Northern Illinois University
Physics 150
Lab 11
Reflection and Refraction
Objective
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Measure a variety of incident and reflection angles off a mirrored surface and verify the
reflection rule that states that the angle of incidence is equal to the angle of reflection.

Take a variety of measurements of light refractions through glass to determine the
refractive index (n) of glass as defined by the equation given in the recitation notes.
Materials
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Corrugated cardboard "table top"
Blank white paper
Straight pins
Straight edge
Protractor
Procedure
Part 1 : Reflection from a (Flat) Mirrored Surface
1. Lay a blank sheet of white paper on the cardboard "table top". Place the mirror in the
middle of the page, marking its location for future reference.
2. Place a pin at a distance from the mirror in the general area indicated by your lab
instructor.
3. View the image of the pin in the mirror (from the level of the table top) and place three
pins in the cardboard directly between you and the image of the pin.
4. Without moving the original pin, repeat Step 3 from two other angles.
5. Remove the pins. Use your marks to draw in the position of the mirror surface. Use the
holes left by the pins to draw in the paths of the image light beams. Label each line with
an identifying number. Carry your lines well behind the original position of the mirror.
6. Where each reflected beam intersects with the mirror surface, draw in the "normal" which
is a line perpendicular to the mirror surface.
Analysis
1. Measure and record each of the three angles of incidence I.
2. Measure and record each of the three angles of reflection R.
3. The intersection of the lines should provide you with a "virtual" location of the original
pin. Measure the perpendicular distance of this location to the mirror surface.
4. Measure the perpendicular distance from the position of the "original" pin to the mirror
surface.
Discussion
1. Compare the incidence and reflection angles for each position. Do your measurements of
incident and reflective angles support the Reflection Rule as stated in the Objective
section (agreement within 5%)? If not, suggest why they don't.
2. What do you notice about the position of the actual object (original pin) compared to the
position of the "virtual" object?
Part 2: Refraction: Air to Glass to Air
1. Using another piece of paper, set the rectangular glass block on the paper and trace its
outline.
2. Place a pin on one side of the block.
3. While viewing the pin over the block (it is helpful if you view the pin from enough of an
angle to clearly separate the original pin from its image), place two pins in line with the
original on the same side of the block as the original pin. The pins should appear in
line from your point of view.
4. Place three more pins between you and the image of the original pin as seen through the
block. There should be a total of six pins (three on each side of the block) pushed into the
table top.
5. Remove the glass block and the pins. Use the holes left by the pins to trace the path of the
light beams into and out of the block.
6. Repeat Steps 3 through 6 for the triangular piece of glass if one is made available.
Analysis
1. Measure and record the angles in air and glass at both sides of the block (you will have
two pairs of angles), noting that the angles are measured at the same boundary between
air and glass
2. Using both sets of angles (one set is the bending of light as it goes into the glass block
and the second set is from the bending of the light as it comes out of the block), calculate
the refractive index using: n = sinA/sinG where A is the angle between the normal and
the beam of light in the air and G is the angle between the normal and the beam of light
inside the glass block.
3. Calculate an average refractive index and record it.
Note: Each lab partner must create and turn in an original diagram for both
parts of the lab. Also, no member of the team is dismissed until everyone on the
team has created both diagrams. Mechanically reproduced copies (Xeroxed) are
not acceptable.