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 or a 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 reflection and refraction.

Newton’s explanation was preferred for a century. 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 theory James Maxwell predicted that light was a high frequency electromagnetic wave that traveled at 3 x 108 m/s

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

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 Diffuse 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

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.

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)

Snell’s Law Snell’s Law

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 (qc), the refracted ray is parallel to the boundary qr = 90o qi = qc 27.35

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

Applications of Total Internal Reflection Diamonds Periscopes Fiber 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 Law Greater 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 (d) 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 source We’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 - 13 Pg. 753