Reflection and Refraction Lab
Introduction:
The law of reflection states that when an incident ray (of light) strikes a
flat mirror in the same medium as the reflected ray, that the angle to the
normal (perpendicular to the mirror) of both rays will be equal. We will
demonstrate this in part I of this lab.
In part two, we will look at refraction, that is, the change in speed of a
light ray as it moves from one medium into another. Snell’s Law illustrates
the differences in angle of incidence and angle of refraction when light
moves from one medium to another. Materials that are very optically dense
have high index of refraction and cause light to move relatively slower. The
index of refraction in air is close to 1.000, the index when light passes
through a vacuum.
Important Formula: ni/nr = sin i/sin r
Materials: Small mirror Protractor Laser Refraction box Water Index card
Procedure, Part I:
1.Stand the small mirror at the edge of the protractor, lined up at zero
degrees.
2.Bend an index card around the protractor and hold or tape it so that it
stands vertically from 900 to 1800. You will use this card to read the angle
of the reflected ray.
3.Set you laser on your table and emit the beam at the center of the mirror,
at the center of the protractor.
4.Observe the resulting laser dot on the index card on the reflection side of
the protractor.
5.Repeat the above with the angles shown in the data table below.
Angle of Incidence Angle of Reflection
200
400
600
800
Procedure, Part II:
1.Draw the normal on your printed protractors found on the back of this page.
2.Tape the index card to the curved side of the refraction box and place the
box over the printed protractors.
3.Place the laser at the center of the protractor and emit its light at 00
(along the normal). The beam should appear at 900, 00 from the normal.
4.Fill the refraction box half-full with water and carefully place it back
over the printed protractor.
5.Move the laser to the angles shown below (angle of incidence) and observe
the location of the resulting beam (angle of refraction).
6.Use Snell’s Law to determine the index of refraction of water at each
angle. Assume the index of refraction in air is 1.000.
7.Using the index of refraction, calculate the speed of light in water.
Assume the speed in air is equal to that in vacuum (c) and is 3.00 x 108 m/s.
8.Find the average index of refraction and average speed of light for water
Angle of Incidence Angle of Refraction Index
of Refraction Speed of Light (m/s)
200
400
600
800
AVERAGES
Questions:
1.The normal is a line that is ____________________ to the mirror.
2.Refraction occurs when a light ray changes ____________________ as it moves
from one medium to another.
3.Light moves fastest in higher / lower optical density materials.
4.Materials with a high index of refraction cause light to move
____________________.
5.If this lab were conducted using ultra violet (UV) light rather than red
visible light, we would find UV light would move faster / slower / the same
speed in water as does red light.
6.A light beam is shined into an unknown material at an angle of 450 to the
normal. The angle of refraction is 350. Calculate the index of refraction
for the material. ____________________
7.How fast does light move in a substance with an index of refraction of
1.66? ____________________
8.A light beam is shined into an unknown material at an angle of 650 to the
normal. The angle of refraction is 600. How fast is light moving after it
enters the material? ____________________
9.The critical angle is the angle of incidence that would make the angle of
refraction 900. This is called total internal reflection. If a light beam
is emitted from water into air, based on your index of refraction for water,
what is water’s critical angle? ____________________
10.Submarines are able to hide under imaginary barriers called thermalclines
in the ocean where different water temperatures change the speed of sound. A
similar phenomena is used in “Doppler radar” to observe weather. Will radar
(short wavelength radio waves) speed up or slow down as they move from warmer
to colder air, moving higher in the troposphere? ____________________
Extension: Repeat part II with transparent or translucent minerals such as
calcite, halite, and quartz to determine how fast light moves in solids.
(BTW: index of refraction in diamond is a gigantic 2.5!) Maybe try mineral
oil or alcohol, two liquids less dense than water.