Diffraction and Interference

Diffraction and Interference

• A wave generated by a single point source spreads in all directions. After traveling a large distance, the arriving wave fronts may appear to be flat.

• A wave generated by a single point source spreads in all directions. After traveling a large distance, the arriving wave fronts may appear to be flat.

How did a wave that “flattened out” begin to spread again after passing

through an opening?

How did a wave that “flattened out” begin to spread again after passing

through an opening?

Huygens’ PrincipleHuygens’ Principle• Wave Fronts are made up of tinier

wave fronts.• http://www.launc.tased.edu.au/online/

sciences/Physics/diffrac.html

• Wave Fronts are made up of tinier wave fronts.

• http://www.launc.tased.edu.au/online/sciences/Physics/diffrac.htmlhttp://www.launc.tased.edu.au/online/sciences/Physics/diffrac.html

Huygen’s PrincipleHuygen’s Principle

• Waves spreading out from a point source may be regarded as the overlapping of tiny secondary wavelets, and that every point on any wave may be regarded as a new point source of secondary waves.

• Waves spreading out from a point source may be regarded as the overlapping of tiny secondary wavelets, and that every point on any wave may be regarded as a new point source of secondary waves.

Huygens’ PrincipleHuygens’ Principle

• Think: Can two or more small waves combine into one big wave?

• Think: Can two or more small waves combine into one big wave?

Huygens’ PrincipleHuygens’ Principle• Think: Can two or more small

waves combine into one big wave?

• Think: Can two or more small waves combine into one big wave?

Huygens’ PrincipleHuygens’ Principle• So can’t you think of one big wave as the

combination of two or more small waves? THAT’S HUYGENS’ PRINCIPLE!

• So can’t you think of one big wave as the combination of two or more small waves? THAT’S HUYGENS’ PRINCIPLE!

DiffractionDiffraction

• A result of Huygens’ principle is that waves are constantly regenerating themselves. This becomes apparent when there is an obstacle or barrier.Diffraction with water waves

• A result of Huygens’ principle is that waves are constantly regenerating themselves. This becomes apparent when there is an obstacle or barrier.Diffraction with water waves

• Notice with the water waves that the diffraction was more obvious when the opening was small compared to the wavelength

• “Kissing Fingers Demo”

• Notice with the water waves that the diffraction was more obvious when the opening was small compared to the wavelength

• “Kissing Fingers Demo”

Helpful DiffractionHelpful Diffraction• The longer the wave compared

with the obstruction, the greater the diffraction.

• This is why AM radio stations get better reception in the mountains! They are longer wavelength, so bend around mountains.

• The longer the wave compared with the obstruction, the greater the diffraction.

• This is why AM radio stations get better reception in the mountains! They are longer wavelength, so bend around mountains.

Annoying DiffractionAnnoying Diffraction

• Ever wonder why you can’t make a microscope to “see” an atom in visible light?

• Ever wonder why you can’t make a microscope to “see” an atom in visible light?

Annoying DiffractionAnnoying Diffraction

• Ever wonder why you can’t make a microscope to “see” an atom in visible light?

• If the size of object is the same as the wavelength of light, the image will be blurred by diffraction. Smaller objects won’t be seen at all

• Ever wonder why you can’t make a microscope to “see” an atom in visible light?

• If the size of object is the same as the wavelength of light, the image will be blurred by diffraction. Smaller objects won’t be seen at all

Annoying DiffractionAnnoying Diffraction• Ever wonder why you can’t make a microscope

to “see” an atom in visible light?• If the size of object is the same as the

wavelength of light, the image will be blurred by diffraction. Smaller objects won’t be seen at all

• No optical microscope can be built big enough or designed well enough to overcome this diffraction limit.

• Electron microscopes used to “see” very small things

• Ever wonder why you can’t make a microscope to “see” an atom in visible light?

• If the size of object is the same as the wavelength of light, the image will be blurred by diffraction. Smaller objects won’t be seen at all

• No optical microscope can be built big enough or designed well enough to overcome this diffraction limit.

• Electron microscopes used to “see” very small things

Check your understanding

Check your understanding

• Why is blue light used to view tiny objects in an optical microscope?

• Why is blue light used to view tiny objects in an optical microscope?

Check your understanding

Check your understanding

• Why is blue light used to view tiny objects in an optical microscope?• Blue light is shorter wavelength,

so diffracts less compared to other colors of visible light.

• Why is blue light used to view tiny objects in an optical microscope?• Blue light is shorter wavelength,

so diffracts less compared to other colors of visible light.

Young’s Interference Experiment

Young’s Interference Experiment

• Is Light a wave or particle? All waves reflect, refract, diffract, and interfere.

• We’ve seen everything but interference so far. Can Light interfere and cancel out?

• Is Light a wave or particle? All waves reflect, refract, diffract, and interfere.

• We’ve seen everything but interference so far. Can Light interfere and cancel out?

Young’s Interference Experiement

Young’s Interference Experiement

• Used monochromatic light (light of single color

• Passed the light through two narrow slits

• Used monochromatic light (light of single color

• Passed the light through two narrow slits

An Interference pattern!! Yet another demonstration of light’s wave

properties!

An Interference pattern!! Yet another demonstration of light’s wave

properties!

Diffraction GratingDiffraction Grating

• Thousands of tiny slits. Diffracts light. Colors are produced by interference between light beams.

• So if you see red, what color is being cancelled?

• Thousands of tiny slits. Diffracts light. Colors are produced by interference between light beams.

• So if you see red, what color is being cancelled?

• x = • L d • d = distance between slits • x = distance between central

and 1st order• L = distance from center of slits

to 1st order

• x = • L d • d = distance between slits • x = distance between central

and 1st order• L = distance from center of slits

to 1st order

• Three of these quantities can be measured in an experiment. What 3?

• X, L and d so with that we can calculate the

• =(xd)/L

• Three of these quantities can be measured in an experiment. What 3?

• X, L and d so with that we can calculate the

• =(xd)/L

• ex• d=0.02cm x=0.184cm L=80cm• =(x*d)/L

• = (0.184*0.02)/80 = 4.6*10-5 cm

• Find freq.• f = c/ = (3*108m/s)/(8.6*10-

5cm*(1m/100cm)) = 6.5*1014

• ex• d=0.02cm x=0.184cm L=80cm• =(x*d)/L

• = (0.184*0.02)/80 = 4.6*10-5 cm

• Find freq.• f = c/ = (3*108m/s)/(8.6*10-

5cm*(1m/100cm)) = 6.5*1014

• CD’s and DVD’s are like diffraction gratings.

• CD’s and DVD’s are like diffraction gratings.

Thin-Film InterferenceThin-Film Interference• Have you ever wondered about

these?• Have you ever wondered about

these?

• Reflections off two surfaces put light waves out of phase. Some colors are cancelled. What’s left is what you see.

• Reflections off two surfaces put light waves out of phase. Some colors are cancelled. What’s left is what you see.

• In the case of oil and soap bubbles, the thickness varies throughout, so different colors cancel in different places

• In the case of oil and soap bubbles, the thickness varies throughout, so different colors cancel in different places