###
Experiment 4

####
**Reflection of Light (Flat Mirrors)**

__Objective__:

The
objective is to verify the law of reflection by forming the image of an object in a flat
mirror.

__Equipment__:

Two mirror holders, a rectangular thin sheet of
glass (about 8"x10"), an optical bench, two bulb holders, two low-watt light-bulbs

__Theory__:

__The Law of Reflection__:

In **flat mirrors**,
the *angle of reflection*
*θ*_{r} equals the *angle of
incidence* *θ*_{i }.
Of course, both angles are measured with respect to the **normal
line N **at the* point
of incidence. *See** Fig.
1***.*

Figure 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 **(**Fig. 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 observer's eye as if it comes from **A'**,
the virtual image of **A****.** The
following argument supports the proof of why angles *θ*_{i} and *θ*_{r} are
equal**.**

** **

*Proof*:

** **Right** **triangles** AHI** and
**A'HI** are congruent because **AH = A'H** and side **HI is common in both** (case
of two sides and the angle in between)**.** Consequently, **Angle α **= *Angle β*.
Since **α
**= *θ*_{i} and **β** = *θ*_{r} **;** *therefore*, **
***θ*_{r} **=** *θ*_{i}
.

__Procedure__:

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 **Fig. 3**)**.**

Figure 3

Mount a light 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'**

__Calculations__:
** N/A**

__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