Physics Question

sci0x · 18-09-2003 7:37pm #1

Physics Question: Can you tell me the answer and how you got it please

A concave mirror has a focal length of 20cm. Find 2 positions where an object can be placed so that the image is 3 times the size of the object.

davej · 19-09-2003 8:06am

Hey we're not here to do your homework for you!

Take a look at this page for some (big) clues...

http://www.physicsclassroom.com/Class/refln/U13L3f.html

davej

MarVeL · 19-09-2003 1:07pm

This reminded me of a question I read in a Spider Robinson book:

Picture an astronaut floating at the centre of a perfect sphere. The inside of the sphere is 100% reflective. He turns on the lamp on his helmet. What does he see?

Haven't yet managed to come up with a reasonable answer, any ideas?

Capt'n Midnight · 19-09-2003 3:39pm

Even if the surface is 100% reflective, the astronaut is not.
So there is no chance of it being really bright...

Sangre · 19-09-2003 3:42pm

Wouldnt he just see himself in the mirror?

Also m=u/v

3=u/v

U=3v

Then put that into 1/f=1/u+1/v

And do the rest.

Sleipnir · 19-09-2003 4:11pm

Originally posted by MarVeL
This reminded me of a question I read in a Spider Robinson book:

Picture an astronaut floating at the centre of a perfect sphere. The inside of the sphere is 100% reflective. He turns on the lamp on his helmet. What does he see?

Haven't yet managed to come up with a reasonable answer, any ideas?

That's a good one alright.
Seeing as it is 100% reflective, and there are no holes in a perfect sphere, then no light escapes. If the light just kept bouncing around without being dissapated or lost, Wouldn't you just see the light.....?
Then again when the light hit the astronaut, would it then change colour or something and then continue it bounce around?

Head expanding........can't........think.....of....anything......else......

sci0x · 19-09-2003 6:59pm

Much appreciated mate

Originally posted by Sangre
Wouldnt he just see himself in the mirror?

Also m=u/v

3=u/v

U=3v

Then put that into 1/f=1/u+1/v

And do the rest.

MarVeL · 19-09-2003 11:41pm

Although a stated answer to the question would be nice :-)

Like I said so far noone has been able to give an answer and prove it and I've heard everything from his own reflection to a halo with him in the centre.

dudara · 20-09-2003 4:06pm

don't forget to take into account virtual and real images, therefore v = +/- 3u. That will give you your two answers once substituted into the lens formula

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