Homework #5 Sign Convention, Lensmakers' Formula and Ray Tracing
Please answer all questions completely. If calculations are required, show enough steps so that I can follow your reasoning.

1. Define:

a) real focal point

b) virtual focal point

2. Identify each of the lenses shown below by giving its name and stating whether it is a positive or negative lens.

a. b.c.d.

3. For the diagram below, indicate whether each of the following quantities is a positive or a negative number:

a. Object height
b. Image height
c. Object distance
d. Image distance

4. The thin lens at left has a radius of curvature on the left side of +20 cm and on the right side of -30 cm. It is made of glass with an index of refraction of 1.58.

a. What is the focal length of the lens when it is used in air?

b. Is this a converging or diverging lens?

c. Suppose the radii are switched-- that is, the left side is -30 cm and the right side is +20 cm. What would the focal length be? What kind of lens would it be?

 

5. For this problem you will need to draw a ray diagram to scale. You can be guided by the sketch below, but it is drawn with a mouse and may not be to scale. (In the next assignment, you will learn the thin lens equation, which allows problems like this to be solved algebraically.)

a. Starting at the tip of the arrowhead, draw the three principle rays: the parallel ray, central ray and focal ray. (See page 40 of the notes.) Follow the rays to the lens, and show their directions after passing through the lens. Using the scale you used to draw the diagram, determine how far from the lens the image forms. (Note that the object is 10 cm to the left of the lens and the focal points are 5 cm to either side of the lens.)

b. If the object is moved closer to the focal point, does the image move in toward the lens or away from the lens? (You can set this up at the web site http://webphysics.ph.msstate.edu/javamirror/explrsci/dswmedia/lenscon.htm)