Reflection And Refraction Of Light

Chapter 22

Introduction

Light is necessary for life on this planet. It is our source of energy. It enables us to see and to communicate.

The Nature of Light

Scientists have always been intrigued by light. Much time was spent arguing over the exact nature of light.Was it a wave or a particle?

The Corpuscular Theory

The Greeks believed that light was made up of particles (corpuscles). Newton used the corpuscular theory to explain the reflection and refraction of light.

The Wave Theory

Christian Huygens believed that light traveled as waves and used this concept to describe many of light’s properties.

The Visible Spectrum

Thomas Young showed that light exhibits interference. This gave support to the wave theory.

Maxwell proposed that light was an electromagnetic wave.

Back to the Corpuscular Theory

Just as scientists began to accept the idea that light was a wave, Max Planck and Albert Einstein returned to the corpuscular theory to explain thermal radiation and the photoelectric effect.

Is light a wave ora particle?

Today, scientists view light as being both a wave and a particle at the same time.

The Nature Of Light

Until the beginning of the 19th century, light was believed to be a particle Newton strongly believed this

He used the particle theory to explain the laws of reflection and refraction

Most scientists agreed with him

B14

Another theory was proposed during Newton’s lifetime. Christian Huygens believed that light was a wave.His wave theory also verified the laws of reflectionand refraction.

Huygen’s Principle was not readily accepted because all known waves required a medium. There was no sign of diffraction of light waves.

Newton’s explanation was preferred for a century.

Demonstrating the Wave Nature of Light

Thomas Young demonstrated the interference of light and gave support to the wave theoryJames Maxwell predicted that light was a high frequency electromagnetic wave that traveled at 3 x 108 m/s

ooεμ1

c=

The Photoelectric Effect

It was discovered that clean metal surfaces emit charges under UV light. This is called the photoelectric effect.

The photoelectric effect could not be explained by the wave theory.

Einstein

Einstein explained the photoelectric effect in terms of corpuscles or quanta of energy. He received the Nobel Prize in 1926.

Einstein’s formula:

h is Planck’s Constant 6.63 x 10-34 J.s

€

E = hf

The Photoelectric Effect

In the photoelectric effect: Photons transfer energy to electrons.Particle nature

Photons have frequency and wavelength.Wave nature

Measurements Of The Speed Of Light

Galileo was unsuccessful in his attempt to measure the speed of light. Two observers, with lanterns, in towers five miles apart

The Ray Approximation In Geometric Optics

Light travels in a straight line path until it encounters a boundary between two different materials. Rays approximate beams of light.

Wavefronts

A wavefront is a surface passing through the points of a wave that have the same phase and amplitude.Rays are perpendicular to wavefronts.

Spherical Wavefronts

The wavefront produced by a point source is spherical.

Reflection

Rays are reflected at the same angle as their incident angle upon the surface.Reflected rays are parallel to each other.

235

Types of Reflection

Specular reflection Reflection from a smooth surface

Wet highways Mirrors

Diffuse reflection Reflection from a rough surface

Dry highways Paintings

22.2

The Law of Reflection

The angle of reflection always equals the angle of incidence. Red eyes in photographs

retroreflection

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θi =θ r

Refraction

Refraction of light Light rays bend when they enter a different medium.

Part of the light is reflected from the surface.

The type of medium affects the angle of refraction.

22.7, 238, 22.6a, 29-3

Refraction at a Boundary

Wave speed changes after refraction.

constantsin

sin

1

2

1

2 ==vv

θθ

Reversibility

The paths of light rays are reversible.Light rays which pass through a slab of transparent material emerge parallel to the original path.

241, 29-4, 29-5, 62, 73

The Law Of Refraction

The index of refraction (n) is the ratio of the speed of light in a vacuum to the speed of light in the medium

Formulas:

n = 1 for a vacuum, >1 for other media

Table 22.1 (Pg. 738)

1

2

2

1

v

v

n

n=

n

o

n

o

n f

f

v

c

λλ

λλ

===n

Snell’s Law

Snell’s Law

2211 sinnsinn θθ =

Total Internal Reflection

This only occurs when light tries to go from a medium with a high index of refraction to one with a lower index of refraction. Example: light going from water to air or glass to air

22.25, 244

Total Internal Reflection

At the critical angle (θc), the refracted ray is parallel to the boundary θr = 90o

θi = θc

27.35

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sin θc =n2

n1

Total internal reflection only occurs at angles greater than the critical angle.

Applications of Total Internal Reflection

DiamondsPeriscopesFiber optics (using lasers) Communications Medicine

(Fiberscopes) Stomach Colon Knee joints

Entertainment equipment

Dispersion And Prisms

The index of refraction in a medium depends upon the wavelength of light. Verified in our lab by using Snell’s LawGreater for red or for violet?

22.13, 246

Prisms

A prism can be used to separate white light into the visible spectrum. ROY G. BIV ( ~ 650 nm to 400 nm)

22.15a

The angle of deviation () is measured between the original path and the new path

22.15a, b

A Prism Spectrometer

A prism spectrometer Used to study wavelengths emitted by a light sourceWe’ll use one in a later lab.

Elements may be identified by comparing with known elements.

Rainbows

When can you see a rainbow?How must you be positioned with respect to the sun?How are rainbows formed?Is it possible to see a complete circle? Niagara Falls ---Rainbow Falls

242, 22.19, 404, 61

Rainbows are formed because of refraction, reflection and dispersion.

Huygen’s Principle

Huygens believed that light was composed of waves.He stated that all points on a given wavefront are sources of new spherical secondary waves.

Questions

1 - 4, 7, 9 - 13Pg. 753