02images and Plane Mirrors

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DIAGNOSTIC /REMEDIAL TEST

02 IMAGES AND PLANE MIRRORS

This test is one of a series in Introductory Physics made available on the Website of theSchool of Physics, Monash University, Australia(www.physics.monash.edu.au/community ).This test is !T for the "ur"oses of assessment. It is to assist you in locatin#misconce"tions and misunderstandin#s and #enerally to assist you in your study of Physics. $ou should %or& by yourself and at your o%n "ace follo%in# the directions#iven. It is not necessary to attem"t the test all at once. $ou may li&e to do it bit'by'bit,%aitin# until you have covered a "articular to"ic in class or in your readin# of your te t

boo& or you may li&e to "lun#e in before you be#in your study of the to"ic.

*uestions are on the left hand (even'numbered) "a#es. While readin# or %or&in# onthese, keep the right hand (odd n!"#ered$ an%&er page 'o ered) DO NOT PEE* AT T+E ANS,ERS ON T+E RIG+T +AND PAGE -

The test %as com"iled by and lar#ely authored by +meritus Professor ill -achin#er %ho %ould a""reciate any comments or su##estions for im"rovement. These could besent to him at [email protected] or+meritus Professor ill -achin#er, School of PhysicsMonash University, P.!. o /, 0ic 1233 Australia

4ia#rams %ere "roduced byMr Steve Mc5ausland, formerly of 4e"artment of Physics, Monash University

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http://www.physics.monash.edu.au/communitymailto:[email protected]://www.physics.monash.edu.au/communitymailto:[email protected]


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5!0+- T6+ -I76T 6A 4 (!44' UM +-+4) PA7+S4! !T P++89

:. In %hich of the follo%in# ob;ects is li#ht made i.e. %hich is a source of li#ht.A. A candle flame

. A screen on %hich a movie is bein# sho%n.5. The sun.4. The screen of a s%itched'on T.0.+. A torch (flashli#ht) #lobe.T *U+STI! . 4! ?T 56+58 $!U- A SW+- $+T.

WAIT U TI@ $!U A-+ I ST-U5T+4 T! 4! S!.

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:. A,5,4 and + are sources of li#ht or luminous ob;ects. @i#ht is "roduced %ithin these ob;ects. The others are ob;ects %hich are seen %hen li#ht from some luminous ob;ect such as the sun isreflected from them and enters your eyes. A mirror or a "oster cannot be seen in a dar&enedroom and the movie screen and the ima#e on it are seen by the reflection of the li#ht comin#from the movie "ro;ector.

. 5 is correct. The %a body of the candle is seen because li#ht is reflected from it. @i#htfrom the candle is reflected, maybe many times by the %alls and ob;ects in the room. Some %ill

be reflected by the candle and enter your eyes.

A is incorrect. Seein# involves li#ht travellin# from the candle to your eyes.

is incorrect. @i#ht does not travel (to any lar#e e tent) throu#h a %a candle. $oucannot see throu#h a %a candle. It is o"a ue.

4 is incorrect. Warm %a is not a source of li#ht. It does not #lo%. If you ble% out thecandle flame you %ould not see the %a candle althou#h the %a %ould still be %arm.

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1. 5onsider t%o situations in %hich a movie is bein# vie%edB(i) on a movie screen in a dar&ened cinema(ii) on a T.0. screen in a dar&ened room.

There are certain "ro"ositions (A'6) listed belo% %hich may relate to one or both of these situations.

Which of these relate to the cinema screen=CCCCCCCCCCC 7ive your reasons for your

choice and comment on the other ans%ers.

Which of these relate to the T.0.screen=CCCCCCCCCCCC 7ive your reasons for your choice and comment on the other ans%ers.

A. @i#ht is bein# made on the screen.. @i#ht from the screen is reachin# your eyes.

5. @i#ht is bein# reflected from the screen.4. The dar& "arts of the ima#e on the screen corres"ond to "laces %here li#ht is hittin# thescreen and bein# absorbed.+. o li#ht is reachin# your eyes from the dar& "arts of the ima#e on the screen.


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1.


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F. A is correct. The main difference bet%een a mirror and a sheet of "a"er is in its surfacerou#hness.

A mirror has a very flat surface and a beam of li#ht is reflected in a very orderly %ay. It is this%hich allo%s you to see nice shar" ima#es in a mirror.


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E. $our friend tells you that you have a red s"ot on your chee& ;ust belo% your eye. Toobserve it you use a mirror. @et?s see ho% this mirror %or&s by studyin# %hat ha""ens toli#ht comin# from the red s"ot and stri&in# the mirror.


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/.In the situation described in the "revious uestion you %ould &no% that it %as only an ima#ethat you %ere loo&in# at because you %ould be a%are of the mirror and &no% that the red s"ot%as really on your chee&.

5onsider the situation in 11


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/.

The li#ht rays from the lam" and the reflected rays are sho%n in the


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.

The fi#ure sho%s t%o li#ht rays %hich leave an ob;ect !. These stri&e a "lane mirror,are reflected and enter the eye as sho%n.4ra% in on the 13


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2.The "osition of the mirror is defined by the "oints at %hich the "airs of rays intersect

($ou should also chec& that the la% of reflection is obeyed here).

The "osition of the ima#e is located by dra%in# bac&%ards the rays A? and ?. As far asthe eye and the observer are concerned the rays A? and ? a""ear to come from this ima#e "oint.

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G @et?s ima#ine an ideal e "eriment. (Thou#ht e "eriments are often useful in Physics.Sometimes real'life situations are too com"le to deal %ith.)

A fine beam of li#ht is introduced throu#h a small %indo%, as sho%n in the


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5omment on the other ans%ers.

56+58 $!U- A SW+-S T! T6IS *U+STI! . -+T !!8

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G. 4 is correct. In this ideal situation the li#ht after bein# reflected by the mirror %ill stri&e the "erfectly blac& %all and be com"letely absorbed there.

is therefore incorrect. The bo %ill not be filled %ith li#ht . It %ill ho%ever containthe beam of li#ht stri&in# the mirror and bein# reflected by it.

$ou %ould ho%ever not see these beams of li#ht as su##ested in Ans%er 5. -emember that you only see thin#s %hen li#ht enters your eye. $ou %ould see the beam from the head'lam"s of a car travellin# on a distant road because some of the li#ht is reflected side%ays by dust

"articles and this li#ht reaches your eye. In our ideal bo there is no air and no dust "articlesand hence you do not see the beams. 5 is therefore incorrect.

A is incorrect. @i#ht does travel throu#h a vacuum for instance in reachin# us from thestars.

As e "lained above, 4 is correct. @i#ht is "resent in the bo as incident and reflected

beams but is absorbed an stri&in# the %all. one of this li#ht reaches your eye.

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:3.

4 is the correct res"onse. In the dia#ram above, %hich is sim"lified to the case of a flat face ,it can be seen that %hether the mirror is close in at A or far distant at the re#ion of the face

%hich is visible (>$) is the same. The #rid lines ma&e it easy for you to see that the rays have been dra%n to obey the la% of reflection.

The rays dra%n from > and $ re"resent the e treme case for rays %hich enter the eye. $ is the field of vision for the "articular arran#ement of eye and mirror.

If the mirror is moved to 5 the field of vision shifts to >?$? but remains the same si e soonce a#ain it is not "ossible to see more of your face. $ou only see a different "art of it.


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::.

A cubical frame made of very fine %ire rests as sho%n on a lar#e flat mirror. Ima#ine that a fine beam of li#ht is sent from the corner of the cube mar&ed ! to stri&e the mirror at the "oint 5 thecentre of the s uare base of the cube.The follo%in# uestions %ill test your &no%led#e of the la%s of reflection. $ou should indicatein your ans%ers %hether the su##estions are allo%ed by these la%s.a) 5ould a reflected beam "ass throu#h the "oint @=

$esJ o7ive your reason.......................

....................................... b) 5ould a reflected beam "ass throu#h the "oint 4=


.......................................c) 5ould a reflected beam "ass throu#h the "oint M=


.......................................

o% ima#ine a beam to be sent from ! to stri&e the mirror at A the mid"oint of the ed#e of thecube.

d) 5ould a reflected beam "ass throu#h the "oint M=$esJ o7ive your reason.......................

.......................................


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e) 5ould a reflected beam "ass throu#h the "oint @=$esJ o7ive your reason.......................

.......................................f) 5ould a reflected beam "ass throu#h the "oint (%here @ K :JD @!)=


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::. These uestions test your &no%led#e of the la%s of reflection.

I. The an#le of incidence is e ual to the an#le of reflection i.e. i K r II. The incident beam, the reflected beam and the normal ("er"endicular) to the mirror must alllie in one "lane.

The ans%ers areB

a) o (@a% II is not satisfied).

b) o ( either la% is satisfied).

c) $es ( oth la%s are satisfied).

d) o ( either la% is satisfied).

e) $es ( oth la%s are satisfied).

f) o (@a% I is not satisfied).


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02images and Plane Mirrors