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Refraction

I'm stuck on another question and hope someone can help me.

A lens will refract rays so that they can be brought to a focus or diverge from a focus
a) if a lens has a virtual focus 5cm from it's centre, what is its focal length and what is it's power in diopetres?
b) if a lens makes a real image at a distance of 20cm when the object is 10cm, what is its focal length in the same units and wha is the magnification?
c) If an object is 5cm from a convex lens of a focal length 12 cm, what percentage lalarger is the image than the object?

I'd be grateful is someone could help
I can only do 1)

1/f = diopter. f=-5cm, which is -0.05m

so the power is 20 diopters.

I need to brush up on this topic ... I seemed to recall not getting it at GSCE and then somehow rote-learning the solutions which I *hate* doing.
Hoping this will bring it someone's attention who knows what they're talking about so it can help me too.
Original post by Deborah Adderley


b) if a lens makes a real image at a distance of 20cm when the object is 10cm, what is its focal length in the same units and wha is the magnification?
c) If an object is 5cm from a convex lens of a focal length 12 cm, what percentage lalarger is the image than the object?

I'd be grateful is someone could help


b/ if 10cm is the distance from the lens... use the thin lens eqn
1/u + 1/v = i/f
magnification is (image distance)/(object distance)

watch out for sign convention
http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/lenseq.html#c2
thank you very much


Original post by DrSebWilkes
I can only do 1)

1/f = diopter. f=-5cm, which is -0.05m

so the power is 20 diopters.

I need to brush up on this topic ... I seemed to recall not getting it at GSCE and then somehow rote-learning the solutions which I *hate* doing.
Hoping this will bring it someone's attention who knows what they're talking about so it can help me too.
thank you very much



Original post by Joinedup
b/ if 10cm is the distance from the lens... use the thin lens eqn
1/u + 1/v = i/f
magnification is (image distance)/(object distance)

watch out for sign convention
http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/lenseq.html#c2

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