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Focal Length Lab -StarterPowerPoint Presentation

Focal Length Lab -Starter

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Focal Length Lab -Starter

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All the lenses on the left are diverging lenses or negative lenses.

All the lenses on the right are converging or positive lenses.

How can you look at a lens and tell if its positive or negative?

A. Design an experiment to locate the focal point of a concave

and convex lens by using a ray box and the lenses provided.

Find a numerical value for the focal length of each lens by

tracing rays.

1. Put a slide on the ray box for three parallel rays.

2. Trace the lens and the three rays on each side of the

lens.

3. Using your drawing, determine where the focal point

is and label it.

4. Measure from the lens center to the focal point

and record the focal length.

B. Determine the radius of curvature of each lens surface

and use the lens maker’s equation.

1. Each radius can be determined by tracing the surfaces

repeatedly until you obtain a circle. Then measure and record

each radius.

2. Look up the index of refraction for the lens material (acrylic).

3. Use the lens maker’s equation and your data to determine f.

Positive Lens

Method 1 : Intersection of three parallel rays

distance from lens center to intersection point = f = _______cm

Method 2: R1 = __________ R2 = ________ n=1.49

1/f = (n-1)( 1/ R1 – 1/ R2) f = ____________cm

Negative Lens

Method 1 : Intersection of three parallel rays

distance form lens center to intersection point = f = _______cm

Method 2: R1 = __________ R2 = ________ n=1.49

1/f = (n-1)( 1/ R1 – 1/ R2) f = ____________cm

1. Two positive lenses have identical shapes, but one is made of glass (n=1.65)

and one is made of acrylic. Which one has the shortest focal length?

2. Draw a lens that has both of its radii of curvature negative.

3. Draw a lens that has both of its radii of curvature positive.

Data Page

Calculations

Drawings

1. Positive lens with intersecting rays

2. Negative lens with intersecting rays

3. Positive lens showing radii

4. Negative lens showing radii

D. Percent differences between focal lengths

determined by each method.

E. Summary