Department of Physics and Applied PhysicsPHYS.1440 Lecture 23 A.Danylov

Lecture 23

Chapter 34

Ray Optics

Course website:http://faculty.uml.edu/Andriy_Danylov/Teaching/PhysicsII

Physics II

Department of Physics and Applied PhysicsPHYS.1440 Lecture 23 A.Danylov

Today we are going to discuss:

Chapter 34:

Section 34.1-3 Ray Optics

Department of Physics and Applied PhysicsPHYS.1440 Lecture 23 A.Danylov

Ray Optics

Wave Optics

∼ (object size)

Ray Optics

≪ (object size)

Department of Physics and Applied PhysicsPHYS.1440 Lecture 23 A.Danylov

The Ray Model of Light

Wave fronts

Ray

Ray

Ray

Ray

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.

Let us define a light ray as a line in the direction along which light energy is flowing.

Any narrow beam of light, such as a laser beam, is actually a bundle of many parallel light rays.

Department of Physics and Applied PhysicsPHYS.1440 Lecture 23 A.Danylov

Types of objects

Ray

Self-luminous object (active)It emits light

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.

Department of Physics and Applied PhysicsPHYS.1440 Lecture 23 A.Danylov

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

Department of Physics and Applied PhysicsPHYS.1440 Lecture 23 A.Danylov

Reflection

Department of Physics and Applied PhysicsPHYS.1440 Lecture 23 A.Danylov

Two types of reflections

Incident ray Reflected ray

Specular Reflection(from a smooth surface)

‐ angle of incidence‐ angle of reflection

Law of reflection: 1. Incident and reflected rays and a normal line

are in the same plane2. Angle of incident equals to angle of reflection

Diffuse Reflection(from a rough surface)

Incident ray

Reflected

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.

It is how you see this slide, the wall, your hand, your friend, and so on.

Most objects are seen by virtue of their reflected light.

Department of Physics and Applied PhysicsPHYS.1440 Lecture 23 A.Danylov

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.

ssmirror

Cat vs Mirror

Department of Physics and Applied PhysicsPHYS.1440 Lecture 23 A.Danylov

Image in a plane mirror II

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

ConcepTest Ray Optics

scre

en

Department of Physics and Applied PhysicsPHYS.1440 Lecture 23 A.Danylov

Refraction

Department of Physics and Applied PhysicsPHYS.1440 Lecture 23 A.Danylov

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

Department of Physics and Applied PhysicsPHYS.1440 Lecture 23 A.Danylov

Indices of RefractionThe index of refraction is used to describe optical properties of a transparent medium.

Speed of light in a medium

Department of Physics and Applied PhysicsPHYS.1440 Lecture 23 A.Danylov

RefractionConsider a smooth boundary between two transparent materials

Incident ray Reflected rayangle of incidence

angle of refraction

Normal

Medium 1Medium 2

Refracted ray

Assume n2 > n1The ray has a kink

at the boundary

Department of Physics and Applied PhysicsPHYS.1440 Lecture 23 A.Danylov

Refraction When a ray is transmitted into a material

with a higher index of refraction, it bends toward the normal.

n2 > n1 n2 < n1It bends away from the normalIt bends toward the normal

When a ray is transmitted into a material with a lower index of refraction, it bends away from the normal.

A laser beam passing from medium 1 to medium 2 is refracted as shown. Which is true?

ConcepTest RefractionA. n1 < n2.B. n1 > n2.C. There’s not enough information to

compare n1 and n2.

n2 < n1

It bends away from the normal

The end of the class

Department of Physics and Applied PhysicsPHYS.1440 Lecture 23 A.Danylov

Thank youSee you next time