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.
- 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.
- 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)
- 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.
- 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
- 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.