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Old 31st October 2009, 17:16   #1
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Smile Objects in the mirror are closer than they appear. Why?

Why do our side RVM have "objects in the mirror are closer than they appear."

Does this not make way for miscalculations on how close the approaching vehicle is?

What purpose does it serve?
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Old 31st October 2009, 17:25   #2
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That's because the mirrors used in rear view mirrors are convex mirrors which are used to increase the field of view.A flat mirror would have a very limited field of view.
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Old 31st October 2009, 17:33   #3
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Quote:
Originally Posted by rishab.k View Post
That's because the mirrors used in rear view mirrors are convex mirrors which are used to increase the field of view.A flat mirror would have a very limited field of view.
Don't you think its possible to have a flat mirror with almost equal field of vision?

There is so much of R&D in the automotive sector and I am sure they could have found out a solution to make zero error mirrors, way back.
Surprising they haven't as yet, or they must have retained the same for a reason.
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Old 31st October 2009, 17:42   #4
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Quote:
Originally Posted by beejay View Post
Don't you think its possible to have a flat mirror with almost equal field of vision?

There is so much of R&D in the automotive sector and I am sure they could have found out a solution to make zero error mirrors, way back.
Surprising they haven't as yet, or they must have retained the same for a reason.
Beejay, the convex mirror is not a manufacturing defect or error. It is intentionally made that way so that the driver gets a wider field of vision. Using a flat mirror will give you accuracy in judgement of distance but narrower field of vision, and a degree of 'concavity' will provide the converse. This is determined by the car mfr and varies from car to car.

If you are not happy replace the mirror (not assembly) with flat mirrors. But it is not advisable.

Rgds,
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Old 31st October 2009, 17:49   #5
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Quote:
Originally Posted by beejay View Post
Don't you think its possible to have a flat mirror with almost equal field of vision?
The convexity is not an error. It's a matter of simple physics which you'd certainly have studied in high school. To encompass the same field of vision, a flat mirror will need to be much larger than a convex one. Since compactness is the order of the day, large mirrors are unacceptable to most car designers. There's also the safety element -- large mirrors break more easily and can cause more damage. An alternative that's often seen is to have a (mostly) flat mirror and a small circular high-convex mirror within it for greater depth of field.

Last edited by Edsel Rulez! : 31st October 2009 at 17:53.
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Old 31st October 2009, 17:55   #6
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Quote:
Originally Posted by R2D2 View Post
Beejay, the convex mirror is not a manufacturing defect or error. It is intentionally made that way so that the driver gets a wider field of vision. Using a flat mirror will give you accuracy in judgement of distance but narrower field of vision, and a degree of 'concavity' will provide the converse. This is determined by the car mfr and varies from car to car.

If you are not happy replace the mirror (not assembly) with flat mirrors. But it is not advisable.

Rgds,
The older cars had normal mirrors but had lots of waves. My palio mirror is flat too and has decent vision.
I understand it would not be wise to change them either. Thanks.

@ edsel rules, your post in bold.
I have see what you mean in high end cars and imports. Thanks.
"An alternative that's often seen is to have a (mostly) flat mirror and a small circular high-convex mirror within it for greater depth of field."

Last edited by beejay : 31st October 2009 at 18:04. Reason: @edsel rules..
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Old 31st October 2009, 19:06   #7
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Quote:
Originally Posted by beejay View Post
The older cars had normal mirrors but had lots of waves. My palio mirror is flat too and has decent vision.
I understand it would not be wise to change them either. Thanks.
Lots of waves in the sense?
See, its not that quality of mirror has anything to do with field of vision (as your posts seem to be suggesting). If you have two mirrors of same quality but one convex or other flat - the convex mirror would provide you with a wider but diminished view. Like most of us have already covered in basic Physics in school, let us recall the good old ray diagram of a convex mirror (notice the size of image is smaller than the object but the field of view is large):
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Old 31st October 2009, 19:16   #8
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Bah! I see the potentials of a nice optics class getting started here now. DCEite you are the designated prof. Bring it on. Start with the focal plain and then proceed on to how light converges to form an image, both in convex and concave mirrors. While you are at it you may also want to cover the lenses.
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Old 31st October 2009, 19:22   #9
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Quote:
Originally Posted by DCEite View Post
Lots of waves in the sense?
If you closely watch RVM of older vehicles, Ambassadors, Jeeps you will be able to see a distortion at some fields. Like a jump in vision. That might be because of the quality of the mirror.

You have been quite informative and thanks for the detailed diagram. I now know why I flunked Physics.

Just for thoughts, so the point is to have a smaller mirror with a larger field of vision. Then why do modern day cars have humongous side mirrors? Must be purely for aesthetics or my Physics might be wrong again.
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Old 31st October 2009, 19:31   #10
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I used to drive a 800 earlier that used to have flat ORVMs, and now I drive a punto with the convex mirrors. I've noticed that the field of view gets better with the convex ones, however I almost rammed into a car thinking it was way too far behind. Needs some time to get used to it.

Why is the inside RVM not convex ?
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Old 31st October 2009, 20:13   #11
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Guys, this is a problem that has me puzzled. As I'm going to prove now, the writing on the mirror is wrong. Objects in the mirror are actually further than they appear.

For a mirror 1/u +1/v =1/f, where u is the object distance, v is the image distance and f is the focal length of the mirror

u>0 and f<0 for convex mirror.

1/v=1/f - 1/u

1/v= -1/|f| - 1/u, |x| is the magnitude of quantity x

|1/v|= 1/|f| + 1/|u|, since the signs only represent whether the object is behind or in front of the mirror and the magnitude is what matters to us

Clearly, |1/v|>|1/u|

|v|<|u|

Thus, the image distance is less than the object distance. It looks like the ray diagram in DCEite's post. Our eyes judge the object distance by equating it to the image distance i.e. our eyes think that |v|=|u|. For instance, if a car is closing up to you, its image appears to come closer to the mirror's surface. Hence |u(observed)|<|u(actual)|. Or in other words, objects in the mirror are further than they appear.

I can't quite see where I may have made a mistake. But every automaker consistently says that objects are closer than they appear. Do they just want to play it safe so that we leave more room for error when we drive?

Last edited by McLaren Rulez : 31st October 2009 at 20:18.
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Old 31st October 2009, 20:19   #12
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I had got confused when I saw this message for the first time. I couldn't understand if the image of the object is closer or the actual object is closer.

At least the message should be reworded to "Objects are closer than they appear in the mirror".
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Old 31st October 2009, 20:28   #13
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Quote:
Originally Posted by sdp1975 View Post
I used to drive a 800 earlier that used to have flat ORVMs, and now I drive a punto with the convex mirrors. I've noticed that the field of view gets better with the convex ones, however I almost rammed into a car thinking it was way too far behind. Needs some time to get used to it.

Why is the inside RVM not convex ?
The two ORVMs are convex and are provided so that we can judge wide angles (traffic in lanes next to us). The IRVM is provided to accurately judge the distance between the next car and yours - thats why they are not convex.

I was taught by my instructor to look into the IRVM (to check the the car behind me was not trying to pass me), then look into left mirror (yes I know I sound wrong, but I learnt driving in a LHD car), just look over the shoulder for the blind spot (as much as you adjust the mirrors, there has to be a blind spot), use the blinkers to change lane into the passing lane/fast lane, pull ahead, and only change back into the earlier lane when the headlights of the car I passed was visible in the IRVM. It was not sufficient for the car to be visible in my right ORVM to pull in the lane ahead of it. So both the mirrors are required to clearly overtake any vehicle in front of you.

Wonder how I had to unlearn everything to drive in Kolkata!
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Old 31st October 2009, 21:14   #14
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Quote:
Originally Posted by blackasta View Post
The two ORVMs are convex and are provided so that we can judge wide angles (traffic in lanes next to us).
Not always. One of the OVRM can be convex and other one can be flat regular. This setting will depend on left hand drive or right hand drive. Driver side mirror in this case would be flat. Reason is very simple here. Driver can judge the accurate distance from a flat mirror and he'll (hopefully) have time and reflex to correct his course. On his other side that convex mirror is to give him a wide angle view of the road and traffic behind. Basically this is for those countries where people obey traffic rules.

I don't know which all cars here in India provide this flat/convex ORVM mirrors combination. But for instance, I my car has them.
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Old 31st October 2009, 21:27   #15
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There is a mistake in my earlier post. Apparently, our eyes do not judge distances based on |v|=|u| as I hypothesized earlier. Rather, our eyes judge the distance by the size of the image in the mirror. Now, we've already proved that |v|<|u|. For a convex mirror producing a virtual erect image, the magnification is

|m|= |v|/|u|.

Since we have proven that |v|<|u|, we conclude that |m|<1.

In layman's terms, the image is smaller than the object. Yes, it is common sense (and the ray diagram posted earlier also agrees with this) but rigourously proven anyway to clear all doubt. If we see a small image, our eyes interpret it as the object being further away. Hence, the object is actually closer than it appears.

A most interesting interaction of biology and physics.

Last edited by McLaren Rulez : 31st October 2009 at 21:36.
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