PASCO OS-8536 OPTICS TABLE - Manual (Page 16)

12
Optics Table 012-06557A
you about the relationship between the focal lengths of these two lenses?
Slide the convex and concave lenses apart to observe the effect of a combination of two lenses. Then reverse the
order of the lenses. Trace the patterns.
Place the convex lens in the path of the five light rays. Put a finger in front of the slit mask to block out the
center three rays and mark the focal point for the outer two rays. Next, adjust the slit mask on the fron t of the
Light Source so three light rays shine across the middle of the Ray Table and mark the focal point for the three
rays. Are the two focal points the same?
Experiment 8: Lensmaker’s Equation
EQUIPMENT NEEDED
– Light Source – Optics Bench
– Ray Table and Base – Concave lens
– metric ruler – pencil
Purpose
To determine the focal length of a convex
lens by direct measurement and by using
the lensmaker’s equation.
Theory
The lensmaker’s equation is used to calcu-
late the focal length of a lens based on the
radii of curvature of its surfaces and the
index of refraction of the lens material.
1
f
= (n–1) 1
R
1
+ 1
R
2
where f is the focal length, n is the relative
index of refraction of the lens material, and
R
1
and R
2
are the radii of curvature of the
lens surfaces.
➤ NOTE: In this notation, R is positive for a
convex surface (as viewed from outside the lens) and R is negative for a
concave surface.
Procedure
Mount the Light Source, Ray Table Base, and Ray Table on the Optics
Bench. Place the Concave Lens on the Ray Table. Shine five light rays
from the Light Source straight into the lens. Trace the outline of the lens
and trace the incident and transmitted rays. Indicate the incoming and
the outgoing rays with arrows in the appropriate directions.
Figure 8.1
R
1
Double
Concave
Lens
R
2
P
A
TTERN
SOURCE
COLOR
DISCONNECT
PO
WE
R
BEFORE CHAN
GING BULB
. 5 3 1
RA
Y
SLITS
Light Source
Five parallel
rays
Concave Lens
Figure 8.2