Pasco OS-8515C Basic Optics System Manuel d'utilisateur télécharger pdf (Page 24)

Basic Optics System Experiment 8: Lensmaker’s Equation

2. Trace around the surface of the lens and trace the incident and transmitted rays.

Indicate the incoming and the outgoing rays with arrows in the appropriate direc-

tions.

3. Remove the lens. To measure the focal length, use a ruler to extend the outgoing

diverging rays straight back through the lens. The focal point is where these

extended rays cross. Measure the distance from the center of the lens to the focal

point. Record the result as a negative value:

f = _______________ (measured directly)

4. 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 effective mirror

(see Figure 8.3).

Trace the surface of the lens and mark the point where the central ray hits

the surface. Block the central ray and mark the point where the two outer

rays cross. Measure the distance from the lens surface to the point where

the reflected rays cross. The radius of curvature is twice this distance.

Record the radius of curvature:

R = _______________

5. For this lens, it is not necessary to measure the curvature of both sides because

they are equal (R

= R

= R). Calculate the focal length of the lens using the lens-

maker’s equation (Equation 8.1). The index of refraction is 1.5 for the acrylic

lens. Remember that a concave surface has a negative radius of curvature.

f = _______________ (calculated)

6. Calculate the percent difference between the two values of f from step 3 and

step 5:

% difference = _______________

Concave lens

1/2 R

Figure 8.3: Reflected rays from

the lens surface

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Pasco OS-8515C Basic Optics System Manuel d'utilisateur Page 24