# Help with Isaac physics optics question

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https://isaacphysics.org/questions/light_circ_mirror

Part D has been confusing me for a while now... I think it's more mathematical in comparison to physics/equations etc.

The SA of the cap is 2πRh, so surely SA of shell is 2πR(x+h), so then one would need to figure out R next, in order to find values for x and h, but how do you find R?

*(the above method is just how I went about trying to solve the question, I have no idea whether it's right or wrong, although there's most likely a better one somewhere as I haven't gotten anywhere with it)

Part D has been confusing me for a while now... I think it's more mathematical in comparison to physics/equations etc.

The SA of the cap is 2πRh, so surely SA of shell is 2πR(x+h), so then one would need to figure out R next, in order to find values for x and h, but how do you find R?

*(the above method is just how I went about trying to solve the question, I have no idea whether it's right or wrong, although there's most likely a better one somewhere as I haven't gotten anywhere with it)

Last edited by user342; 1 month ago

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(Original post by

https://isaacphysics.org/questions/light_circ_mirror

Part D has been confusing me for a while now... I think it's more mathematical in comparison to physics/equations etc.

The SA of the cap is 2πRh, so surely SA of shell is 2πR(x+h), so then one would need to figure out R next, in order to find values for x and h, but how do you find R?

*(the above method is just how I went about trying to solve the question, I have no idea whether it's right or wrong, although there's most likely a better one somewhere as I haven't gotten anywhere with it)

**user342**)https://isaacphysics.org/questions/light_circ_mirror

Part D has been confusing me for a while now... I think it's more mathematical in comparison to physics/equations etc.

The SA of the cap is 2πRh, so surely SA of shell is 2πR(x+h), so then one would need to figure out R next, in order to find values for x and h, but how do you find R?

*(the above method is just how I went about trying to solve the question, I have no idea whether it's right or wrong, although there's most likely a better one somewhere as I haven't gotten anywhere with it)

https://www.thestudentroom.co.uk/sho...php?p=88232022

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(Original post by

The following thread may help.

https://www.thestudentroom.co.uk/sho...php?p=88232022

**Eimmanuel**)The following thread may help.

https://www.thestudentroom.co.uk/sho...php?p=88232022

If so, should I change R to (x^2+15^2)^1/2 then open up brackets?

Last edited by user342; 1 month ago

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(Original post by

https://isaacphysics.org/questions/light_circ_mirror

Part D has been confusing me for a while now... I think it's more mathematical in comparison to physics/equations etc. ...

**user342**)https://isaacphysics.org/questions/light_circ_mirror

Part D has been confusing me for a while now... I think it's more mathematical in comparison to physics/equations etc. ...

(Original post by

... The SA of the cap is 2πRh, so surely SA of shell is 2πR(x+h),...

**user342**)... The SA of the cap is 2πRh, so surely SA of shell is 2πR(x+h),...

(Original post by

...although I think I'm a bit more confused now...

**user342**)...although I think I'm a bit more confused now...

(Original post by

...If so, should I change R to (x^2+15^2)^1/2 then open up brackets?

**user342**)...If so, should I change R to (x^2+15^2)^1/2 then open up brackets?

It would better that you write your thinking logically or your working with some explanation for others to understand you better instead just "spitting out" formula or equation.

Last edited by Eimmanuel; 1 month ago

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(Original post by

Agree. Perhaps is just geometry.

I don't really see the logic. Isn't x+h = R? What do you understand by SA of shell or SA of spherical shell?

I would say the reverse. R is already given in the question.

**Eimmanuel**)Agree. Perhaps is just geometry.

I don't really see the logic. Isn't x+h = R? What do you understand by SA of shell or SA of spherical shell?

I would say the reverse. R is already given in the question.

i got to this (x^2+15^2)^1/2 as x R and 1/2 of d (d is 30) are right angled triangle, so wanted to use pythagoras to try to get R out of the way

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(Original post by

Good point, I didn't recognise that R=x+h. How would you find SA of shell then? And what do you mean when you say R is already given in the question?

i got to this (x^2+15^2)^1/2 as x R and 1/2 of d (d is 30) are right angled triangle, so wanted to use pythagoras to try to get R out of the way

**user342**)Good point, I didn't recognise that R=x+h. How would you find SA of shell then? And what do you mean when you say R is already given in the question?

i got to this (x^2+15^2)^1/2 as x R and 1/2 of d (d is 30) are right angled triangle, so wanted to use pythagoras to try to get R out of the way

(Original post by

….How would you find SA of shell then? ….

**user342**)….How would you find SA of shell then? ….

(Original post by

And what do you mean when you say R is already given in the question? …

**user342**)And what do you mean when you say R is already given in the question? …

Yes, R is not given for this question. I would edit the post.

(Original post by

…i got to this (x^2+15^2)^1/2 as x R and 1/2 of d (d is 30) are right angled triangle, so wanted to use pythagoras to try to get R out of the way

**user342**)…i got to this (x^2+15^2)^1/2 as x R and 1/2 of d (d is 30) are right angled triangle, so wanted to use pythagoras to try to get R out of the way

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