# Optics

#### logearav

Joined Aug 19, 2011
243
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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#### Georacer

Joined Nov 25, 2009
5,182
Do they confuse you? You can always not use them to reach the solution, as long as you prove your answer.

#### davebee

Joined Oct 22, 2008
540
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.

#### logearav

Joined Aug 19, 2011
243
Thanks for the replies Georacer and davebee.