two coherent light sources

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley

Two sources S1 and S2 oscillating in phase emit sinusoidal waves.

Point P is 7.3 wavelengths from source S1 and 4.3 wavelengths from source S2. As a result, at point P there is

Q35.1

A. constructive interference.

B. destructive interference.

C. neither constructive nor destructive interference.

D. not enough information given to decide.




A35.1








Q35.2








A35.2






Interference from two radio stations• Two radio antennas are separated by 2.0 m. Both broadcast

identical 750 MHz waves. If you walk around the antennas in a circle of radius 10 m, how many maxima will you detect?


Two coherent light sources


Interference from two radio stations revisited• Radio station operating at 1500 kHz has two antennas spaced

400m apart. In which directions is the intensity greatest in the resulting radiation pattern far away (>> 400m) from the antennas? How many total regions of high intensity are there?


As the waves interfere, they produce fringes A red laser produces the following fringe pattern:• What happens to fringe pattern if the spacing

between slits increases?• What happens if you shine a green laser (higher

frequency) through the same slits?• What happens if you move the screen farther away

from the slits?

Spacing between fringe pattern• A) increases• B) decreases• C) stays the same


Coherent light passing through two slits (S1 and S2) produces a pattern of dark and bright areas on a distant screen. If the wavelength of the light is increased, how does the pattern change?

Q35.3

A. The bright areas move closer together.

B. The bright areas move farther apart.

C. The spacing between bright areas remains the same, but the color changes.

D. any of the above, depending on circumstances

E. none of the above


Coherent light passing through two slits (S1 and S2) produces a pattern of dark and bright areas on a distant screen. If the wavelength of the light is increased, how does the pattern change?

A35.3

A. The bright areas move closer together.

B. The bright areas move farther apart.

C. The spacing between bright areas remains the same, but the color changes.

D. any of the above, depending on circumstances

E. none of the above


Coherent light passing through two slits (S1 and S2) produces a pattern of dark and bright areas on a distant screen.

What is the difference between the distance from S1 to the m = +3 bright area and the distance from S2 to the m = +3 bright area?

Q35.4

A. three wavelengths

B. three half-wavelengths

C. three quarter-wavelengths

D. not enough information given to decide



What is the difference between the distance from S1 to the m = +3 bright area and the distance from S2 to the m = +3 bright area?

A35.4

A. three wavelengths

B. three half-wavelengths

C. three quarter-wavelengths




What is the difference between the phase from S1 to the m = +3 bright area and the distance from S2 to the m = +3 bright area?

Q35.4

A. 3p/2

B. 3p

C. 6p




What is the difference between the phase from S1 to the m = +3 bright area and the distance from S2 to the m = +3 bright area?

A35.4

A. 3p/2

B. 3p

C. 6p



Diffraction gratingWhat is the first order diffraction peak (angle) for a grating with 600 slits per mm for red (700 nm) and violet (400nm) light? For a screen 1 m away, what distance away from the central peak is the first order peak?

By what angle (max angle minus min angle) is the rainbow spread out for the first order diffraction?

How many constructive interference peaks are there?


Fraunhofer diffraction and an example of analysis• A red laser (700nm) is shown through a single slit.• What is the slit width for this diffraction pattern?


Q36.1

A. Double the slit width a and double the wavelength l.

B. Double the slit width a and halve the wavelength l.

C. Halve the slit width a and double the wavelength l.

D. Halve the slit width a and halve the wavelength l.

Light of wavelength l passes through a single slit of width a. The diffraction pattern is observed on a screen that is very far from from the slit.

Which of the following will give the greatest increase in the angular width of the central diffraction maximum?


A36.1

A. Double the slit width a and double the wavelength l.

B. Double the slit width a and halve the wavelength l.

C. Halve the slit width a and double the wavelength l.

D. Halve the slit width a and halve the wavelength l.

Light of wavelength l passes through a single slit of width a. The diffraction pattern is observed on a screen that is very far from from the slit.

Which of the following will give the greatest increase in the angular width of the central diffraction maximum?


Intensity maxima in a single-slit pattern• The expression for peak

maxima is iterated for the strongest peak.


In a single-slit diffraction experiment with waves of wavelength l, there will be no intensity minima (that is, no dark fringes) if the slit width is small enough.

What is the maximum slit width a for which this occurs?

Q36.2

A. a = l/2

B. a = l

C. a = 2l

D. The answer depends on the distance from the slit to the screen on which the diffraction pattern is viewed.


In a single-slit diffraction experiment with waves of wavelength l, there will be no intensity minima (that is, no dark fringes) if the slit width is small enough.

What is the maximum slit width a for which this occurs?

A36.2

A. a = l/2

B. a = l

C. a = 2l

D. The answer depends on the distance from the slit to the screen on which the diffraction pattern is viewed.


Intensity from single slit• Single slit pattern expands as slit width decreases


Multiple slit interference


Several slitsMore slits produces sharper peaks


In Young’s experiment, coherent light passing through two slits separated by a distance d produces a pattern of dark and bright areas on a distant screen.

If instead you use 10 slits, each the same distance d from its neighbor, how does the pattern change?

Q36.3

A. The bright areas move farther apart.

B. The bright areas move closer together.

C. The spacing between bright areas remains the same, but the bright areas become narrower.

D. The spacing between bright areas remains the same, but the bright areas become broader.


In Young’s experiment, coherent light passing through two slits separated by a distance d produces a pattern of dark and bright areas on a distant screen.

If instead you use 10 slits, each the same distance d from its neighbor, how does the pattern change?

A36.3

A. The bright areas move farther apart.

B. The bright areas move closer together.

C. The spacing between bright areas remains the same, but the bright areas become narrower.

D. The spacing between bright areas remains the same, but the bright areas become broader.


Michelson and Morley’s interferometer• In this amazing experiment at Case Western Reserve, Michelson and Morley

suspended their interferometer on a huge slab of sandstone on a pool of mercury (very stable, easily moved). As they rotated the slab, movement of the earth could have added in one direction and subtracted in another, changing interference fringes each time the device was turned a different direction. They did not change. This was an early proof of the invariance of the speed of light.

two coherent light sources

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