# Optics

Discussion in 'Physics' started by logearav, Oct 9, 2011.

1. ### logearav Thread Starter Member

Aug 19, 2011
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0
Revered members,
Kindly see my attachment. I don't understand the role of dotted lines in the diagram. Actually , DA, EC and NB are perpendicular bisectors. In the first place, why should we draw perpendicular bisectors? Can't we solve the problem with incident ray and reflected ray alone? Please help, members.

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2. ### Georacer Moderator

Nov 25, 2009
5,151
1,266
Do they confuse you? You can always not use them to reach the solution, as long as you prove your answer.

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3. ### davebee Well-Known Member

Oct 22, 2008
539
46
The law of reflection is defined using the angles to a normal to the reflecting surface.

Lines DA and NB show the normal at the mirror for the lines of sight that the problem is asking about, so these are the lines that the reflecting angles are measured from.

Line EC does not need to be used.

For a plane mirror, the angles could just have well have been measured to the mirror surface instead of to the normal, but a curved surface has no plane to measure from, so using the normal makes sense because it will work for a mirror regardless of the nature of the surface.

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4. ### logearav Thread Starter Member

Aug 19, 2011
248
0
Thanks for the replies Georacer and davebee.

5. ### logearav Thread Starter Member

Aug 19, 2011
248
0
I don't understand this line. Could you please elaborate?