Essential Question: How does the lens in your eye focus light?
Ask the question: How many of you have used lenses to see today? Explain that if they have used their eyes, then they have used lenses because we all have a built-in lens in our eyeball. Now ask, "What is a lens?" (An object that bends or refracts light.) Allow the students a few minutes to discuss this question with a partner. Explain that lenses are found in many devices we use with our eyes. Depending on a lens's shape, it can bend light rays to a point and bring them into focus, or it can spread light rays far apart.
Hold up a magnifying glass. Explain that a magnifier is similar to the lens of your eye, except the magnifier has a hard, rigid lens, while the lens of your eye is soft and flexible. Both lenses, however, are convex, which means that the sides are curved outward. Convex lenses bend light toward a point to focus.
Darken the room and shine a flashlight toward a wall so that it makes a spot. Invite the class to watch the spot as you place the magnifier in front of the flashlight. Slowly move the magnifier back and forth. The spot of light should get brighter and dimmer. Explain that as you move the magnifier, you are changing the focal point of the light. The spot will be brightest when the beam is in focus. The light is focused onto the back of your eye on a layer of light-sensitive cells called the retina. The retina then gathers the information and sends it to the brain to be processed into an image.
Turn the lights back on. Ask students to hold one hand in front of their faces so that they are staring at their palm. Now tell them to quickly look up to the other side of the room. Explain that as they change their view from near to far, their eyes adjust its focus. Ask, "How do you think the lens does this?" Allow a few minutes for the students to collaborate with a partner or group to answer this question.
Students will complete the Eye Spy lab sheet activity with a group of 3-4 students. Each group will need a mirror and water-filled zipper bag.
Have each student complete the following journal entry: Your eye has a flexible lens, much like the bag of water. Based on your experiments, explain how you think the lens in your eye adjusts to change the focus from nearby objects to far away objects.