Light

Light Facts Visible Light is an electromagnetic wave that the human eye can detect. Different light wavelengths are seen as different colors. A ray is a straight line that represents the path of a light beam. Speed of Light (c) = 3.0 x 108 m/s

Speed of Light Physicist Albert Michelson determined the speed of light in the 1880s. He wanted to determine the speed of the “ether” through which light traveled. His experiment showed that the speed of light is about 300 000 km/s or 3.0 x 108 m/s

Nature of Light Light has a dual nature because it behaves as both a wave a particle Photons are bundles of electromagnetic energy that do not have mass The wave nature of light can be seen from interference patterns. EM waves are transverse waves.

How is light like a wave? Light reflects in the same manner a wave would. Light refracts in the same manner a wave would. Light diffracts in the same manner that a wave would.

Light will undergo interference in the same way a wave would.

Light exhibits the Doppler effect just as a wave would.

What happens when light reaches the end of a medium? 3 things will happen when light reaches the end of a medium. 1. Reflection: It bounces off an object 2. Refraction: If it approaches the boundary at an angle. 3. Diffraction: Boundary is an obstacle within the medium. And obstacle is smaller than wavelength.

1. Reflection Reflection: When an electromagnetic wave (or any other type of wave) “bounces” off of a surface. All waves show reflection. Law of Reflection- when a light ray strikes a reflecting surface, the angle of reflection is equal to the angle of incidence. Ex . Laser beam

Regular vs diffuse reflection Regular Reflection- when light rays are reflected in one direction. Ex. Smooth surface Diffuse Reflection- when light rays are reflected in many directions. Ex Rough surface

Regular Reflection Example

Diffuse Reflection Example

2. Refraction Refraction: The bending of a wave as it passes from one medium to another medium. All waves refract when they enter new mediums Refraction of light- bending of light due to the speed of light in different media. Ex Fish tank

Snell’s Law Snell’s Law- a ray of light bends in such a way that a ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant. ni sin I = nr sin r n = the index of refraction of the medium i = incident (“first”) r = refracted (“second”)

Index of Refractions for various substances Indices of Refraction Medium n Vacuum 1.0 Air 1.0003 Water 1.33 Ethanol 1.36 Crown glass 1.52 Quartz 1.54 Flint glass 1.61 Diamond 2.42

3. Diffraction Diffraction is the spreading of waves into a space or region behind an obstacle. Diffraction usually occurs when waves pass through small openings (like slits), around obstacles, or sharp edges Diffraction occurs with all waves

Electromagnetic waves An electromagnetic wave is (simplified) a self propagating wave that is part electric and part magnetic. It is caused by a moving charge. The range of electromagnetic waves can be seen in the electromagnetic spectrum. Notice the small portion that is visible light

EM wave

EM spectrum

Frequency will increase to right Higher frequency = higher energy Wavelength increases to left Inverse Relationship Broken up into sections Visible light is only a small portion

Color Colors of the spectrum are associated with specific light wavelengths Primary colors – red , green , blue Secondary colors – yellow, cyan, and magenta

The visible spectrum of Light

Calculation for spectrum C=λv λ lamda= wavelength v = frequency of the light wave c = speed of light (3x108 m/s)

Example What is the frequency of electromagnetic radiation having a wavelength of 210 x 10-7 m. 1.4x10 13

EM Waves Radiation: the transfer of energy by EM waves High frequency EM waves have the greatest amount of energy High frequency EM waves behave like a photon and are high energy waves Low frequency EM waves behave more like a wave and are lower energy waves Visible light is the only part of the EM spectrum that humans can see

Polarization The aligning of vibrations of a transverse wave. This is usually accomplished by filtering. Example: polarized sun glasses.

Polarization of light Polarization of light – light in which the electric fields are all in the same line.Light can be polarized because it is a transverse wave. Ex. Sunglasses

Common conversions 1 MHz = 106 Hz 109 nm = 1 m 100 cm = 1 m 1000 g = 1 kg 1000 m = 1 km

Light and Matter Transparent- objects that transmit light waves Ex. Glass, plastic, Translucent – transmit light but objects are not seen clearly. Ex. Frosted glass Opaque- absorb or reflect all light waves that fall on them. Ex. Brick

Index of Refraction and Speed of Light The index of refraction is the ratio of the speed of light in a vacuum to the speed in the medium. Equation: nsubstance = c / vsubstance or vs = c / ns