Interference and Diffraction Preview Section 1 Interference Section

Interference and Diffraction Preview Section 1 Interference Section 2 Diffraction Section 3 Lasers © Houghton Mifflin Harcourt Publishing Company Section 1

Interference and Diffraction Section 1 What do you think? • Sound waves and water waves can interfere constructively and destructively. Examples are the wake behind a boat, beats used to tune instruments, and dead spots in auditoriums. Can light waves interfere as well? • If so, can you think of any examples? • Suppose two light waves were to interfere destructively and cancel, so no light was seen. How might this occur? © Houghton Mifflin Harcourt Publishing Company

Interference and Diffraction Section 1 Light Wave Interference • Like other waves, light waves can add constructively and destructively as shown above. • Examples: – Colors seen in soap bubbles – Colors seen in a thin film of oil on water © Houghton Mifflin Harcourt Publishing Company

Interference and Diffraction Section 1 Requirement for Interference • Waves must maintain a constant phase relationship, or be coherent. – In figure a, the waves are in phase. – In figure b, the waves are out of phase. • Incoherent light has phase relationships that shift. – Interference will not be observed. © Houghton Mifflin Harcourt Publishing Company

Interference and Diffraction Section 1 Comparing Noncoherent and Coherent Light Click below to watch the Visual Concept © Houghton Mifflin Harcourt Publishing Company

Interference and Diffraction Section 1 Demonstrating Interference • Light from a single source passes through two slits, creating two coherent sources. • The two waves then spread out in all directions (Huygen’s Principle) and interfere with each other. – Constructive interference creates bright spots on a screen. – Destructive interference creates dark spots on a screen. © Houghton Mifflin Harcourt Publishing Company

Interference and Diffraction Section 1 Constructive Interference • The two waves travel the same distance to the center of the screen. – They arrive in phase. – Constructive interference occurs. – A bright spot is seen in the middle of the screen. © Houghton Mifflin Harcourt Publishing Company

Interference and Diffraction Section 1 Constructive Interference • The top wave travels a distance of 1 farther than the bottom wave. – They arrive at the screen in phase. – Constructive interference occurs. – A bright spot is seen. • What distances other than 1 would result in a bright spot on the screen? © Houghton Mifflin Harcourt Publishing Company

Interference and Diffraction Section 1 Destructive Interference • The top wave travels a distance of (1/2) farther than the bottom wave. – They arrive at the screen out of phase. – Destructive interference occurs. – A dark area is seen. • What distances other than (1/2) would result in a dark area on the screen? © Houghton Mifflin Harcourt Publishing Company

Interference and Diffraction Interference Arising from Two Slits Click below to watch the Visual Concept © Houghton Mifflin Harcourt Publishing Company Section 1

Interference and Diffraction Section 1 Calculating Maxima and Minima • Because the screen is distant, l 1 and l 2 are nearly parallel. • This makes the angles almost equal. – d sin is the extra distance the lower wave travels while moving to a common point on the screen. © Houghton Mifflin Harcourt Publishing Company

Interference and Diffraction Section 1 Calculating Maxima and Minima • For constructive interference, d sin (path length difference) is a whole number of wavelengths. • For destructive interference, the path length difference is an odd number of half-wavelengths (1/2, 3/2, 5/2 and so on). © Houghton Mifflin Harcourt Publishing Company

Interference and Diffraction Section 1 Classroom Practice Problem • The distance between the two slits is 0. 0050 mm. Find the angles of the zeroth-, first-, second- and thirdorder bright fringes of interference produced by light with a wavelength of 550 nm. – Answers: 0°, 6. 3°, 19° © Houghton Mifflin Harcourt Publishing Company

Interference and Diffraction Section 1 Classroom Practice Problem • When monochromatic light (light of a single wavelength) falls on two slits with a separation of 0. 010 mm, the zeroth-order dark fringes are observed at a 2. 0° angle. Find the wavelength. – Answer: 0. 00070 mm or 7. 0 10 -7 m or 7. 0 102 nm © Houghton Mifflin Harcourt Publishing Company

Interference and Diffraction Section 1 Now what do you think? • Sound waves and water waves can interfere constructively and destructively. Examples are the wake behind a boat, beats used to tune instruments and dead spots in auditoriums. Can light waves interfere as well? • If so, can you think of any examples? • Suppose two light waves were to interfere destructively and cancel, so no light was seen. How might this occur? © Houghton Mifflin Harcourt Publishing Company

Interference and Diffraction Section 2 What do you think? • Suppose you are standing off to the side of a door on your friend’s porch. She is inside the house speaking to you. Can you hear her, even though you are off to the side of the door? • Using your knowledge of waves, explain how this might occur. • Consider the same situation described above. Can you see your friend when standing off to the side of the door? • Why or why not? © Houghton Mifflin Harcourt Publishing Company

Interference and Diffraction Section 2 Diffraction • A change in direction for waves when they encounter an obstacle or pass through an opening – Sound waves around a tree or through a door – Huygens’ principle states each point on a wave is a source for new waves. • Diffraction occurs to a greater extent if the wavelength and opening size are appropriate. – Openings much larger than the wavelength show very little diffraction. • Light waves through a door © Houghton Mifflin Harcourt Publishing Company

Interference and Diffraction Single Slit Interference • Occurs because one part of the wave interferes with the other parts • The diagram shows 5 sample points, each producing waves. – The center line will be a point of constructive interference. – Waves 1 and 5 travel the same distance to this line as do waves 2 and 4. © Houghton Mifflin Harcourt Publishing Company Section 2

Interference and Diffraction Section 2 Single Slit Interference • In order to reach a point above the center line, waves travel different distances. – Wave 3 travels 1/2 farther than wave 1, so destructive interference occurs. – Similarly, wave 5 travels 1/2 farther than wave 3. • Thus, at the angle shown, destructive interference occurs and the screen is dark. © Houghton Mifflin Harcourt Publishing Company

Interference and Diffraction Section 2 Diffraction Around Obstacles • Suppose a laser shines on a screen. The round head of a pin (like those used to pin clothing) is placed between the laser and screen. You would expect to see a round shadow on the screen. – What will you see in the center of the shadow? Why? – You will see a bright spot if you look very carefully. The waves diffract and all waves travel the same distance to the center of the shadow, creating constructive interference. © Houghton Mifflin Harcourt Publishing Company

Interference and Diffraction Section 2 Diffraction Gratings • Many slits very closely spaced – Behaves like the double slit, but maxima and minima are much brighter. • Monochromatic light produces bright and dark fringes. • White light produces a full spectrum. – Similar to reflection off a CD surface © Houghton Mifflin Harcourt Publishing Company

Interference and Diffraction Section 2 Diffraction Gratings • Diffraction gratings are used in spectrometers to separate light into its component colors. – Used to study the makeup of distant stars – Helium’s spectrum was first observed on the sun, and helium was later discovered on Earth. © Houghton Mifflin Harcourt Publishing Company

Interference and Diffraction Function of a Spectrometer Click below to watch the Visual Concept © Houghton Mifflin Harcourt Publishing Company Section 2

Interference and Diffraction Section 2 Problem Solving - Diffraction Gratings • Equations are identical to those for two slit interference. – d is the distance between adjacent slits or lines. – If there are 8000 lines per cm, then d = (1/8000) cm © Houghton Mifflin Harcourt Publishing Company

Interference and Diffraction Section 2 Classroom Practice Problems • Monochromatic light shines on the surface of a diffraction grating with 5. 0 103 lines/cm. The first order maximum is observed at an angle of 15°. Find the wavelength. – Answer: 5. 2 102 nm • Find the first-order and the second-order angles of diffraction observed through a 1. 0 104 lines/cm diffraction grating with light of wavelengths 400. 0 nm and 600. 0 nm – Answers for 400 nm light: 23. 6°; 53. 1° – Answers for 600. 0 nm light: 36. 9°; the second order does not occur because it would be greater than 90° (sin = 1. 2 does not occur) © Houghton Mifflin Harcourt Publishing Company

Interference and Diffraction Section 2 Instrument Resolution • If observing two distant objects, the diffraction patterns could overlap as shown. • Without diffraction, there would be two bright spots on the screen. © Houghton Mifflin Harcourt Publishing Company

Interference and Diffraction Section 2 Resolving Power • The wavelength ( ) and the opening size (D) determine the resolving power. – is the limiting angle between the two resolved objects measured in radians. – Longer wavelengths require a larger aperture (D) to resolve distant objects. – Because radio waves are long waves, radio telescopes are very large to accommodate the need for a large aperture. © Houghton Mifflin Harcourt Publishing Company

Interference and Diffraction Section 2 Instrument Resolution • Imagine the opening is the pupil of your eye and the two sources are adjacent red and green pixels on your television screen. – What will you see? – How would it change if your pupil opening was larger? – How would it change if you were closer? – How would it change if the pixels were farther apart? © Houghton Mifflin Harcourt Publishing Company

Interference and Diffraction Section 2 Now what do you think? • Suppose you are standing off to the side of a door on your friend’s porch. She is inside the house speaking to you. Can you hear her, even though you are off to the side of the door? – Using your knowledge of waves, explain how this might occur. • Consider the same situation described above. Can you see your friend when standing off to the side of the door? – Why or why not? © Houghton Mifflin Harcourt Publishing Company

Interference and Diffraction Section 3 What do you think? • Lasers are quite common in today’s world. List all applications for lasers that come to mind. • How does the use of a laser in each application improve on prior methods? • For example, a laser pointer can be used from anywhere in the room. • How is laser light different from other forms of light, such as that produced by a flashlight? • Is it produced differently? Explain. • Does it behave differently? Explain. © Houghton Mifflin Harcourt Publishing Company

Interference and Diffraction Section 3 Coherent Light • Light bulb filaments emit light of many different wavelengths and phases. – Hot filaments emit incoherent light. – Like water waves on a pond during the rain • Lasers produce a narrow beam of coherent light. – Laser light is not produced by a hot filament. © Houghton Mifflin Harcourt Publishing Company

Interference and Diffraction Section 3 Laser • LASER is an acronym. – Light Amplification by Stimulated Emission of Radiation • Lasers use light, electrical energy, or chemical energy to produce coherent light. • The active medium can be a solid, liquid, or gas. – The medium determines the wavelength of the light. © Houghton Mifflin Harcourt Publishing Company

Interference and Diffraction Section 3 Producing Light with a Laser (Step 1) • Energy is added to the medium. – When absorbing the energy, atoms move into higher energy states. – Excited atoms release this energy as light or some other EM radiation when they return to lower energy states. © Houghton Mifflin Harcourt Publishing Company

Interference and Diffraction Section 3 Producing Light with a Laser (Step 2) • Light emitted by one atom can induce adjacent atoms to emit light with the same properties (wavelength and phase). – The process continues, and light intensity begins to increase. – Called stimulated emission © Houghton Mifflin Harcourt Publishing Company

Interference and Diffraction Section 3 Producing Light with a Laser (Step 3) • Mirrors at the ends reflect the light back into the medium, and intensity continues to increase. • The mirror on the right is not fully reflective. Thus, some light passes through once the intensity is great enough. • The light is monochromatic and in phase (coherent). © Houghton Mifflin Harcourt Publishing Company

Interference and Diffraction Laser Click below to watch the Visual Concept © Houghton Mifflin Harcourt Publishing Company Section 3

Interference and Diffraction Section 3 Applications of Lasers • Measuring distance is possible because the beam does not spread out very much. – Mirrors left on the moon are used to reflect laser light back to Earth to determine the distance. • Accurate to within a few centimeters • Able to measure the rise and fall of Earth’s crust © Houghton Mifflin Harcourt Publishing Company

Interference and Diffraction Section 3 Applications of Lasers • Medical uses – Certain lasers that pass though the cornea and lens can be used to treat tears in the retina or to seal bleeding vessels in the retina. – Lasers can be used for incisions. • This cauterizes the wound while cutting to reduce bleeding. – Laser light can pass through optical fibers and treat internal problems without massive surgery. © Houghton Mifflin Harcourt Publishing Company

Interference and Diffraction Section 3 Now what do you think? • Lasers are quite common in today’s world. List all applications for lasers that come to mind. – How does the use of a laser in each application improve on prior methods? • For example, a laser pointer can be used from anywhere in the room. – How is laser light different from other forms of light, such as that produced by a flashlight? • Is it produced differently? Explain. • Does it behave differently? Explain. © Houghton Mifflin Harcourt Publishing Company