PASCO OS-8459 Beginning Optics System - Manual (Page 21)

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M o d e l N o . O S - 8 4 5 9 E x p e r i m e n t 8 : L e n s m a k e r ’ s E q u a t i o n
21
Experiment 8: Lensmaker’s Equation
Purpose
In this experiment you will determine the focal length of a concave lens in two ways:
a) by direct measurement using ray tracing and b) by measuring the radius of curva-
ture and using the lensmaker’s equation.
Theory
The lensmaker’s equation is used to calculate the focal length (in air or a vacuum), f,
of a lens based on the radii of curvature of its surfaces (R
1
and R
2
) and the index of
refraction (n) of the lens material:
(eq. 8.1)
In this notation, R is positive for a convex surface (as viewed from outside the lens)
and R is negative for a concave surface (as in Figure 7.1).
Figure 8.1
Procedure
1. Place the light source in ray-box mode on a white sheet of paper. Turn the wheel
to select three parallel rays. Shine the rays straight into the convex lens (see Fig-
ure 8.2).
Note: The lens has one flat edge. Place the flat edge on the paper so the lens stands stably
without rocking.
Required Equipment from Beginning Optics System
Light Source
Concave Lens from Ray Optics Kit
Other Required Equipment
Metric ruler
1
f
--- n 1 –
( )
1
R
1
------
1
R
2
------ –
⎝ ⎠
⎜ ⎟
⎛ ⎞
=
Double
Concave
Lens
R
2
R
1
Incoming rays
Concave lens
Figure 8.2