PHYS 1407 – Conceptual Physics II
Geometric Reflection

 

Leader: _____________________________  Recorder: ___________________________

Skeptic: _____________________________ Encourager: _________________________

 

Materials

Pasco Basic optics Bench with light source and table with protractor

Pasco Ray Optics Kit

Ruler

White paper

Flashlight

Laptop (for graphs)

 

Introduction

      In this lab we will study the law of reflection.  Figure 1 shows the geometry of rays incident on a flat surface.  The rays make an angle of qi with the normal to the surface.  The reflected ray makes an angle qr with the normal to the surface.  In part 1 of this activity, we will examine the law of geometric reflection.  The law of geometric reflection, relates the incident angle to the reflected angle. We will measure several incident and reflected angles and plot them on a graph, to examine the relationship between the angles.  In part 2, we will investigate reflection from a concave and a convex spherical mirror.

 

Figure 1  Geometry for the law of reflection

 

Part 1 – Law of Reflection

Procedure

1.  Set-up        

Mount the light source and the horizontal table on the optics bench.  Rotate the table so that the 0° mark faces the light source.  Position the flat mirror so that its surface runs along the line extending for 90° to 270°.  Adjust the light source so that the slits face the horizontal table.  Turn on the light source and adjust the mask so that a single beam is directed towards the mirror. 

 

2. Data Acquisition

For 5 different incident angles, differing by at least 5°, measure the incident angle and the reflected angle and record in a data table below.  In each case, the ray must hit the mirror at center of the optics table.  Both the incident and the reflected angles should be measured from the vertical as shown in figure 1.

 

Data

Trial

θi

θr

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Data Analysis

      Use whatever software you want to construct a graph of the incident angle versus the reflected angle.  Add a best fit line that goes through your data points. 

 

Questions

1)  What is the value of the slope of your best-fit line?  Is the value close to 1?

 

 

2)  What are the units of the slope of your line?

 

 

3)  Write the equation implied by your line.

 

 

4)  Summarize the law of reflection in your own words.

 

 

 

Part 2 -  Reflection from Curved Mirrors

In part 2 of this activity we will investigate reflection from curved mirrors.  Curved mirrors obey the law of reflection, only the relevant surface is the plane tangent to the mirror at the point the light is incident on the mirror.  Figure 2 illustrates this.

 

Figure 2  Reflection from a curved surface

 

Procedure and Questions

1.  Adjust the mask so that 4 rays appear.  Place a sheet of white paper on the optics table.  Place the mirror from the ray optics kit on the sheet of paper so that the concave side faces the light source.  Trace the incident and reflected rays on the paper.

 

1)  Sketch the appearance of the incident and reflected rays

 

 

 

 

2)  Note that the rays come together at a point.  This point is called the focal point.  Measure the distance from the focal point to the point on the mirror where the central ray hits and record your value.  (Don’t forget units.)  This distance is called the focal length of the mirror.

 

 

Place the mirror on the clean side of the piece of white paper with the convex side towards the rays.  Trace the front edge of the mirror onto the piece of paper.  Use a straight edge to mark the path of the incident and reflected rays.

 

3)  How do the rays reflected from the convex mirror compare to the rays reflected from the concave mirror?

 

 

When rays are reflected so that they come together we call them converging and when they are reflected so that the spread apart, we call it diverging.

 

4)  Fill in the following blanks with either diverging or converging.  A concave mirror produced _________________ rays, and thus a concave mirror is also known as a ____________________ mirror.  A convex mirror produced _________________ rays, and thus a convex mirror is also known as a ____________________ mirror.

 

5)  On the sheet of the paper, use a straight edge to trace the reflected rays you have marked backwards.  Do they seem to trace back to a point (or at least fairly close)? This is the focal point for a diverging mirror.

 

 

 

6)  Is the focal point for a diverging mirror located in front of or behind the mirror (with respect to the incident rays)?

 

 

7)  Measure the focal length for the convex mirror.

 

 

In general, the focal length of a mirror depends only on its radius and is given by

| f | = R/2 where f is the focal length and R is the radius of the mirror. 

 

 

We refer to something if it is on the same side of the mirror as the light source as in front of the mirror, and if it is on the opposite side as behind the nmirror.

 

 

8)  Is the focal point of a concave mirror in front or behind the mirror?

 

 

 

9)  Is the focal point of a concave mirror in front or behind the mirror?

 

 

10)  Fill in the following blanks with converge or diverge. 

When the focal point is in front of the mirror, parallel rays of light hitting the mirror ________ to a point in front of the mirror.  When the focal point is behind the mirror, parallel rays of light hitting the mirror ________in front of the mirror.  Backtracked rays ______ to a point behind the mirror.