Light and Optics  The Electromagnetic Spectrum  Interference, Diffraction, and Polarization Wave Properties of Light

Light Objectives  Calculate the frequency or wavelength of light when given one of the two.  Describe the relationship between frequency, energy, color, and wavelength.  Identify at least three different waves of the electromagnetic spectrum and an application of each.  Describe a ray of light and explain how it reflects.  Explain refraction and use Snell’s Law to determine the angle in which a ray will bend.  Explain the concept of Diffraction.

Vocabulary Terms  x-ray  spectrum  microwave  index of refraction  electromagnetic wave  spectrometer  gamma ray  radio wave  transmission axis  diffraction grating  polarization  destructive interference  ultraviolet  infrared  speed of light  constructive interference  visible light  wavelength

A Brief History of Light  1000 AD —It was proposed that light consisted of tiny particles  Newton —Used this particle model to explain reflection and refraction  Huygens —Explained many properties of light by proposing light was wave-like

The Electromagnetic Spectrum Key Question: So what is Light???

 Light is an electromagnetic wave in that it has wave like properties which are created due to electric and magnetic fields.  E and B Fields oscillate perpendicular to each other as the wave moves.  These waves are called electromagnetic waves. The Electromagnetic Spectrum

 Electromagnetic waves have both an electric part and a magnetic part and the two parts exchange energy back and forth.  A 3-D view of an electromagnetic wave shows the electric and magnetic portions. The Electromagnetic Spectrum  The wavelength and amplitude of the waves are labeled λ and A, respectively.

 The higher the frequency of the light, the higher the energy of the wave.  Since color is related to energy, there is also a direct relation between color, frequency, and wavelength. The Electromagnetic Spectrum

Speed of Light c = f Wavelength (m) Frequency (Hz) Speed of light 3 x 10 8 m/sec Did you know that light can travel about 7.5 times around the world in 1 second!

Calculate wavelength  Calculate the wavelength in air of blue-green light that has a frequency of 600 × Hz.

Waves of the electromagnetic spectrum  Visible light is a small part of the energy range of electromagnetic waves.  The whole range is called the electromagnetic spectrum and visible light is in the middle of it.

Waves of the electromagnetic spectrum —Radio waves are on the low- frequency end of the spectrum. —Microwaves range in length from approximately 30 cm (about 12 inches) to about 1 mm. —The infrared (or IR) region of the electromagnetic spectrum lies between microwaves and visible light.

Waves of the electromagnetic spectrum —Ultraviolet radiation has a range of wavelengths from 400 down to about 10 nm. —X-rays are high-frequency waves that have great penetrating power and are used extensively in medical and manufacturing applications. —Gamma rays are generated in nuclear reactions.

We see things because they reflect light into our eyes: Homework

Light ray  Narrow beam of light that travels in a straight line. Laser

Law of Reflection Suppose A ray of light known as the incident ray travels in a medium until it encounters a boundary with a second medium. Then part (or all) of the incident ray is reflected back into the first medium… LAW OF REFLECTION: The angle of incidence is equal to the angle of reflection when the angle is measured relative to the normal (perpendicular)

Refraction Occurs when light bends when crossing a surface or moving through a material.

n 1 sin θ 1 = n 2 sin θ 2 θ 1 is the angle of incidence 30.0° in this diagram θ 2 is the angle of refraction

Light may refract into a material where its speed is lower The angle of refraction is less than the angle of incidence The ray bends toward the normal

Light may refract into a material where its speed is higher The angle of refraction is greater than the angle of incidence The ray bends away from the normal

Interference, Diffraction, and Polarization Key Question: What are some more ways light behaves like a wave?

Interference, Diffraction, and Polarization  In 1807, Thomas Young ( ) did the most convincing experiment demonstrating that light is a wave.  A beam of light fell on a pair of parallel, very thin slits in a piece of metal.  After passing through the slits, the light fell on a screen.  A pattern of alternating bright and dark bands formed is called an interference pattern.

Diffraction gratings  A diffraction grating is a precise array of tiny engraved lines, each of which allows light through.  The spectrum produced is a mixture of many different wavelengths of light.

How a Diffraction Grating Works When you look at a diffracted light you see: —the light straight ahead as if the grating were transparent. —a "central bright spot". —the interference of all other light waves from many different grooves produces a scattered pattern called a spectrum.

Grating Formula = d sin  dw L distance between grating lines (m) distance between screen and glasses wavelength of light (nm) d= 13,500 lines/inch = ? lines/m distance between 2 first order bright spots

Spectrometer  A spectrometer is a device that measures the wavelength of light.  A diffraction grating can be used to make a spectrometer because the wavelength of the light at the first-order bright spot can be expressed in a mathematical relationship.

Polarization  Polarization is another wave property of light.  The fact that light shows polarization tells us that light is a transverse wave.

Polarization  Polarization is a vector.  A wave with polarization at 45 degrees can be represented as the sum of two waves.  Each of the component waves has smaller amplitude.

Polarization  A polarizer is a material that selectively absorbs light depending on polarization.  A polarizer re-emits a fraction of incident light polarized at an angle to the transmission axis.

Applications of Polarizers  Polarizing sunglasses are used to reduce the glare of reflected light  The LCD (liquid crystal diode) screen on a laptop computer uses polarized light to make pictures.