Experiment 4

Reflection of Light (Flat Mirrors)



To verify the law of reflection by forming the image of an object in a flat mirror.




            Two mirror holders, a rectangular thin sheet of clear glass (approximately 8x10), an optical bench, two light-bulb sockets, two (identical) socket holders, and two (identical) low-watt light-bulbs            




Law of Reflection:


            In a flat mirror, the angle of incidence( i ) is equal to the angle of reflection( r ), with both angles  measured with respect to the normal line (N) at the point of incidence (Figure 1).


Fig. 1


This can be proven by simple geometry by accepting the fact that in a flat mirror the object and image are equidistant from the mirror (Figure 2).



Fig. 2


Note that infinite rays of light emerge from point A of object AB.  Only one ray is shown.  In order for this one ray (AI) to be seen by the observer, it must reflect at point I on the mirror and reach the observers eye as if it comes from A, the virtual image of A.  The following argument supports the proof of why angles i and r are equal.


Proof: Right triangles AHI and AHI are congruent because AH = AH and side HI is common in both ( case of two sides and the angle in between).  Consequently, angle α = angle β.

But α= i and β = r ; therefore, i = r.




            Place an optical bench on a flat and horizontal table.  Use the two flat mirror holders to hold the flat piece of glass perpendicular to the table and the optical bench as well (See figure.)



Fig. 3


Mount a bulb with a holder on one side of the glass plate at a certain distance ( d ) from it and turn it on.  Mount another bulb and holder on the other side of the glass plate and keep it unlit.  Now looking at the image of the lit bulb into the glass plate (of course from the lit-bulb side), slide the unlit bulb on the other side until it takes the position of the image of the lit bulb and appears lit itself.   Measure the distance of both bulbs from the mirror (glass plate) and see how close they are.


Data:           Given:              Object distance ( d ) as selected by students in your group


Measured:      Image distance ( d )





Comparison of the Results:


Using (d ) as the accepted value, calculate the %error using the usual %error formula.


Conclusion:          To be explained by students


Discussion:             To be explained by students