Dr. Jie Zou
PHY 1161

4. Conceptual Checkpoint 26-4
A horizontal ray of light encounters a prism, as shown in the first diagram. After passing through the prism, is the ray (a) deflected upward, (b) still horizontal, or (c) deflected downward?
Dr. Jie Zou
PHY 1161

5. Optical Illusions Caused by Refraction
Apparent depth: an object appears to be closer to the water’s surface than it really is.
Dr. Jie Zou
PHY 1161

6. Total Internal Reflection
For angles of incidence greater than the critical angle, it is observed that all the light is reflected back into the water. This phenomenon is referred to as total internal reflection.
The critical angle for total internal reflection can be found by sin c = n2/n1.
Total internal reflection can only occur on the side of an interface that has the greater index of refraction.
Dr. Jie Zou
PHY 1161

7. Real World Applications
Example: An optical fiber
Dr. Jie Zou
PHY 1161

8. Dispersion and the rainbow
Dispersion: The “spreading out” of light according to color is known as dispersion.
Explanation:
The index of refraction for a given material depends on the frequency or wavelength of the light.
In general, the higher the frequency, the higher the index of refraction.
Violet light bends more than red light does when refracted by a given material.
Light of different frequencies is bent at different angles when incident on a refracting material.
Dr. Jie Zou
PHY 1161

9. Formation of a rainbow Refraction of sunlight by a spherical raindrop.
The formation of a rainbow.
Dr. Jie Zou
PHY 1161