Chapter2: Light and EM Spectrum

• 18.1 The Electromagnetic Spectrum

• 18.2 Interference, Diffraction, and Polarization

• 18.3 Special Relativity

Professor Mohammad Sajjad AlamUniversity at AlbanySeptember 28, 2010Adapted from Web

Chapter 18 Objectives

1. Calculate the frequency or wavelength of light when given one of the two.

2. Describe the relationship between frequency, energy, color, and wavelength.

3. Identify at least three different waves of the electromagnetic spectrum and an application of each.

4. Interpret the interference pattern from a diffraction grating.5. Use the concept of polarization to explain what happens as light

passes through two polarizers.6. Describe at least two implications of special relativity with

regards to energy, time, mass, or distance.

Chapter 18 Vocabulary Terms• x-ray

spectrum microwave index of

refraction electromagnetic

wave spectrometer gamma ray radio wave transmission axis diffraction

grating special relativity

polarization polarizer rest energy destructive interference ultraviolet time dilation infrared speed of light constructive

interference visible light wavelength

18.1 The Electromagnetic Spectrum

Key Question:What is the electromagnetic

spectrum?

*Students read Section 18.1 BEFORE Investigation 18.1

• The energy field created by electricity and magnetism can oscillate and it supports waves that move.

• These waves are called electromagnetic waves.

18.1 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.

18.1 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.

18.1 The Electromagnetic Spectrum

18.1 Speed of Light

c = f l

Wavelength (m)

Frequency (Hz)

Speed of light3 x 108 m/sec

18.1 Calculate wavelength

• Calculate the wavelength in air of blue-green light that has a frequency of 600 × 1012 Hz.

18.1 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.

18.1 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.

18.1 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.

18.2 Interference, Diffraction, and Polarization

Key Question:What are some ways

light behaves like a wave?

*Students read Section 18.2 AFTER Investigation 18.2

18.2 Interference, Diffraction, and Polarization• In 1807, Thomas Young (1773-1829) 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.

18.2 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.

18.2 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.

18.2 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.

18.2 Grating Formula

l = d sinq ~ dw

L

distance between grating lines (m)

distance between

screen and glasses

wavelengthof light (nm)

d= 13,500 lines/inch = ? lines/m

distance between

2 first order bright spots

18.2 Polarization

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

transverse wave.

18.2 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.

18.2 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.

18.2 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.