Lensorama!

Purpose

The purpose of this activity is to familiarize you with the different types of images that may be created with any converging lens (or mirror).  You will be using two lenses popularly called “magnifying glasses”.  A more scientific term for it is double convex lens, which refers to the shape of both sides of the lens.  Inspect each lens – any converging lens is thicker at the center than at the edges.  It is “converging” in the sense that it can focus light such that rays converge on a point in space.

Procedure

1. Use the small lens, sit up straight, and lay this paper on the desk.  View the above “stick man” through the lens, starting with the lens sitting on top of it, and then moving the lens slowly toward your eye.

(a) Under what circumstances do you see an enlarged image of the stick man?  a reduced image?


(b) Under what circumstances do you see an inverted image?  an erect image?


(c) Under what circumstances do you see no image?

2. Repeat the above process with the large lens.

3. Use each lens as a magnifying glass to magnify part of the grid shown below.  Compare the width of the squares seen through the lens to the width of the squares seen without the lens.  The magnification is the ratio of these widths.  Adjust for maximum magnification and record the factor – how many times bigger do the squares appear through the lens?

4. Hold the small lens a few centimeters above a piece of blank paper and move the lens up and down until you focus an image of the overhead lights on the piece of paper.  This is called “projecting an image onto a screen”.  Light passes through the lens and is focused onto the paper.  Repeat this process using the large lens.  What do you notice?  How are the images different?

5. Repeat the steps from #2 above but this time form an image of the door that leads outdoors (not the door to the hallway).  You will have to hold up a piece of paper to form a “screen” for the image.  Place the lens between the screen and the object to be imaged – in this case the bright doorway is the “object” to be imaged.  Describe the image of the doorway formed by each lens.

6. Any image formed by a lens can be viewed with the naked eye – if your eye has the right vantage point.  But only real images may be projected onto a screen as was done in the case of the doorway and the overhead lights.  Try looking at a real image without the screen:  As before, form an image of the doorway on a piece of paper, but this time view it from behind the paper with one eye, looking toward the lens and the doorway.  Slowly remove the paper to the side, from between the lens and your eye.  Watch the image as you do this.  How does the image on the screen compare to the image seen with the naked eye?

7. Not every image formed by a lens may be projected onto a screen.  An image which cannot be projected on a screen is called a virtual image.  Which images created in this activity do you think were virtual?  You can test this by attempting to project on a screen – however some real images may be too dim to be noticeable on the screen. 
Note:  light is actually focused and converges in the formation of a real image but this is not the case with a virtual image.  Light appear to diverge from a certain location in the formation of a virtual image.