Mini-experiment (Module: Light and Colour)
IJSO Training: Light and Colour
Mini-experiment
Refractive Index and Snell’s Law
Objective
In this experiment, you are required to determine the refractive index of an acrylic
trapezoid (or any block with parallel faces) by using Snell’s Law.
Theory
Recall the Snell’s Law:
n1 sin 1  n2 sin 2 ,
where
(1.1)
1 is the angle of incidence,
 2 is the angle of refraction,
n1 and n2 are the respective refractive indices of the two media.
Procedure
1.
2.
Place the laser pointer on a sheet of paper. Turn on the laser pointer.
Place the trapezoid on the paper so that the beam passes through the parallel
sides as shown in Figure 1.1.
1
Incident Ray
Transmitted Ray
Figure 1.1
Mini-experiment (Module: Light and Colour)
3.
4.
5.
6.
Mark the positions of the parallel sides of the trapezoid and trace the incident and
transmitted beams. Mark the rays on the graph paper carefully.
Remove the trapezoid and draw a line on the paper connecting the points where
the beams entered and left the trapezoid.
Measure the angle of incidence and the angle of refraction. Record the angles
in Table 1.1.
Repeat above steps by using other angles of incidence. Record the angles in
Table 1.1 again.
Angle of Incidence ( 1 )
Angle of Refraction (  2 )
Refractive Index of acrylic ( n2 )
Mean =
Table 1.1: Data and Results
Analysis
1.
Use Snell’s Law to calculate the refractive index by assuming that the refractive
index of air ( n1 ) is 1.
2.
Find the mean of the three values of refractive indices and calculate the
percentage error comparing with the true value n  1.5
Question
What is the angle of the ray that leaves the trapezoid relative to the ray that enters it?
Mini-experiment (Module: Light and Colour)
IJSO Training: Light and Colour
Mini-experiment
Refractive Index and Apparent Depth
Objective
In this experiment, you are required to determine the refractive index of a prism (or
any block with parallel faces) by measuring the apparent depth.
Theory
nair = 1
n>1
d
t
Figure 2.1
Referring to Figure 2.1, the apparent depth, d, of the bottom surface of the block is
related to the actual depth, t, by the following equation (can be proved by using
Snell’s Law):
d t n,
(2.1)
where n is the refractive index of the material.
Procedure
1.
2.
Draw a straight line on the paper. Place the prism on the paper over the line.
Look down through the top of the prism with both eyes. Judge if the line
viewed through the prism appear to be closer. Close one eye and move your
head side to side. You will see parallax（視差）between the line viewed
through the prism and the line viewed directly.
Mini-experiment (Module: Light and Colour)
3.
Hold a pencil near the side of the prism and move your head side to side to
check for parallax. Move the pencil up or down and check again until no
parallax is observed (see Figure 2.2).
Look down
Move eye side to side
Pencil
Figure 2.2
4.
Mark the position of the pencil pointing on the side of the prism. Measure the
distance from the top of the prism to the position. Record this apparent depth, d,
in Table 2.1 and record the apparent depth.
5.
Measure the thickness, t, of the prism and record it in Table 2.1
Apparent depth (d)
Actual thickness (t)
Refractive index (n)
Table 2.1: Data and Result
Analysis
Use Equation (2.1) to calculate the refractive index of the prism and record your result
in Table 2.1.
Mini-experiment (Module: Light and Colour)
IJSO Training: Light and Colour
Mini-experiment
Focal Lengths of Convex and Concave Lenses
Objective
In this experiment, you are required to determine the focal lengths of convex and
concave lenses and investigate the difference between the two lenses.
Theory
Recall that when the rays parallel to the principal axis pass through a thin lens, they
emerge either converging or diverging. The point of convergence (or origin of
divergence) is the focus of the lens. The focal length is then the distance of the focus
from the center of the lens. If the rays diverge, the focal length is negative.
Procedure
1.
Place the laser pointer on a sheet of paper. Turn on the laser pointer.
2.
Place the convex lens on the paper so that the incoming beam is parallel to the
principal axis as shown in Figure 3.1a.
Outgoing ray 1
Incoming ray 1
Incoming ray 1
Outgoing ray 2
Incoming ray 2
Incoming ray 2
Outgoing ray 2
Outgoing ray 1
Convex lens
Figure 3.1a
3.
4.
Concave lens
Figure 3.1b
Place the laser pointer at several different positions and trace the incident and
transmitted beams. Indicate the incoming and outgoing rays with arrows in the
appropriate directions.
Locate the focus of the lens. Measure the focal length and record the result in
Table 3.1.
Mini-experiment (Module: Light and Colour)
5.
Repeat the procedure with the concave lens as shown in Figure 3.1b and fill
Table 3.1.
Convex lens
Concave lens
Focal length
Table 3.1: Results
Further Investigation
Place the convex and concave lenses together as shown in Figure 3.2 and repeat the
procedure. Check if the outgoing rays are converging, diverging or parallel.
Discuss if the result is consistent with the focal lengths found for the two lenses.
Incoming ray 1
Outgoing rays = ?
Incoming ray 2
Figure 3.2
Mini-experiment (Module: Light and Colour)
Safety Precautions (General)

Do not touch any optical surface. If a component is dirty, please consult
Teacher/Lab Technician.

Optical components should be covered when experiments are finished.
Precautions when Handling Laser Pointers

Never shine a laser pointer at anyone.
Laser pointers are designed to illustrate
inanimate objects.

Do not allow minors to use a pointer unsupervised.

Do not point a laser pointer at mirror-like surfaces.
like a direct beam on the eye.
Laser pointers are not toys.
A reflected beam can act