Ray Optics. Lecture 23. Chapter 34. Physics II. Course website:

Transcription

1 Lecture 23 Chapter 34 Physics II Ray Optics Course website:

2 Today we are going to discuss: Chapter 34: Section Ray Optics

3 Ray Optics Wave Optics (object size) Ray Optics (object size)

4 The Ray Model of Light Ray Ray Let us define a light ray as a line in the direction along which light energy is flowing. Ray Ray Wave fronts Now, we will model light with rays, straight lines to wave fronts. This model is valid as long as an object is very large compared to the wavelength. Any narrow beam of light, such as a laser beam, is actually a bundle of many parallel light rays.

5 Types of objects Self-luminous object (active) It emits light Ray Reflective object (passive) It reflects light Objects can be either self-luminous, such as the sun, flames, and lightbulbs, or reflective. Most objects are reflective.

6 How Rays interact with media Light interacts with matter in four different ways: At an interface between two materials, light can be either reflected or refracted. Within a material, light can be either scattered or absorbed. Medium 1 Medium 2 Reflection Refraction

7 Reflection

8 Two types of reflections Specular Reflection (from a smooth surface) Incident ray Reflected ray Diffuse Reflection (from a rough surface) Incident ray Reflected angle of incidence angle of reflection Law of reflection: 1. Incident and reflected rays and a normal line are in the same plane 2. Angle of incident equals to angle of reflection For a rough surface, the law of reflection is obeyed at each point but the irregularities of the surface cause the reflected rays to leave in many random directions. Most objects are seen by virtue of their reflected light. It is how you see this slide, the wall, your hand, your friend, and so on.

9 Image in a plane mirror I Consider P, a source of rays which reflect from a mirror. The reflected rays appear to emanate from P, the same distance behind the mirror as P is in front of the mirror. That is, s = s. mirror s s Cat vs Mirror

10 Image in a plane mirror II

11 ConcepTest Two point sources of light illuminate a narrow vertical aperture in a dark screen. What do you see on the viewing screen? Ray Optics screen

12 Refraction

13 Refraction Two things happen when a light ray is incident on a smooth boundary between two transparent materials: 1. Part of the light reflects from the boundary, obeying the law of reflection. 2. Part of the light continues into the second medium. The transmission of light from one medium to another, but with a change in direction, is called refraction. Refracted ray

14 Indices of Refraction The index of refraction is used to describe optical properties of a transparent medium. Speed of light in a medium

15 Refraction Consider a smooth boundary between two transparent materials Incident ray angle of incidence Normal Reflected ray The ray has a kink at the boundary Medium 1 Medium 2 Assume n 2 > n 1 angle of refraction Refracted ray

16 When a ray is transmitted into a material with a higher index of refraction, it bends toward the normal. Refraction When a ray is transmitted into a material with a lower index of refraction, it bends away from the normal. n 2 > n 1 n 2 < n 1 It bends toward the normal It bends away from the normal

17 ConcepTest Refraction A laser beam passing from medium 1 to medium 2 is refracted as shown. Which is true? A. n 1 < n 2. B. n 1 > n 2. C. There s not enough information to compare n 1 and n 2. n 2 < n 1 It bends away from the normal The end of the class

18 Thank you See you next time