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Welcome to blog help for PHY 122! Here's how it works. For each homework question and lab report we will make a post, this will probably contain a few tips on what the problems are about and how to solve them. If you are stuck on something then instead of emailing us directly you should post a comment in reply to the relevant post. We will try to guide you through tough points and help you understand the problems and the concepts behind them.
8 comments:
For these problems make sure you understand the sign conventions for mirrors summarized in Table 23.1 on p. 765 of SV8. I suggest you go carefully through the worked-out examples in Ch. 23.3.
I cannot figure out number 9 on the webassign
I have tried 1/p+1/q=1/f and it does not work.
My numbers:
n=1.70
r=10cm
p (as given): 20cm, 8.0cm, 4.00cm, 2.00cm.
When I do the problem for 20, I get the right answer. When I attempt to for the other portions I don't come close to the answer.
I 13.33 when I use p=8.0cm. someone help!
Thanks!
-Vito
Vito,
Are you sure you mean WA prob. 9 (which is SV8 prob. 22 in Ch. 23.)If so, you're using the wrong formula; this is not a thin-lens problem. Look at SV8, Fig. 23.15 for what this problem looks like. The relevant formula for such a problem is [23.7].
Prof. Koch
Professor,
I attempted to use that formula, first practicing with it using example 23.5 in the text. However, when I plugged in my numbers I got 7.0165. Now I know that my answer should be close to 79.0165, because it tells me I'm within 10% of the correct answer. I am not sure what I did wrong.
I set it up as such:
(1/8.0cm)+(1.70/q)=[(1.70-1.0)/10]
I don't understand what I'm doing wrong.
-Vito
Vito,
You must used the exact value for the Index of Refraction of air which is n= 1.000293.
Hope this helps!
Jen! Thank you thank you thank you!!!!!!!!!!!!!!!!!!!!!!!! it worked!!!! =D
-Vito
Jen-TA and Vito, Thanks to Jen-TA for helping out, but I have to disagree with Jen-TA's comment. I can see the results of the WebAssign HW for each student. I looked at Vito's and worked his problem 9 (the randomization of input given to him by WA for SV8 Ch.23, Prob. 22) that you both discussed in the blog entries above. Using n=1 for air or n=1.000293 for air, I get the same result for q to 3 significant figures. This result agrees with the result that WA posts (WA gives it to 3 significant figures) for the Vito's problem when p=8 cm. That's why I disagree. Vito entered his answer with 6 significant figures that round off to the WA answer at 3 significant figures. It would be poor practice for WA to require use of n_air to 7 significant figures when SV8 routinely uses n=1 for air. Prof. Koch
We are aware of a bug in WebAssign HW problem 9 (SV8 Ch 23 Prob 22). This only occurs for people who are given randomized numbers n=1.60, R=12 cm, but it causes WA to mark all parts wrong. If this affects you, just enter what you believe are correct answers and we will adjust your score later. WebAssign sincerely apologizes for the error, and they are working hard to make it right. As far as we know, this only affects people who are given those specific numerical values.
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