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Question Number 176966 by mr W last updated on 28/Sep/22

Commented by mr W last updated on 29/Sep/22

is it possible that a light ray starting  from point O returns back to the same  point after being reflected at the  points P, Q, R on the three mirror  walls?  if yes, find out the positions of the  reflection points.  the edge length of the cube is 1.

isitpossiblethatalightraystartingfrompointOreturnsbacktothesamepointafterbeingreflectedatthepointsP,Q,Ronthethreemirrorwalls?ifyes,findoutthepositionsofthereflectionpoints.theedgelengthofthecubeis1.

Answered by mr W last updated on 29/Sep/22

Commented by mr W last updated on 29/Sep/22

P_1 =plane containing incident ray          and reflected ray  P_2 =plane containing the mirror  we have:  P_1 ⊥P_2   incidence angle = reflection angle =θ

P1=planecontainingincidentrayandreflectedrayP2=planecontainingthemirrorwehave:P1P2incidenceangle=reflectionangle=θ

Commented by mr W last updated on 29/Sep/22

Commented by mr W last updated on 29/Sep/22

due to symmetry the point Q must lie  on the diagonal. its position can be  described through the value a.  BC=a  AD=1−a−a=1−2a  ((BC)/(AD))=((PC)/(AP))=((CQ)/(AO))  (a/(1−2a))=(a/1)  ⇒1−2a=1  ⇒a=0  that means it is not possible that  the light ray returns to its starting  point after being reflected by three  mirrors as shown.

duetosymmetrythepointQmustlieonthediagonal.itspositioncanbedescribedthroughthevaluea.BC=aAD=1aa=12aBCAD=PCAP=CQAOa12a=a112a=1a=0thatmeansitisnotpossiblethatthelightrayreturnstoitsstartingpointafterbeingreflectedbythreemirrorsasshown.

Commented by Tawa11 last updated on 02/Oct/22

Great sir

Greatsir

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