DOUBLE CONVEX LENS

OBJECT:

To observe and study the kind of images that are formed, using a Double Convex Lens. ( A typical magnifying lens.)

MATERIALS:

Use caution, when working with a compass! Sheet of wide ruled line paper, Sharp pencil, Compass and a Protractor. ( Protractor not necessary but helpful )

PROCEDURE AND OBSERVATION:

SAFETY FIRST, Be sure to take every possible precaution to prevent any INJURIES!

1. Take a sheet of wide ruled note book paper measuring 10 and 1/2 inches by 8 inches, be sure to use sharpen pencils for all work.

2. Come down to the fourth line, put your ruler on that line, place a neat single dot at exactly the 4 inch position. Make a light circle around that dot, you should get in the habit, when drawing graphs or diagrams, by using this procedure; making a sharp dot and a light circle around the dot, this way you can always find a very small dot. Now, make a dot and a light circle around the dot at 2, 3, 5, and 6 inch positions.

3. Take your compass and open it to exactly 1 and 1/4 inches. The best way, to open your compass, is by placing the point at exactly 1 inch position and opening the pencil to exactly 2 and 1/4 inch position. You should always avoid using the very edge of the ruler, for measuring, because sometimes it is cut a bit to long and at other times, it is slightly shorten because of use.

4. Place your point of the compass at exactly the 3 inch position and make an neat arc up to a semi circle.

5. Now, place your point of the compass at the 5 inch position, and makea neat arc forming another semi circle. You have just drawn a double convex lens.

6. Neatly connect the intersecting arcs, you have just made a perpendicular line, to the line with the numbers, in other words, if you were to take a protractor and place it on that perpendicular line, it should read 90 degrees, and you should be very close to the 4 inch position mark.

7. Now, label your 2 inch position as 2F, then label your 3 inch position as F. Have you ever tried to start a fire with a magnifying lens by using the sun rays? That fine point of bright light that you get, is located at exactly the focal length of the lens, called F, where all light rays converge. Then 2F represents twice the Focal length.

8. Now, take your ruler and make a dot at about 1/2 inch before the 2 inch position or 2F, at exactly that position, draw a vertical line that is perpendicular,( at this point you may use your protractor). It should be from the second line to the 6 th line. Place an arrow head at the top of this new line. Underneath the arrow, label it as your OBJECT.

9. Light rays always travel in a straight line. Retrace the blue line from the top of the arrow head to the middle of the Lens. Now since all rays pass through the F ( focal ) point, place your ruler at the point where the ray line meets the middle of the lens, and at F ( focal ) point or the 5 inch position. Draw your line, from the top middle of the lens, passing through F, and a little past the 6 th line of the paper.

10. Now, repeat step # 9 but this time, work with the bottom of the arrow ( or tail ).

11. All light rays passing through the center of the lens ( 4 inch position ) and are not affected. There are an infinite number of light rays from the head of the arrow to the tail of the arrow passing through the center of the lens. It would be an impossibility to draw these lines. So we are limiting it to the outer most rays, meaning the tip of the arrow head and the tip end of the tail. Place your ruler on the tip of the arrow head, and at the center of the lens ( 4 inch position ) and draw a line, extend it to, at least past the 2F position. Now, repeat this same procedure for the tip of the tail. Line from the tip of the tail, through the center of the lens, extended it to, a bit past the 2F position.

12. Draw a line between the two sets of intersecting lines ( rays ), that you have just made. Notice that the light ray coming from the head of the arrow is located at the bottom, and the tail of the arrow is now at the top. Draw your new arrow head, which is pointing down. You have now formed an REAL IMAGE, a real image, is an image that can be projected on a screen. Notice that this image is smaller, then the object.

13. This is the way our eyes see objects, smaller, and upside down, but our brain turns the picture right side up.

14. To summarize the way light rays travels from an object, through the lens to form an image.

A. All rays from the object pass through the center of the lens with out being affected.
B. All other rays coming from the object, pass through the lens and have to pass through the Focal point ' F ' .
C. Where you have light rays passing through the center of the lens, and crossing rays that pass through the Focal point, you are in the process of forming a Real Image.
1. Real Image is always inverted.
2. Real Image can result in being: Smaller, Same size or Larger then the object.

QUESTIONS:

1. Now, using your lab book, construct the images that would be formed, when an Object is located at exactly 2F, then between 2F and F. Describe your images.

Results are found at the following page:

Object at 2F and between 2F and F

2. Now, try to draw an Object at F and see what image you get!

Answer found at the following page:

Object at F

3. This is the big one, now, try placing your Object between F and the Lens and see what you get!

The answer is found at following page:

Object between F and the lens

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