Alfred F. Leung

California State University at Long Beach

Department of Physics and Astronomy

CSULB

1250 Bellflower Boulevard

Long Beach, CA 90840-3901

562-985-4923

afleung@csulb.edu

Inner Diameter of a Narrow-mouth Glass Bottle Measured with a Laser

Abstract (50-75 words)

The beam from a laser pointer illuminates a narrow-mouth glass bottle perpendicular to the vertical axis of the bottle and at an incidence angle of about 55^o. Three bright spots are observed on the surface of the bottle. The positions of these spots and the outer diameter of the bottle determine the inner diameter. This simple demonstration shows how optics can be applied to obtain a parameter that is seemingly impossible to measure.

Construction of Apparatus:

A support that shapes like an L is made of ¾” thick wood. The horizontal part of the L-shape support has a dimension of 13 cm x 25 cm. The vertical part of the support has a dimension of 13 cm x 19 cm. A narrow-mouth glass bottle is placed on the horizontal part of the support. A laser pointer is mounted horizontally on the vertical part of the support. This mounting allows the beam from the laser pointer to illuminate the bottle perpendicular to the vertical axis of the bottle and at an incidence angle of about 55^o.

Use of Apparatus:

The laser pointer illuminates the bottle at point A, as shown in the figure below. The subsequent reflection and refractions give rise to two bright spots at C and E. The arc

length between A and C and that between C and E are measured along with the circumference of the bottle. With the index of refraction of the bottle taken to be 1.5 we can determine the inner diameter of the bottle with these length measurements. The accuracy of the determination is about 5%.

Equipment and costs required to construct apparatus:

Item	Source	Part number	Cost
¾” thick wood (not essential)	Home Depot		$2
Glass bottle	Chemistry Department garbage		0
Laser pointer	Fry’s Electronics	Outpost #: 3969718	$7.99
Total Cost			$9.99