Investigation 22.3: Images from Lenses Name:

________________________Purpose: to learn how lenses form images

Refracting Light Through a Lens

Like a prism, a converging lens bends light. Because the shape of a lens is curved, rays of light hit different

places along the lens, and bend different amounts.

Rays that approach a lens parallel to the axis meet at a point called the focal point.

The distance between the center of the lens and the focal point is called the focal length.

1. Click the “geometric optics” icon on your desktop or

http://phet.colorado.edu/en/simulation/geometric-optics

2. Select “marginal rays.”

3. Adjust the objects and draw the rays and the image for each of the following setups.

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A. Curvature radius: 0.37 Refractive index: 1.44

Diameter: 0.49

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B. Curvature radius: 0.37 Refractive index: 1.44

Diameter: 0.49

Background

A lens has an index of refraction different than air, so it bends (or refracts) light differently than air does, so that it can form images.

An image is a picture of an object formed in space where light rays meet. The place where the light rays meet is called the focal point.

The focal length is the distance between the center of the lens to the focal point.

The exact shape of a lens depends on how much and in what way the lens needs to bend light.

Analysis

1. What is the focal point of a lens? ___________________________________________________

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2. Identify the focal point on your drawings by drawing a star where the focal point is.

3. Why did the path of the ray change after passing through the lens? Use the phrase “index of

refraction” in your answer.

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4. Why does the diagram look so different when we click ‘2nd point?”

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C. Curvature radius: 0.37 Refractive index: 1.78 Diameter: 0.49 Click “2nd point”

Near/Far-sightedness and glasses

Approximately 75% of Americans use some sort of corrective lenses to help them see better. When a person has difficulty seeing, it is primarily due to the fact that the image their eye is making is not focused on the retina (the back of the eye). The image needs to be refocused on the retina in order for a person to see clearly.

People who are nearsighted can see things that are near. They have an elongated (longer) eye shape that causes the image that their eye creates to have a focal point too far in front of the retina.With lenses, the image’s focal point can be adjusted so that it is ON the retina instead.

Divergent lenses cause the light rays to spread out, causing the focal length to increase. An increase in focal length helps a nearsighted person see because their eye shape is longer than normal.

On the other hand, people who are farsighted can see things that are far. They have a shortened eye shape – so they have the opposite problem of what nearsighted people have.

1. What is the main reason that people have difficulty seeing, and need need glasses or contacts?

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2. Why are focal points and focal lengths important in understanding how corrective lenses work?

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3. Explain how convex/convergent lenses help farsighted people see more clearly. Use the terms

focal point, refraction, light rays, focal length, retina in your answer.

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