The Refractive Index of Water
Grade Level: 9 - 12
Overview
An understanding of both the basic physics of light and the basic physiology
of light perception (vision) is necessary for understanding the more complicated
subject of interpretive visual perception. The unique environmental variables
of the space environment (microgravity, vacuum, radiation, etc.) can produce
adaptive responses that may alter visual perception. This is particularly
the case when visual cues conflict with the normal 1 G (Earth gravity)
postural cues, that contribute to our notions of "up" and "down."
When a light ray passes from one transparent medium to another, the it's
speed and direction are altered. This phenomenon is known as refraction
and is often thought of as the bending of light rays. It is boggling how
many things in our lives are affected by this simple physical phenomenon-from
the lenses inside all our eyes to the lenses in automobile headlights;
from distortions in things seen through a glass of water to the photographed
images in books and magazines. Refraction inherently causes problems for
telescopes on Earth, since light passing from space through the Earth's
atmosphere is subjected to variable of refraction. In fact, the primary
reason for placing the Hubble space telescope in orbit around Earth was
to eliminate this undesirable effect of Earth's atmosphere.
This activity aims to help students achieve understanding of refraction
by allowing them to study the refractive properties of water. They make
light rays pass from air to water and measure the angle of the incoming
ray (the incident ray) and the angle of the exiting ray (the refracted
ray). They find the mathematical relationship between the two angles,
a relationship known as Snell's Law.
Key Questions
- How does light behave when it goes from one clear medium into another?
- Can that behavior (refraction) be characterized mathematically?
- What does the refraction of light have to do with astronomical studies?
Time Frame:
1 or 2 class periods
Materials
For each pair of students:
- 2 protractors or 1 photocopy of the protractor (master below)
- water
- starch powder or milk
- flashlight and batteries
- black construction paper (about 6 cm square)
- tape
- black marker
- 1 half circular dish or a two-well petri dishes
For each student:
Getting Ready
Before beginning the activity, be sure that you have gathered all of the
necessary materials.
Semi-circular plastic containers may be hard to obtain, whereas rectangular
containers are more readily available in candy stores.
Glass containers create interference by their refraction.
A light sprinkle of starch powder or one drop of milk will make the beam
more visible as it crosses the liquid medium.
Classroom Activity
1. Ask your students to recall any experience they have had with
magnifying lenses. Ask, "What properties of a magnifying lens makes
it magnify things?" One important aspect is its shape-a curved surface.
The light is "bent" by the lens.) Ask the students to brainstorm
and list all the things they encounter on a day to day basis that use
lenses. (Some of the things volunteered in the Overview may be mentioned.)
Once you feel that the class is beginning to appreciate the importance
of lenses in their lives, tell them that they will be making some observations
to understand the phenomenon underlying lenses-refraction.
2. Have the students work in teams with 4 - 6 students in each
team. Distribute materials and have them set up the apparatus for measuring
refraction as follows:
a. Put a clear semicircular dish on top of the protractor. Line
up the straight edge of the dish on the 90°-90° line
b. Now half fill the container with water. If using a petri dish
with two wells, fill only one of the wells.
c. Place a circular piece of black construction paper over the
flashlight head and tape it in place. The light should be well blocked.
With a needle, make a pin-size hole through the paper.
d. The flashlight is placed on the table toward the upper left
quadrant of the protractor with the beam aimed toward the center of
the protractor. Align the beam with one of the lines aiming toward the
protractor center. It should make a spot of light on the container as
it enters right at the center of the protractor.
3. Dim the room lights. Ask the students to observe that as the
beam enters the water-filled container, it is bent-refracted. Tell the
students to refer to the beam from the flashlight before it hits the dish
as the incident beam. They can refer to the beam after it is "bent"
as the refracted beam. It creates a spot of light somewhere on the curved
part of the dish as it exits the container.
4. Instruct the students to position the flashlight at various
angles for the incident beam, and observe the angle of the refracted beam.
Distribute a Data Table to each student and have them record the values
of the incident angles and the corresponding refracted angles in the Table.
Wrap-up Session
1. Prepare a line graph of the results from your corrected data.
Incident angles are placed on the y-axis and refracted angles on the x-axis.
Prepare the best straight-line plot based on the points. Based on the
resulting graph, ask the students what patterns they can see? Is there
a range of incident angles for which the refracted angles are very pronounced?
For which range of incident angles is the refraction effect minimal?
2. Advanced students can calculate the sine for all incident and
refraction angles collected for a respective liquid. If they plot the
incident angle sine values on the y-axis and the refracted angle sine
values on the x-axis, students can calculate the slope of their graph
and derive the formula that relates incidence to refraction. The formula
should be in the y = mx + b format taught in algebra classes. The slope
is the index of refraction in Snell's Law, as given by the following equation:
sin øi ÷ sin ør = constant (index of refraction).
3. Have a discussion about the properties of other clear objects,
for example:
The face plate of an astronaut's helmet-does it refract light?
How does a prism make a rainbow?
When a bear goes fishing, does the bear need to compensate for refraction
of the image of the fish under the water?
What causes near/farsightedness?
More Activity Ideas
1. Substitute another liquid in the same container and obtain incident
and refraction angle data for this substance. Plot the results and compare
the graph with that of water. Which substance gave a steeper line-graph?
What relationship is there between the steepness of the line and the degree
of refraction of liquids tested. Does there seem to be any relationship
between the degree of refraction and the viscosity of the liquid?
2. Investigate the refractive ability of lenses. Students can investigate
the relationship between lens curvature and focusing ability.
3. Have students research how solids and gases (or dispersed water
droplets) might affect light by refracting it. Can light be affected by
gravity? The unit of light is the photon. Does gravity act on these particles?
(Yes) Do we observe gravitational effects on photons on Earth? (Not within
the limits of our vision.) Where can we observe gravity acting on light?
(Black holes!)
Background for Teachers
Prerequisites:
- Basic geometric principles
- Measurement of angles / use of a protractor
Vocabulary:
- Refraction - "bending" of light as it goes from one
substance into another
- Angle of incidence - the angle between the light beam striking
a surface and the line perpendicular to that surface
- Medium - a substance
- Light ray - a beam of light
- Angle of refraction - the angle that the refracted beam makes
with the line perpendicular to the refracting surface
- Protractor - instrument for measuring angles
- Sine - a trigonometric function
- Snell's Law - a wave refracted at a surface makes angles relative
to the normal to the surface, related by the equation ni sin øi
= nr sin ør , where ni and nr are the refractive indices on each
side of the surface
Skills:
- Physical measurement of natural phenomena
- Constructing and maintaining data tables
- Data comparison and analysis
- Designing effective scientific experiments
Concepts:
- Refraction of light
- Experimental models
- Scientific "law"
Keywords: Visual Perception, Refraction, Snell's Law
|
