PASCO OS-8536 OPTICS TABLE - Manual (Page 17)

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012-06557A Optics Table
Remove the lens. To measure the focal length, use a ruler to extend the
outgoing diverging rays straight back through the outline of the lens.
The focal point is where these extended rays cross. Measure the dis-
tance from the center of the lens to the focal point.
To determine the radius of curvature, put the concave lens back in the
path of the rays and observe the faint reflected rays off the first surface
of the lens. (The front of the lens can be treated as a concave mirror
having a radius of curvature equal to twice the focal length of the effec-
tive mirror.) Trace the incident rays and the faint reflected rays. Mea-
sure the distance from the center of the front curved surface to the point
where the faint reflected rays cross. The radius of curvature of the sur-
face is twice this distance.
Note that the lens is symmetrical and it is not necessary to measure the
curvature of both sides of the lens because R is the same for both. Calculate the focal length of the lens using
the lensmaker’s equation. The index of refraction is 1.5 for the lens. Remember that a concave surface has a
negative radius of curvature.
Experiment 9: Apparent Depth
EQUIPMENT NEEDED
– Light Source – Optics Bench
– Convex lens – Rhombus
– masking tape – metric ruler– pencil
PART I
Purpose
To determine the index of refraction using apparent depth.
Theory
Light rays originating from the bottom surface of a block of material
refract at the top surface as the rays emerge from the material into the
air. When viewed from above, the apparent depth, d, of the bottom
surface of the block is less than the actual thickness, t, of the block.
The apparent depth is given by
d = t/n, where n is the index of refraction of the material.
Procedure
Mount the Light Source, Ray Table Base, and Ray Table on the Optics Bench. Place the Convex Lens on the
edge of the Ray Table nearest to the Light Source. Adjust the slit mask on the front of the Light Source so five
light rays shine straight into the Convex Lens. Use a strip of masking tape to block the center three light rays.
Mark the place where the two outer light rays cross each other.
Next, place the Rhombus as shown in Figure 9.2. The surface of the Rhombus facing the Light Source must be
exactly at the point where the two rays cross. The crossed rays simulate the rays that emerge from the bottom of
the Rhombus block discussed in the theory.
Air
n
t
d
Figure 9.1
Figure 8.3
P
A
TTERN
COLOR
5 3 1
RA
Y
SLITS
Light
Source
1/2 R
Incident rays
Faint Reflected
Rays
Concave
Lens