Optics question (I hate kaplan)

[Blondnuttyboy]

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So I'm having trouble visualizing this...

If you have are in front of a convex mirror, what happens to your image as you walk away from it? Does it shrink or grow?

And what about for a concave mirror? Shrink or grow?

I'd like to see someone explain this with the thin lense equation - thanks!!

 

[RPedigo]

Sure... the thin lens equation is 1/i + 1/o = 1/f

So lets put the object at infinity.

1/i + 1/(infinity) = 1/f

1/infinity is zero

1/i = 1/f

i = f

So if you have an object at infinity, the image is at the focal point.

The opposite is also true. If the image is at infinity, that means the object has to be at the focal point.

So therefore, if you have an object infinitely far away, as you move it towards the focal point, the image retreats from the focal point, eventually hitting infinity as the object reaches the focal point. Does that make sense?

http://www.edumedia-sciences.com/m103-p2_l2-lenses-and-mirrors.html

These are trial versions, so they boot you off real quick, but hopefully this can help you visualize what I'm saying.

 

[Blondnuttyboy]

hmm yeah i understand how that works for a convex mirror...b/c f is negative and as o approaches infinity (making 1/0 = o), then i must get smaller so 1/i will get larger

BUT how do you use the thin lense to explain the fact that with a concave mirror, an object moving backwards will have an image that gets larger and then shrinks..how the hell are we suppossed to deduce that? I hate kaplan.

 

[FlowLimited]

Continuing from where RPedigo left off, you can easily figure out how the image size varies. Magnification is m = -i/o.


FOR A CONVEX MIRROR:
So if you increase the object distance the image distance decreases (as RPedigo said). Looking at the magnification equation, and you see if you put a small distance in for "i" and a large distance in for "o" the image size is small.

Conversely, like RPedigo said, decrease the object distance and the image distance increases. Plugging into the magnification equation again, large "i" and small "o" gives you a larger image size.

So therefore, as you increase the object distance from the focal point, the image size decreases and as you move the object closer, it increases.

The same is true for a concave mirror because the lens equation is identical and the only difference is that the focal length is negative.

 

[FlowLimited]

hmm yeah i understand how that works for a convex mirror...b/c f is negative and as o approaches infinity (making 1/0 = o), then i must get smaller so 1/i will get larger

BUT how do you use the thin lense to explain the fact that with a concave mirror, an object moving backwards will have an image that gets larger and then shrinks..how the hell are we suppossed to deduce that? I hate kaplan.

Okay, this happens because when you are inside the focal point with a concave mirror, the image is VIRTUAL. As you pass through the focal point moving away from the mirror, the image switches to being REAL. The increase in size followed by a decrease is just an effect due to this change from a virtual to a real image and the resulting object and image distances.

 

[Blondnuttyboy]

dont concave mirrors have positive focal lengths?

 

[Blondnuttyboy]

Okay, I think you meant to say a convex lens. This happens because when you are inside the focal point with a convex lens, the image is VIRTUAL (same side of lens as the object). As you pass through the focal point moving away from the lens, the image switches to being REAL. The increase in size followed by a decrease is just an effect due to this change from a virtual to a real image and the resulting object and image distances.

no i meant concave - http://www.edumedia-sciences.com/m103-p2_l2-lenses-and-mirrors.html - you can see with the concave mirror illustration.

 

[FlowLimited]

no i meant concave - http://www.edumedia-sciences.com/m103-p2_l2-lenses-and-mirrors.html - you can see with the concave mirror illustration.

Yeah, i misworded it. I edited my post but what I said is still valid.

 

[FlowLimited]

dont concave mirrors have positive focal lengths?

You're right. I didn't read your post close enough and just assumed you were talking lenses. That's why I had to edit.

 

[Blondnuttyboy]

i really hope we dont something this confusing on the real deal...signs change way too much

for lenses, if object/image is to the left, then theyre + but focal to the left is -

and this bull**** with focal length is opposite with mirrors...f to the left is +

ARGH

 

[Oceaner]

if you have one of those makeup mirrors and/or a spoon (although the spoon's extreme curve makes this harder to visualize)... you can see for yourself.

 

[FlowLimited]

i really hope we dont something this confusing on the real deal...signs change way too much

for lenses, if object/image is to the left, then theyre + but focal to the left is -

and this bull**** with focal length is opposite with mirrors...f to the left is +

ARGH

Are there sign changes with image in mirrors??

I hear you. I'm a physics major whose taken a number of classes in geometric and physical optics and it still gets me sometimes. For the mirrors at least, it's intuitive. If you imagine the mirror as a part of a circle, the light gets focused right at the center of that circle. Using that you can always remember which side of the mirror the focal point is on.

For the lenses, just remember that it's the reverse of the mirrors.

 

[Blondnuttyboy]

hahaha i like that..nice spoon idea...

If any of you guys are curious about where I'm getting these questions, it's from KAPLAN MCAT 45.

ps in there is horrid.

if it were a real MCAT, a 10 would be like a 32/52

 

[RPedigo]

Glad everything got worked out... don't stress, the real thing won't be so bad, and its not even a sure thing you'll get an optics passage at all.

I'm taking my MCAT tomorrow at 8:00am, so I'll let you know 😛