Converging Lens

This experiment uses a double convex lens (a magnifying glass) on your desk. Each student works with their own lens (identified on the lens as student 1 and 2). The lens and screen are held in place by magnetic clamps that can be switched on and off.

A converging lens can produce a real image (where the light focuses to an image). Film that is placed at the image plane would be exposed and could be developed into a picture. Real images are also found with concave mirrors and holograms with images projecting out in front.

Experiment a

• Determine the focal length of the lens directly by obtaining the image of a very distant object on a white card and measuring the image distance.
• Write down a value (an uncertainty) of how accurate you feel this focal length was measured.

Experiment b

• Write your name along with a color drawing on a card. Attach the card with masking tape to the front of a light. This back-lit card will form the object. Place the light into position at one end of the board.
• Adjust the lens height so it coincides with your name on the card. Now, position the lens at some distance (30-60cm) from the object (your light with the card)
• Now find the position of a screen (card on a post) for which an image is in sharp focus
• Measure the object and image distances from the lens as well as the object and image sizes.
• Determine how exact you can measure distance and also how exact the screen can be positioned at the image location. Write down these values as uncertainties (a ± value)
• What effect does covering part of the lens have on the image. (Try covering ½, ¼, or ¾ of the lens and see what happens with the image)

Experiment c

• Separate the object card and the screen by as much as bench will allow.
• Start with the lens close to the screen and find the position at which there is a sharp image on the screen. Measure the object and image distances from the lens as well as image and object sizes.
• Now leaving the object card and screen in the same position, find a second (different) position of the lens where there is a sharp image on the screen and measure all positions and sizes.

Experiment d

• With the lens and screen positioned as they should be from the end of experiment c (with the lens close to the object card and a large focused image on the screen) place the small aperture card directly after the lens.
• This should produce a dimmer image on the screen which should still be in focus.
• Move the screen back and forth to find the range where the image is still in focus (this is referred to as the depth of field)
• Record the screen positions where the image just begins to loose focus.
• ​Remove the small aperture card and compare how the depth of field changes.  Comment on the difference between the depth of field with and without the small aperture card

Analysis

• Determine the focal length of the lens from the measurements of experiment b and c.
• Considering all three experiment values for the lens focal length, write a single statement that states an overall focal length and an overall accuracy for this value.
• For all parts of experiments b and c, determine the expected magnification using the object/image distances and compare to the magnification found from the object/image sizes. Write the comparison statements using percent difference.