real image vs virtual image

[blankguy]

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I am not clear on what they are. Real image a projection and virtual is just a reflection??? 😕

 

[FutureDrKt]

It depends. If you're talking about mirrors, the real image will be in front of the mirror since it should be reflected (that's what mirrors do, reflect things). When you look in a normal plane mirror, it looks like the image is behind the mirror, so that is a virtual image. With lenses, you expect the image to go through since the surface is not reflective. So a real image lies behind a lens, while a virtual image is in front of it. That's how I remember those two: what should the image be doing, based on common sense?

I believe real images are always inverted, while virtual are always upright.

Ah, E&M.

 

[theblastopore]

I am not clear on what they are. Real image a projection and virtual is just a reflection??? 😕

I used to understand this subject when I was taking physics. By the time I got to study for the MCAT, I had completely forgotten it. Fortunately, I only find optics problem in stand-alone questions, which means you don't have to completely understand it--just know how to work the formula. You only need to know a few formulas for light and optics. The most important one being

1/p + 1/q = 1/f​

Make a chart for yourself to see when f, p, q, and M (magnification) are negative or positive (p is almost always positive) and know what what each positive or negative quality means. That way you'll be able extract the correct value from info given information to plug into the equation. For instance, if given a concave mirror, then I know it has negative f. The reverse is also true. Or if M is negative, then the image is inverted AND the image is real AND q is positive??? (not absolutely sure about the M part, can't remember everything on the top of my head). The idea is to know the basic formular and be able to extract plugging-in value from information given.

If I find my chart, I'll scan it and send it to you. Let me know if you are interested.

Good luck

 

[Gentle]

I used to understand this subject when I was taking physics. By the time I got to study for the MCAT, I had completely forgotten it. Fortunately, I only find optics problem in stand-alone questions, which means you don't have to completely understand it--just know how to work the formula. You only need to know a few formulas for light and optics. The most important one being

1/p + 1/q = 1/f​

Make a chart for yourself to see when f, p, q, and M (magnification) are negative or positive (p is almost always positive) and know what what each positive or negative quality means. That way you'll be able extract the correct value from info given information to plug into the equation. For instance, if given a concave mirror, then I know it has negative f. The reverse is also true. Or if M is negative, then the image is inverted AND the image is real AND q is positive??? (not absolutely sure about the M part, can't remember everything on the top of my head). The idea is to know the basic formular and be able to extract plugging-in value from information given.

If I find my chart, I'll scan it and send it to you. Let me know if you are interested.

Good luck

concave mirrors have +ve f while convex has -f.
convex lenses have +f while concave has -f.

Converging optics (mirrors and lenses) p'duce real images, inverted
Diverging optics (mirrors and lens) p'duce virtual images, uprigt.
It's important to note that convex lenses are converging whereas concave mirrors are converging; concave lenses are diverging whereas convex mirrors are diverging.
You could think of the eye, convex (converging) lens. It p'duces real, +ve but inverted images on the retina. Concave should behave oppositely.

 

[theblastopore]

concave mirrors have +ve f while convex has -f.
convex lenses have +f while concave has -f.

Converging optics (mirrors and lenses) p'duce real images, inverted
Diverging optics (mirrors and lens) p'duce virtual images, uprigt.
It's important to note that convex lenses are converging whereas concave mirrors are converging; concave lenses are diverging whereas convex mirrors are diverging.
You could think of the eye, convex (converging) lens. It p'duces real, +ve but inverted images on the retina. Concave should behave oppositely.

Thanks for the correction and sorry for the confusion. Just like I said, don't remember everything on top of my head Just the ideas. Sorry again

 

[Shrike]

real images are generally inverted and virtual images are generally upright. Maybe Shrike can come out of where ever he's hiding and help with this one.

Real images are always inverted and virtual images are always upright, but that's not the point. It is: real image: light goes there. Virtual: light does not go there. (Hence, for mirrors, real = on the object side of the mirror, because light bounces back; virtual is behind the mirror, where light can't go. For lenses, the opposite: light really goes through the lens, so real images are on the far side; virtual on the near side, because light doesn't bounce back.)

Be nice to me and soon I'll post my definitive, incredibly simple guide to all MCAT optics problems.

 

[deuist]

Here's an easy way to think of the difference: If you can shine the image on a sheet of paper, it's real. If not, it's virtual. For example, shining a light through a magnifying glass will result in a light on a surface on the other side. That image from the magnifying glass is real. A mirror, on the other hand, cannot produce a light image onto a surface behind the mirror---that's virtual

 

[Shrike]

Here's an easy way to think of the difference: If you can shine the image on a sheet of paper, it's real. If not, it's virtual. For example, shining a light through a magnifying glass will result in a light on a surface on the other side. That image from the magnifying glass is real. A mirror, on the other hand, cannot produce a light image onto a surface behind the mirror---that's virtual

Is this really easier than "real image: light goes there. Virtual image: light does not go there"?


I actually am not being quite as sarcastic as I sound: if there's a better way to explain something, I like to know it.

 

[theblastopore]

Is this really easier than "real image: light goes there. Virtual image: light does not go there"?


I actually am not being quite as sarcastic as I sound: if there's a better way to explain something, I like to know it.

I think that's as simple as it gets. And I think that answers the orinal question well.

But I'm just curious to know if we need to know that to solve mcat light and optic problems. What type of problem would require us to know that, besides showing us that ray diagram and asking what type of image formed

Thanks

 

[Shrike]

The easiest way to solve MCAT optics problems is to know two equations (which must be memorized):

• 1/o + 1/i = 1/f
• m = -i/o

... eight easy definitions (try them; you already know most of them):

• o = distance (from the lens/mirror) to the object
• i = distance to the image
• f = focal length (sometimes hiding in the problem, as Radius of Curvature/2)
• m = magnification (which, oddly, includes being right side up and upside down)
• converging = bringing together
• diverging = spreading apart
• real = light goes there
• virtual = light does not go there

... three things you already know about the world, for translating MCAT problems and figures into the proper form:

• magnifying glasses, which are convex, make light come together
• mirrors are different from lenses (they have the opposite effect on light)
• concave is different from convex (it has the opposite effect on light)

... and four easy, but perhaps novel, principles for setting up and interpreting the equations, based on the idea that positive is good:

• object o --> always + (because it's always real, and real is good)
• image i --> real is + (because real is good)
• focal length f --> converging is + (because it's good to come together)
• magnification m --> upright is + (because it's good to be upright)

This knowledge will enable the solution of every MCAT optics problem. To answer the question about why we should know which images are real and which are virtual, it's because it enables finding where the image is in a figure, without having separate rules for lenses and mirrors. (General principle: distinct cases suck, and exceptions suck more.)

That's all there is. When I publish Shrike's Guide to Acing MCAT Physics, it will be a cornerstone. For now, I hope this helps.

The above is the bulk of my optics lecture. Incidentally, this presentation is not standard TPR fare. I first delivered it this way in Norman, OK, earlier this year, with apparent success, and have since refined it slightly for classes in Dallas, Arlington, and Sherman.

 

[gotgame83]

Well get to publishing! I have until august 20th to learn this stuff.

 

[Shrike]

Well get to publishing! I have until august 20th to learn this stuff.

So, learn it from this post; when I publish it I'll charge for it. A cheap little paperback, undoubtedly.

 

[gotgame83]

Ha, its funnie you should mention that. I was going over the physics section in the big kaplan comprehensive book last night. The optics section is the last one before the nuclear crap which im fine with. I started reading the optics section but i was like, " some bird guy(i couldnt remember ur sn just ur avatar lol) posted some good stuff on optics" so i signed back online to read this lol

 

[Shrike]

some bird guy... posted some good stuff on optics

I can live with being some bird guy, I guess. The bird in the avatar is a shrike, fwiw.

I hope to put the other publishers out of business when I get everything together. Even TPR (though we'll have issues regarding intellectual property; I mostly don't teach TPR material straight, but it's tough to avoid using some of their stuff).

Shrike
TPR physics, verbal, biology; LSAT; GMAT; GRE

 

[DarkLordofSith]

So, learn it from this post; when I publish it I'll charge for it. A cheap little paperback, undoubtedly.

Allright, get on it. I have the optics down, and it's all good. Now teach me about the rest of physics 🙂 What would you suggest is the best method for learning this again by August? Good God, I swear I'm never gonna let my kids take AP classes. I'm dying right now trying to relearn physics in 2 months along with trying to study for the rest of the MCAT. Down with AP!!

 

[gotgame83]

I can live with being some bird guy, I guess. The bird in the avatar is a shrike, fwiw.

I hope to put the other publishers out of business when I get everything together. Even TPR (though we'll have issues regarding intellectual property; I mostly don't teach TPR material straight, but it's tough to avoid using some of their stuff).

Shrike
TPR physics, verbal, biology; LSAT; GMAT; GRE

Haha, well that just goes to show everyone how little i know about birds. Well i havent been on SDN long but I really enjoy your posts and they are very informative so keep them comin.

 

[gotgame83]

Allright, get on it. I have the optics down, and it's all good. Now teach me about the rest of physics 🙂 What would you suggest is the best method for learning this again by August? Good God, I swear I'm never gonna let my kids take AP classes. I'm dying right now trying to relearn physics in 2 months along with trying to study for the rest of the MCAT. Down with AP!!

Those AP classes in highschool killed me. I was exempt from college inorganic chem and physics. Granted i was killer at them in highschool but now i have 3 more years of useless crap cluttering my brain and its become rather difficult to retrieve some of that old information.

 

[Shrike]

Good God, I swear I'm never gonna let my kids take AP classes. I'm dying right now trying to relearn physics in 2 months along with trying to study for the rest of the MCAT. Down with AP!!

I submit that it wasn't the AP tests that caused the problem, it was the decision to use the AP scores to test out of otherwise-required classes. No way is the average AP class gong to be as comprehensive or effective as the average college equivalent. I believe some colleges are agreeing, and beginning lately to reduce the number of credits granted for certain AP scores.

 

[deuist]

When I publish Shrike's Guide to Acing MCAT Physics, it will be a cornerstone.

hmmmm.... maybe we could start an SDN writers' guild---or possibly convince the SDN gurus to start a publishing company.