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Lab 2: Thin Lens Equation

Real Images and the Thin Lens Equation



The lab was designed to demonstrate the way lenses affect the behavior of light. A mathematical formula that relates object distance, image distance and focal distance was also derived.


Terms and Definitions

Real Image: Can exist even if no observer is present (object is farther than the focal length) Can be projected so that multiple people can see the image, even if the image disappears.
Virtual Image: Requires the visual system of an observer (object is closer to the lens than the focal length) Cannot be projected
Focal Distance: Distance between the lens and the point where the light rays converge
Object Distance: Distance from the objective lense to the physical object
Image Distance: Distance from the objective lense to the image
Magnification: A vector quantity used to determine the degree at which a visual object is magnified
Concave: A surface that curves in on itself like the interior of a circle
Convex: A surface that bulges outward like the exterior of a circle
Appearance: The way something looks (ie. shape)


Overview of Lab

Convex lenses were used to explore the properties of light and the projection of real images on screens in setups involving a screen, ambient light and lenses of differing focal lengths. The lens used to determine the relationship between object distance and image distance was a convex lens with a 10cm focal length.

The purpose was to observe the properties of real images, specifically magnification and orientation of images, projected on a screen.



When Image is Most Focused

Object Distance 50 cm
Image Distance 12.5 cm
Size Smaller
Shape Same
Orientation Upside down and flipped about the vertical axis
Analysis: The image projected is smaller and upside down than the original.

Object Distance vs. Image Distance

When Projected Image is Focused (cm)
Analysis: Object distance and image distance are inversely proportional

Equations and Calculations

The illumination decreases until the ambient light makes the light variance, due to distance, negligible.

1/d + 1/i = 1/f
(the focal distance was 10 cm in this lab) Where d is the object distance, i is the image distance and f is the focal distance


Analysis and Conclusion

We learned that object distance and image distance are inversely proportional. The light rays through the convex lens converge on a surface, creating a real image, even if the image produced is distorted from the original one. The equation allows us to calculate the focal distance, image distance or object distance when given two of the three values.


Lab Extensions

1. Use one of the converging lenses (in the plastic bin in the classroom) to make a macro lens for your phone camera. Try taking pictures. Fun!
The lens appeared to dramatically zoom the camera’s image. When positioned correctly, the macro lens caused the object to be clearly magnified many times its size. This lab extension revealed how cameras can zoom so closely into small objects. By adding a macro lens, we essentially added a magnifying glass onto the object. It helped us understand how a camera’s macro lens attachment worked.

2. Place a drop of water over your camera lens and it should be another way to make a macro lens. Neato! Due to the strong intermolecular forces of water (hydrogen bonding), the liquid has a high amount of surface tension (yay chemistry). Therefore, a drop of water forms a dome shape when on a surface. The dome, or convex shape, acts as a secondary lens that magnifies the image (kind of like a contact lens for your camera).



Our lab setup with the light source, lens, and screens to project the images on.

A closeup of the upside down, real image the lens produced. It is an upside down '4'.