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I recently got in an argument with a friend while I was driving. When I moved to a different lane, I did not turn my head to check for any vehicles in my blind spot. Instead, how I checked was by leaning toward the rightmost region of the drivers wheel, looking into the left side mirror to see the field that is normally the blind spot when sitting upright. I've been doing this for several years, I imagine if I was wrong then surely I'd have gotten in an accident by now.

My friend, however, refuses to believe that works. He thinks the blind spot remains fixed no matter where the observer (driver) looks at the mirror or that at least it is not safe/effective as turning your head to look into your blind spot.

I understand mirrors in vehicles are convex mirrors, they slightly bulge out to increase the field of view for the observer (which is why objects appear closer in the mirror and why there is slight distortion). And I know as light hits the mirror, the mirror reflects it back outward. What I'm struggling with is how to explain that the position of the observer changes the blindspot.

How should I make my case?

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  • $\begingroup$ en.wikipedia.org/wiki/… $\endgroup$
    – BowlOfRed
    Nov 8, 2021 at 22:46
  • $\begingroup$ Can't tell if you're talking about the driver's side mirror (left-hand drive automobile) or the passenger's side (left side mirror, right-hand drive.) You said, "I understand mirrors in vehicles are convex mirrors." Here in the U.S.A., The driver's side mirror always is flat. Only the passenger-side mirror is curved. We presume that the driver's eye is close enough to the flat, driver-side mirror to provide a sufficient field of view. The passenger-side mirror, being much further from the driver's eye, would give only a very narrow view if it were not curved. $\endgroup$ Nov 9, 2021 at 1:33
  • $\begingroup$ im talking about the drivers side mirror, the left side mirror $\endgroup$
    – Lex_i
    Nov 9, 2021 at 1:34

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I suggest that you start by drawing a small plane mirror in roughly the right position in relation to the driver, and draw a couple of rays reaching the driver by reflection (with angle of incidence = angle of reflection) to show how the blind spot arises. Then repeat for a convex mirror with exaggerated curvature. Even without using ruler and protractor (or a drawing application) you should be easily convinced that the blind spot phenomenon will be mitigated. Perhaps your friend will be convinced too.

But turning your head is never bad – unless you hit something in front of you, or injure your neck...

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  • $\begingroup$ For me personally, I don't like turning my head because I still want to be always aware of what's going on in front of me. What happens if the vehicle in front abruptly stops? etc. How I adjust my side mirrors is by rotating them out just enough that my vehicle itself is not in view. By the time another vehicle reaches this new blind spot, while my mirror does not show the vehicle it's already close enough to be in my peripheral vision. So I look by glancing, or if there are many vehicles around I lean $\endgroup$
    – Lex_i
    Nov 9, 2021 at 0:06
  • $\begingroup$ the issue is my friend agrees that a convex mirror offers greater field of view than a flat mirror, except he does not agree that leaning in and away from the side mirror removes the blind spot. $\endgroup$
    – Lex_i
    Nov 9, 2021 at 0:07
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    $\begingroup$ If you show the rays that reach you, reflected off the near and far parts of the mirror, then repeat with your head in a different position on the diagram, you'll see that the field of view is different. $\endgroup$ Nov 9, 2021 at 0:13

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