Physics 208 – Introductory Physics II

Nazareth College

Department of Chemistry & Biochemistry

Robert F Szalapski, PhD – Adjunct Lecturer

www.CallMeDrRob.com

Spring 2012

Exam 3

Sound & Geometric Optics

## § Problem 1

One firecracker makes a sound.

### § (a)

What is the sound level for three firecrackers? (15 points)

The most direct route is to triple the intensity and to then use .

Alternatively we can first compute as in part (b):

### § (b)

What is the intensity of the one firecracker? (15 points)

## § Problem 2

At the very edge of a swimming pool you shine a flashlight at the water making an angle of with the vertical. At what distance does the beam hit the bottom if the pool is deep? (20 points)

Begin with an application of Snell’s Law:

From the figure

## § Problem 3

The diagram shows a large convex security mirror superimposed on a grid where each box is wide.

### § (a)

If the mirror has a curvature that would be equal to a diameter sphere, what is the focal length, ? Plot and label on the diagram. (5 points)

It will be a negative focal length, and its magnitude will be one-half of the radius or one-quarter of the diameter.

### § (b)

Suppose an object is placed in front of the mirror. Calculate the position of the image. Clearly indicate the position of the object and the image on the diagram. Is the image real or virtual? (10 points)

The image is virtual.

### § (c)

What is the magnification? (5 points)

### § (d)

Use ray tracing with three rays on the figure above using a straight edge to accurately show the position of the image and magnification of the object. (10 points)

## § Problem 4

The diagram shows a convex lens with a focal length .

### § (a)

Where should the object be placed if a real image is to be half the size of the object? Indicate the computed object and image on the diagram. (10 points)

The image is real, and hence it will be inverted, so the magnification is negative.

Hence

### § (b)

Will the image be upright or inverted? (5 points)

Inverted.

### § (c)

Use ray tracing with three rays on the diagram above using a straight edge to clearly show the position of the image and the magnification. (10 points)