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Foundation year

General PhysicsPHYS 101

Chapter 4 : Light and Optics

Instructor: Sujood Alazzam 2015/2016

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CHAPTER OUTLINE

The Nature of Light Reflection and Refraction. Lenses.

4.1 The Nature of Light

Before the beginning of the nineteenth century, light was considered to be a stream of particles that either was emitted by the object being viewed or emanated from the eyes of the viewer. Newton, the chief architect of the particle theory of light, held that particles were emitted from a light source and that these particles stimulated the sense of sight upon entering the eye. Using this idea, he was able to explain reflection and refraction.

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• The quantization model assumes that the energy of a light

wave is present in particles called photons; hence, the

energy is said to be quantized.

• According to Einstein’s theory, the energy of a photon is

proportional to the frequency of the electromagnetic

wave:

• where the constant of proportionality

is Planck’s constant

E h f

346.63 10 J.sh

Cont.

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Why does the bee’s image appear at left?

Ans: Total internal reflection; fiber optics.

4.2 Reflection

When a light ray traveling in one medium encounters a boundary with another medium, part of the incident light is reflected. This Figure shows several rays of a beam of light incident on a smooth, mirror-like, reflecting surface.

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• The reflected rays are parallel to each other, as

indicated in the figure. The direction of a reflected ray is

in the plane perpendicular to the reflecting surface that

contains the incident ray.

• Reflection of light from such a smooth surface is called

specular reflection. If the reflecting surface is rough, as

shown in last Figure, the surface reflects the rays not as

a parallel set but in various directions.

• Reflection from any rough surface is known as diffuse

reflection. A surface behaves as a smooth surface as

long as the surface variations are much smaller than the

wavelength of the incident light.

Cont.

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Consider a light ray traveling in air and

incident at an angle on a flat, smooth

surface, as shown in this Figure. The

incident and reflected rays make angles Ɵ1

and Ɵ1’, respectively, where the angles are

measured between the normal and the rays.

(The normal is a line drawn perpendicular

to the surface at the point where the

incident ray strikes the surface.)

Experiments and theory show that the angle

of reflection equals the angle of incidence:

Cont.

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Ɵ1= Ɵ1’

This relationship is called the law of reflection.

4.3.1 Refraction

When a ray of light traveling through a transparent medium encounters a boundary leading into another transparent medium, as shown in the next Figure, part of the energy is reflected and part enters the second medium. The ray that enters the second medium is bent at the boundary and is said to be refracted. The incident ray, the reflected ray, and the refracted ray all lie in the same plane. The angle of refraction, Ɵ2 in this Figure, depends on the properties of the two media and on the angle of incidence through the relationship

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where v1 is the speed of light in the first medium and v2 is the speed of light in the second medium.

4.3.2 Refraction Facts

• Refraction Fact #1: As light goes from one medium to another, the velocity CHANGES!

• Refraction Fact #2: As light goes from one medium to another, the path CHANGES!

4.3.3 Index of Refraction

In general, the speed of light in any material is less than its

speed in vacuum. In fact, light travels at its maximum speed

in vacuum. It is convenient to define the index of refraction

n of a medium to be the ratio:

From this definition, we see that the index of refraction is a

dimensionless number greater than unity because v is

always less than c. Furthermore, n is equal to unity for

vacuum.

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• The experimental discovery of this relationship is usually credited to Willebrord Snell (1591–1627) and is therefore known as Snell’s law of refraction.

Cont.

Example 4.1:

A beam of light of wavelength 550 nm traveling in air is incident on a slab of transparent material. The incident beam makes an angle of 40.0° with the normal, and the refracted beam makes an angle of 26.0° with the normal. Find the index of refraction of the material.

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Example 4.2:

• A light ray of wavelength 589 nm traveling through air is incident on a smooth, flat slab of crown glass at an angle of 30.0° to the normal, as sketched in this Figure. Find the angle of refraction.

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