Areas of Science Physics
Time Required Very Short (≤ 1 day)
Prerequisites None
Material Availability Readily Available
Cost Very Low (under $20)
Safety No hazards


Have you ever looked through a magnifying lens? Why do things look bigger when you look at them through the magnifying lens? Even though the object appears to get larger, it really stays the same size. Each lens has its own unique power of magnification, which can be measured with a ruler. How powerful is your lens?


In this experiment you will measure the apparent size of an object through a magnifying lens at different optical distances compared to the actual size of the object and discover a relationship between magnification and optical distance of a lens.

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Sara Agee, Ph.D., Science Buddies

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Last edit date: 2020-11-20


Optics is the study of light and how light travels through different objects in different ways. Have you ever looked through a glass of water at something? You will notice that the object looks smaller through the glass than when you look through the air. This is because light travels differently through the glass and the water, causing the image to bend. Your eyes perceive this as a smaller object, even though your brain knows that the object is still the same size.

Physicists use the power of optics to make many different kinds of lenses to see different kinds of things better than the naked eye. Powerful telescopes can see far into space, making far away objects look closer and brighter. Powerful microscopes make tiny microscopic objects look much bigger, so that our eyes can see them in more detail. Many important discoveries in biology, chemistry, astronomy and other sciences have been made using the power of optics.

We also use optics in some things we use around the house. If you or your parents wear glasses, your doctor used optics to find a prescription for a lens to help you see better. If you like to go bird watching, then you have used a pair of binoculars to see birds in far off distances. A magnifying glass can be used to read tiny print, look at insects or to do a small hobby or craft.

One important factor for any lens is the power of magnification, a measurement of how much bigger the lens will make an object appear. When you use a magnifying lens to look at an object you can compare the actual size of the object (the real size of what you are looking at) to the apparent size of the object (the size your object looks like it is when you look through the lens). The magnification power of a lens can change depending upon how close the lens is to the object you are looking at, something called optical distance.

In this experiment, you can use a magnifying glass to make your own apparatus to discover the relationship between the optical distance and magnification of a lens.

Terms and Concepts

To do this project, you should do research that enables you to understand the following terms and concepts:
  • magnification
  • optics
  • lens
  • objective
  • optical distance
  • actual measurement
  • optical measurement


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Materials and Equipment

  • large magnifying glass with a sturdy handle
  • ruler
  • permanent marker
  • scotch tape
  • wax paper
  • drinking straw
  • string or twisty tie
  • chunk of clay
  • small objects to measure (penny, typed or printed letter, bean, rice grain, etc.)

Experimental Procedure

  1. Pull out a piece of scotch tape and place it on a non-stick surface, like a piece of wax paper.
  2. Using your ruler and permanent marker, mark points along the tape indicating centimeters and millimeters.
  3. Using scissors cut the length of the tape to fit inside the diameter of your magnifying lens.
  4. Stick the tape to the diameter of the lens. You will use this as a guide to make your optical measurements.
    Diagram of a strip of tape placed on the lens of a magnifying glass
  5. Now fold a drinking straw in half around the ruler. Tightly tie a piece of string around the ends of the straw so that it is tied around the ruler. The straw should slide up and down along the ruler without slipping.
    Drawing of a straw wrapped around a ruler
  6. Now tape the handle of your magnifying glass onto the ends of the straw, so that the magnifying glass is perpendicular to the ruler. Jab the end of the ruler into a chunk of clay and secure the clay to the table so that your apparatus will not slip or topple over. Make sure that the end of the ruler is touching the surface of the table. This will ensure that your measurements of the height of the lens will be accurate.
    Diagram of a ruler held vertically by clay and a magnifying glass attached horizontally to the ruler with straw and string
  7. Now you are ready to measure your objects with your magnifying lens apparatus. First collect items to measure and organize them into a data table. You will need a data table to keep track of your measurements:
    Object Actual Size
    Height of Lens
    Optical Size
  8. Now take one of your objects and measure the actual size of the object along the length (if it is a long object) or diameter (if it is a round object like a penny). Write down your measurement in a data table.
  9. Place the object on the table beneath your apparatus.
  10. Adjust your magnifying glass lens to a fixed height above the table using your ruler as a guide by moving the adjustable straw. Write down the height of the lens in your data table.
  11. Now look through the lens of the magnifying glass. Using your taped on ruler as a guide, indicate the size of the object as it appears in the lens. This measurement may or may not match the actual size of the object. Write the optical size measurement in the data table.
  12. Continue to change the height of your magnifying lens, and taking optical measurements of your objects. Remember to write both the height of the lens and measurement of the object in your data table each time.
  13. After collecting your data, it will be helpful to make a graph to help you interpret your data and think about the relationship between the height of the objective and the optical size of each object.

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  • Magnification is the ratio between the apparent size of an object and its true size. From your graph, you can actually calculate the magnification of your lens by using a formula. A formula is a description of how to use math to calculate something from data you have collected. Here is a simplified formula to calculate the magnification of a lens. Use it to calculate the magnification of the lens in your experiment.
    Apparent Size
    Actual Size
  • Different types of lenses bend light in different ways, which make a big difference to how your eyes will see objects through a lens. Some shapes of lenses will make objects smaller rather than larger, or will cause the object to appear upside down. There are many different shaped lenses, which fall into two main categories: concave or convex. Experiment with different shapes and combinations of lenses to see how they change how an object will appear.

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