Lecture 12Light: Reflection and Refraction

Chapter 22.1 22.4

Outline

• History of Studies of Light• Reflection of Light• The Law of Refraction. Index of Refraction.

Towards the Nature of LightNewton considered light as a stream of particles and explained reflection and refraction (1660s).

Huygens proved that these phenomena can be understood if light is waves (1670s).Grimaldi (ca. 1660) and Young (1801) experimentally demonstrated that light exhibits diffraction and interference.Maxwell (1865) predicted that light is a form of electromagnetic waves.Work of Hertz (1880s), Planck (1900), and Einstein (1905) showed that light has dual nature.

What is Light?

Dual nature means that under some conditions light behaves as a wave, while under others as a particle.

Light does not show properties of both wave and particle in the same experiment.

Particles of light are called photons.

E = h fE energy of a photonh = 6.63 1034 J s Planck constantf frequency of the electromagnetic wave

Geometric Optics

Light travels in a straight-line path in a homogeneous medium.

At a boundary between 2 different media the light:• is reflected• passes into the other medium (material)• does both (partially)

After entering a different medium, light continues traveling in a straight-line path.

Definition: wave front is a surface of a wave with the same phase and amplitude.

Reflection of LightThere are types of reflection: specular and diffuse.

Picture

=

Reflection off a smooth surface (average distance between irregularities is smaller than the wavelength of the incident light) is specular.

A line perpendicular to the surface is called a normal.

The angle of incidence equals the angle of reflection.

Refraction of Light

Refraction is bending of light at a boundary between two different media.

The incident, reflected, and refracted rays all lie in the same plane.The path of a light ray through a refracting surface is reversible.

sin 2 v2

= = constantsin 1 v1

The Law of RefractionAny medium is characterized by an index of refraction (n).

c speed of light in vacuumn = v speed of light in a medium

The frequency (f) of light, traveling between different media is constant.

Since f = v, the wavelength of a light ray in media with different indices of refraction should change.

The Law of Refraction

v1=f 1

v2=f 2

f1 = f2

1 v1 c/n1 n2

= = = 2 v2 c/n2 n1

n = 0/ n

Index of refraction is the ratio of the wavelength of light in vacuum and its wavelength in a medium

The angle of refraction is related to the index of refraction by Snell’s law of refraction.

n1 sin 1 = n2 sin 2

A Refraction Problem

1 = 30o

l =2cm

Problem: A ray of light strikes a flat block of glass (n=1.50) at an angle of 30o with the normal.

Find: angles of incidence and refraction at each surface and the distance (d) of shift from the initial direction.

d

x2

x1

sin 2 = (n1/n2) sin 1 d = (x1 x2) sin (90o 1)

1

2

n1 = 1n2 = 3/2

x1= l tan 30o

x2= l tan 2

2 = 19.5o, x1x2=0.45cmd = 0.39 cm

Summary

• Light is a form of electromagnetic waves. It shows properties of both a wave and a particle.

• Reflection and refraction are wave properties of light.

• For specular reflection, the angle of incidence equals to the angle of reflection

• Index of refraction is a property of a medium. It is a measure of the speed of light in a certain medium.