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An exploration of lenses that curve outward and how they can make objects look larger.
What if you could hold a piece of 'magic glass' that turns a tiny, fuzzy ant into a giant, detailed monster? This isn't magic—it's the power of the convex lens!
A convex lens is a piece of transparent material, like glass or plastic, that is curved outward. The easiest way to remember its shape is that it is thicker in the middle and thinner at the edges. If you ran your fingers across it, it would feel like a smooth, shallow hill. This specific shape is why some people call it a 'positive lens.' You can find these lenses in many everyday items, including your own eyes, cameras, and telescopes. Because they curve outward like a lentil bean, the word 'lens' actually comes from the Latin word for 'lentil'!
Quick Check
Where is a convex lens the thickest?
Answer
A convex lens is thickest in the middle.
When light rays travel through a convex lens, something amazing happens. Instead of going straight through, the light rays refract (or bend) inward toward the center. Because the lens is curved, all these bending rays are steered to meet at one specific spot on the other side. This spot is called the focal point. The distance from the center of the lens to this point is called the focal length, often written as the variable . If you've ever used a magnifying glass to create a tiny, bright dot of sunlight on the pavement, you were finding the focal point!
Imagine you are holding a magnifying glass under the sun. 1. You move the lens up and down above the ground. 2. As you move it, the circle of light gets smaller and brighter. 3. When the light is at its smallest, sharpest point, you have found the focal point. 4. The distance between the lens and that bright dot is the focal length .
Quick Check
What do we call the single spot where light rays meet after passing through a convex lens?
Answer
The focal point.
A magnifying glass is just a convex lens in a handle. When you hold it close to an object, like a leaf, it bends the light rays so that they enter your eye at a wider angle. Your brain is used to light traveling in straight lines, so it 'tricks' you into thinking the light is coming from a much larger object. This creates a virtual image. The closer the object is to the lens (within the focal length), the larger it will appear. However, if you move the lens too far away, the image might actually flip upside down!
Try this: Hold a magnifying glass at arm's length and look at a distant window. 1. Notice that the image of the window looks tiny and upside down. 2. This happens because the light rays have crossed past the focal point and started spreading out again. 3. When the object is farther than the focal length , the lens creates a 'real image' that is inverted (flipped).
Which of these objects most likely uses a convex lens?
What happens to light rays as they pass through a convex lens?
A convex lens is thinner in the middle than at the edges.
Review Tomorrow
Tomorrow morning, try to explain to a friend why a magnifying glass has to be 'curved' rather than flat to work.
Practice Activity
Water Drop Lens: Place a single drop of water on a piece of clear plastic wrap over a newspaper. Does the water drop make the letters look bigger? (Hint: The water drop is a natural convex lens!)