We use lenses everyday whether we know it or not. There are two lenses in each of our eyes: the cornea and the crystalline lens which focus light onto our retinas. Lenses are used to not only correct our vision, but to enhance it by letting us see worlds we normally wouldn't be able to see, via microscopes and telescopes. The following are activities you can carry out at home.

Magnification and focal length

A simple microscope

a) Put a drop of water on the clear sheet to form a water lens 5 millimeters (mm) or greater in diameter. Use a bright lamp (desk lamp) as a light source and hold it over a paper so you can estimate the focal length with a ruler. You'll find the focal length to be about 5 millimeters -- maybe more, maybe less. Can you estimate the magnification by measuring a known object (hair is about 70 microns in diameter)? Compare the width of the image with the diameter of your water lens which you can measure.

b) An improved water-drop magnifier. Take a stiff card 10 centimeters (cm) long and 3 cm wide - about the size of a microscope slide. Punch out or cut a hole about 5 mm wide (standard paper hole punch is 7 mm in diameter and maybe too big). Put a piece of clear plastic over the hole, tape it down, and then put a drop of water on the plastic over the hole with the droplet big enough to extend beyond the hole. Now you have a water-Leeuwenhoeck microscope. See if you can determine focal length and magnification.

Real and Virtual lmages

a) What to do
1. Get the A lens from your Optics Kit.
2. Place the magnifying lens over the drawing of an arrow.
3. Hold the magnifying lens at about arm's length away from your eye, and slowly lift the magnifying lens off the picture.
4. Keep lifting the magnifying lens until the image you are watching turns upside down.
5. Repeat for the B image

b) What happened

As you first began to move the magnifying lens further away from the page, the image grew larger. At one point the image became fuzzy and then came back into focus upside down. This is because of the way the magnifying lens bends the light traveling to your eye from the page. When the magnifying lens is close to the page, you see what is known as a virtual image. This is an upright image that is larger than the image on the page. As you move the magnifying lens closer to your eye and away from the page, you see what is called a real image, which is upside down. Look at the diagrams below to see how the light is bent by the lens to form the two types of images you saw.




c) Summary: If the object is inside the focal plane (F) the image is virtual and erect; if the object is outside the focal plane (F) the image is real and inverted.

Continue on to the Optical Microscopy Readings Page