Braingle: 'Close Shave' Brain Teaser

Close Shave

Science brain teasers require understanding of the physical or biological world and the laws that govern it.

Puzzle ID:	#36934
Fun:	(2.98)
Difficulty:	(1.57)
Category:	Science
Submitted By:	drussel3

Homer had suffered a bizarre accident that affected his eyesight. The doctor said it would be temporary, but for the next 4 weeks, he had to adjust how he did some things. The accident had affected his focal length. He was only able to focus on objects that were 6 or more feet away from him, anything closer than 6 feet was just a blur.

Homer was used to shaving up close in front of his bathroom mirror. Now after the accident, how close could Homer get to the mirror to see his face clearly enough to shave?

Answer

3 feet

Your focal length in a mirror is your distance away from the mirror, plus the distance of the object away from the mirror. When you look at an object in a mirror, you are seeing a virtual image the exact distance it is away from the mirror. So, if you are standing 3 feet away from a mirror, and focus on your face, you are actually focusing at 6 feet.

To try this out, put a piece of tape on a mirror, about eye level. Standing in front of the mirror, focus your eyes on the tape, now your face is out of focus. Focus now on your face, and the tape is out of focus.
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Comments

cheeny13 Jun 14, 2007	Very interesting bit of trivia! I may have to try that when I get home!! Nice job!
cheeny13 Jun 14, 2007	By the way, I will try the tape thing, not the shaving thing...
Kamazar Jun 14, 2007	That was easy. I've often thought about a mirror and depth, so it took me no effort, but it was a good teaser. 1 full star! √
cooltigerr1 Jun 15, 2007	haha i will try that!! good one!!
dshebo Jun 15, 2007	I got it right away, but it was an awesome teaser! 1 full star from me too!
dshebo Jun 15, 2007	Quick question, I am new and want to earn points fast so I can do more stuff like submit my own teasers. How do I earn more points? Thanks again for the awesome teaser!
Kamazar Jun 15, 2007	For points, you rate teasers. I points per teaser. And your account needs to be 5 days old and with 250 points to submit a teaser, then you work yourself up from their. Look at the FAQs.
vikingboy Jun 16, 2007	cool, I tested it by holding a book up next to my head...the words were too small to read.
dshebo Jun 16, 2007	Thanks Kamzar! Will do!
hockeyfan94 Jun 16, 2007	Thank you too Kazamar. I only have 44 points and I too want to be able to do stuff on the site. thanks!
Brock Jun 18, 2007	cool
FutureMD Jun 18, 2007	It was very easy, but also very fun. An aside note: Did you ever try stereographs (AKA 3D pictures)? Because they use the same concept: in order for the 3D picture to pop out, you need to be focusing as if you were looking at an object that is twice as far as the picture you are looking at (which takes some time to get used to). This is why some people find it easier to print the stereograph on glossy paper and focus on their reflection, which is twice as far as the picture. Try googling stereographs-they are very cool.
bmarshal Jun 21, 2007	This would be great for kids in a science class to try. The comment about using glossy paper for stereographs is cool!
lmurray Jul 15, 2007	EASY, EASY, EASY. THIS IS A QUESTION OR PROBLEM FOR PRIMARY SCHOOL STUDENTS. TEASER OF THE DAY???
solotravlr Jul 15, 2007	that answer for me was ...a million miles away.... im not a mathie....gimme some spelling bees...
mosca Jul 15, 2007	Great teaser! I went off in the wrong direction and tried to set up an arrangement of multiple mirrors!
kauphi1976 Jul 15, 2007	i had the answer but then thought there was a twist somewhere till i saw it was meant to be easy ! i need to checkout the stereographs next ! Genie-Ass
jennrenee23 Jul 15, 2007	Very neat! I got it right, but not because of how the answer was to be attained! Oh well, very good!
doehead Jul 15, 2007	You would have a hard time finding one any easier
bradon182001 Jul 15, 2007	Fascinating. I'll try this experiment also.
jabdr Jul 15, 2007	I had no clue Learned something though.
speedqueenkmw Jul 15, 2007	guess ya either know this stuff or ya don't.
krishnaswamiv Jul 15, 2007	nice and easy teaser.
2ndhandrose Jul 15, 2007	To all the geniuses who think this was easy there are a few of us who didn'texcel in science. I will look up stereograhs & mayble learn something. I don't ever want to be a know it all. Thanks for sending this in
puttumup Jul 15, 2007	fun teaser you dont have to be a genied to figure it out
mondayschild59 Jul 15, 2007	Well I guess I'm an idiot then, cause I missed it. Of course I've been out of school for 30 years. Maybe I'll get the next one. Thanks for the post. Monday~
InsaneShelton Jul 15, 2007	That was an awesome Teaser. Glad it got picked as Teaser Of The Day. I guessed the right answer. I guess I missed that day in school (no surprise there), so it was nice to hear why it was three feet from the mirror. Keep 'em comin'! ~Insane Shelton
Trishgal Jul 15, 2007	Ilearned something. It was nice Thx.
Fhempen Jul 15, 2007	me too i figured it was around 3 feet because you dont necessarily need crystal-clear vision to shave....but i had no idea that that's y. very very cool!
cloughme Jul 15, 2007	Cool!!!
Castle Jul 15, 2007	Sorry, I found this one far, far too basic. The only thing that tricked me was its simplicity, I spent about 5 minutes re-reading the puzzle, looking for the catch, thinking "it couldn't be the obvious answer, could it?"
smiling2bsweet Jul 15, 2007	awesome. glad i finially got one. didn't take me long either.
RedPython Jul 16, 2007	Although to some this might be easy, it was in fact a very interesting fact that many people don't know. So don't feel dishartened by the negative comments. I have some extra things people might like to think about regarding mirrors. When you look in a convex mirror (such as a car mirror) do the images in it look further away or smaller? Also when you look through a pair of binoculars do images look closer or bigger? The answer is not both. CLUE - Think about the point where your eyes are focusing in each case and how the light rays travel in each system (A pair of binoculars have more than one lens (or mirror) and a car mirror, obviously only has only focus, which in the case of a convex mirror is behind it's surface)
RedPython Jul 16, 2007	Sorry! I made a tipo, In the above post "only has only focus" should read "only has one focus"
dancer4crs Jul 19, 2007	nice i never new tht
HarryPutter Jul 29, 2007	this was so easy though, it is basically common sense. geez...... (as Americans say)
jazzyca7 Sep 16, 2007	cool
Qrystal Sep 21, 2007	Neat idea for a teaser, and nice to teach people new stuff with the answer. I don't agree with the first sentence in the explanation of the answer, though. The concept of "focal length" has a very specific meaning that isn't quite the same as what was meant, although most laypersons wouldn't notice the difference. A change of wording such as this might be more appropriate: "When looking at your reflection in a mirror, your eyes focus to a distance equal to your distance from the mirror plus the apparent distance that your reflection is from the mirror. In a flat mirror, the image is the same distance away from the mirror as the object that is reflected. So when standing three feet away from the mirror, and focusing on your face which appears to be three feet past the mirror, you are focusing at a distance of six feet." I'm too lazy to figure out where to email this proposed correction, and I don't have enough points to correct it myself. Guess I need to submit some teasers, eh?
rockstar1234567 Jan 31, 2008	quite easy
wordmama Jul 15, 2010	Thanks for the posting. It made me aware of something I don't usually think about. As for the comments from previous years stating how easy/elementary this was, 1) I'm not a scientist, esp. not physics; 2) I skipped 5th grade; maybe that's where you all studied mirrors!! Always happy to learn new things.
auntiesis Jul 15, 2010	I never learned this in school. I thought it was hard, but I knew the answer would be less than 6 ft., just didn't understand the science behind it. Verrrry interesting.
craniac Jul 15, 2010	Qrystal's explanation of the answer (above) is definitely superior. Easy but fun teaser.
princess2007 Mar 09, 2011	I dont really want to be negative but these are just way too easy! Could someone please come up with something a little higher than primary school level?
royale-ninja Jul 28, 2012	i got that correct