Here are some things to think about and do as we start the unit on images.   These questions can be answered by simple investigations that will only take a few minutes. Do all of these. You should also discuss what you predict will be observed (if it is a situation with which you are already familiar).  

For these activities you should be standing in a fairly dim area, such as inside a hallway looking towards the windows in the front door, or under a shady tree.   You want the scene you are looking at to be fairly bright (so look outside - cloudy or sunny conditions are both fine), but you and your lens and card should be in a dim area.

Cathy and Laura are making images of the view through the window that is behind them

Laura is holding two lenses at once Why aren't there two images?

1. Facing away from the window (or brightly lit scene), hold up an index card (or any other white flat stiff object), at about eye level and at nearly arm's length.  Then hold a positive lens against the card, and slowly move it away towards the window or scene behind you, keeping it parallel to the card, like the people in our photo.  (They are facing away from the window. Notice how they are holding their lenses and cards.  They are making images of the landscape outside the windows that are behind them.)  Meanwhile, watch what happens on the card (do not look through the lens, but rather look around it at the card). You will see meaningless smudges of light, which will become a meaningful pattern when you are holding the lens the right place and the right way.  We will then say, "The image is on the card." There is only a narrow range of distance between lens and screen where this will happen.

   The other activities in this unit assume you have gotten this to work -- that you have seen a well-defined picture of something on your card, and know how you did it and can do it again easily. If you are having trouble with this, don't hesitate to email us or even give us a telephone call ((859) 257-3197 or (859) 257-5131).

2. Once you find a focused image, interpret the pattern: how is it related to the scene/window?
3. Predict what will happen to the image on the card if you cover part of the lens with your hand.  Will half of the image go away?  Or something else happen?  Try it!  Can you tell whether the covering is on the right, on the left, or a finger across the middle?
4. Label the positive lenses A,B,C,D. Make an image of a distant scene (that is, something more than 3 meters away) with each of them. Measure the distance between lens and card, that gives an image on the card. This is called the focal length for the lens. Enter this in the table below. While you are measuring the focal lengths, notice how the images on the card are alike, and how are they different. In particular, note which lens gives the largest image, and which gives the smallest, and record this on the table (by writing "largest" or "smallest" on the second row). Indicate the lens that has the largest focal length by writing "largest" at the top of the column; similarly note the lens with the shortest focal length.     

   	Lens
   	A	B	C	D
   focal length

   
   * Use each lens as a magnifier, holding it right at your eye. Which lens is the strongest magnifier (gives the biggest view), and which is the weakest? Enter this in the table, too.
   Summarize: How does the focal length of a lens affect the things it can do?

5. Use a concave mirror (spherical mirror) to form an image on a card.  To do this you will need to orient the mirror facing the scene that you wish to image. Look at the drawing in the curved mirrors unit for a hint on how to arrange the mirror relative to card and scene. You will need to place the screen between the scene and the mirror - but doesn't that block the light from getting to the mirror?  Yes, but if you use an index card for your screen you can cut a small hole (about 1/2") in the middle of it to let some light get through to the mirror.  The shape of the hole doesn't matter, and an irregular shape is actually more interesting. Place the concave side of the mirror against the hole in the index card and slowly move it towards you, while peeking around the mirror, looking at the side of the card facing the mirror to see what is happening.  A focused image should appear on the card before you have moved the mirror any further than 6" from the card.  There is again only one distance that gives an image of the scene, though there is another distance that gives an interesting image of something: what is this image?

All done?