AS (Optics) question help needed!
Hello, I am self teaching A Level Physics, and have hit a brick wall with a question (hence why i'm here!)
The question is on topic 13.6: Diffraction.
Figure 3 of topic 13.5 (attached) shows the interference fringes observed in a Young's fringes experiment.
A) Explain why the fringes at the outer edges are dimmer than the fridges near the centre.
For this i am really lost - i know that intensity falls as you get further away from the central maxima but i have no idea why!
B) Describe the appearance of the fringes when the slits were made wider without changing the slit-to-slit separation
Again - i'm pretty sure that the fringes would be wider due to the equation for W, but i have no idea why.
C) Sketch a graph to show how the intensity of the fringes varies with distance from the central fringes
The intensity falls, using a double middle slit doesn't it? So the first peak is the most intense, which covers two fringes, then the intensity for each peak gets smaller and smaller? Confused as to how it would look though - if anyone has the link to a diagram it would be greatly appreciated!
D) Explain why the wider slits at the same spacing could produce fringes at the outer edges brighter than some of the fringes near the centre.
I have absolutely NO idea with this one.
Any help appreciated. Thanks!
The question is on topic 13.6: Diffraction.
Figure 3 of topic 13.5 (attached) shows the interference fringes observed in a Young's fringes experiment.
A) Explain why the fringes at the outer edges are dimmer than the fridges near the centre.
For this i am really lost - i know that intensity falls as you get further away from the central maxima but i have no idea why!
B) Describe the appearance of the fringes when the slits were made wider without changing the slit-to-slit separation
Again - i'm pretty sure that the fringes would be wider due to the equation for W, but i have no idea why.
C) Sketch a graph to show how the intensity of the fringes varies with distance from the central fringes
The intensity falls, using a double middle slit doesn't it? So the first peak is the most intense, which covers two fringes, then the intensity for each peak gets smaller and smaller? Confused as to how it would look though - if anyone has the link to a diagram it would be greatly appreciated!
D) Explain why the wider slits at the same spacing could produce fringes at the outer edges brighter than some of the fringes near the centre.
I have absolutely NO idea with this one.
Any help appreciated. Thanks!
This simulation might help
http://www.wiley.com/college/halliday/0470469080/simulations/sim48/sim48.html
http://www.wiley.com/college/halliday/0470469080/simulations/sim48/sim48.html
A) The fringes are brighter in the centre because this is the point at which the two sources of light constructively interfere and the greatest intensity, as there is no path difference. If you think about it, this is the closest to the source as it gets, and the waves, (or you can model as photons if it helps), spread out over distance, which is one of the standard laws which explain the behaviour of light. That's why this happens.
B) The slits being made wider means that we are less certain as to the position of the photons as they traverse the slits, which basic quantum mechanics will dictate that uncertainty of momentum will increase. Because the momentum is more likely to be of higher value in these circumstances, the light has a greater inertia and so diffracts less. So there is a much smaller angle of diffraction in this case.
C)Don't know. Sorry. We might be using different boards (mine's AQA). Would be interested in any answer to this.
D)There may be an uneven diffraction effect if the light source is not monochromatic or not exactly central compared with the slits. Or simply if light coming from two separate angles can have said effect on the pattern of interference.
Hope this was helpful.
B) The slits being made wider means that we are less certain as to the position of the photons as they traverse the slits, which basic quantum mechanics will dictate that uncertainty of momentum will increase. Because the momentum is more likely to be of higher value in these circumstances, the light has a greater inertia and so diffracts less. So there is a much smaller angle of diffraction in this case.
C)Don't know. Sorry. We might be using different boards (mine's AQA). Would be interested in any answer to this.
D)There may be an uneven diffraction effect if the light source is not monochromatic or not exactly central compared with the slits. Or simply if light coming from two separate angles can have said effect on the pattern of interference.
Hope this was helpful.
Original post by Nuclear Ghost
A) The fringes are brighter in the centre because this is the point at which the two sources of light constructively interfere and the greatest intensity, as there is no path difference. If you think about it, this is the closest to the source as it gets, and the waves, (or you can model as photons if it helps), spread out over distance, which is one of the standard laws which explain the behaviour of light. That's why this happens.
B) The slits being made wider means that we are less certain as to the position of the photons as they traverse the slits, which basic quantum mechanics will dictate that uncertainty of momentum will increase. Because the momentum is more likely to be of higher value in these circumstances, the light has a greater inertia and so diffracts less. So there is a much smaller angle of diffraction in this case.
C)Don't know. Sorry. We might be using different boards (mine's AQA). Would be interested in any answer to this.
D)There may be an uneven diffraction effect if the light source is not monochromatic or not exactly central compared with the slits. Or simply if light coming from two separate angles can have said effect on the pattern of interference.
Hope this was helpful.
B) The slits being made wider means that we are less certain as to the position of the photons as they traverse the slits, which basic quantum mechanics will dictate that uncertainty of momentum will increase. Because the momentum is more likely to be of higher value in these circumstances, the light has a greater inertia and so diffracts less. So there is a much smaller angle of diffraction in this case.
C)Don't know. Sorry. We might be using different boards (mine's AQA). Would be interested in any answer to this.
D)There may be an uneven diffraction effect if the light source is not monochromatic or not exactly central compared with the slits. Or simply if light coming from two separate angles can have said effect on the pattern of interference.
Hope this was helpful.
Take a look at the link in my previous post for the answers.
Original post by Stonebridge
Take a look at the link in my previous post for the answers.
Honestly, I don't know how I didn't see that. Thinking too hard, I guess.
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