Focal Length of Lenses
Date Lab Conducted: June 19, 2012
Date Due: June 25, 2012
Date Submitted: June 25, 2012
1. To demonstrate that converging lenses form real images while diverging lenses form virtual images. 2. To determine the equivalent focal length of two joined lenses using their individual focal lengths. 3. To measure the focal length of diverging lens by combining it with a converging lens and forming a real image.
List of Apparatus:
- Optical bench containing clamps for lenses and a screen
- White screen upon which to project images
- Lap with a geometric object painted upon its face to serve as an illuminate object - Three lenses: A, B, and C with focal lengths of approximately +30cm, +10cm, and -30cm - Meter stick
- Masking tape
Theory: Lenses are transparent devices made of glass or plastic and have at least one curved surface. They work by refracting, or bending, light rays passing through them. Parallel waves are caused to either converge or diverge. Convex lenses have surfaces which bulge outwards in the center. This shape converges light by bending the light rays inward and causing them to intersect at a point just beyond the lens known as the focal point. The distance from the center of the lens to the focal point is the focal length and is a positive number for converging lenses. Concave lens have an outer surfaces curving inward forming a shallow cavity. This shape diverges light by bending the light rays outward. By tracing these light rays backwards and treating them as linear, unbent waves, a focal point can be found in front of the lens. The distance from this point to the center of the lens is the focal length and it is a negative number for diverging lenses. Lenses form two types of images – real and virtual. Real images can be focused on a screen and result from light waves passing through the points in space in which the image is formed. Virtual images cannot be focused on a screen and the light does not actually pass through the points at which the image is formed. Converging lenses can form both real and virtual images, but diverging lenses can only form virtual ones. Virtual images formed by one lens can be used by another to form a real image. The distance between and object and a lens is called the object distance s. The distance between the lens and the projected and focused image is distance s’. The focal length of the lens is f and these three elements are related by the equation: 1s+ 1s' = 1f
When two lenses are joined together, the effective focal length, feff, is determined by the equation: 1f1+ 1f2 = 1feff
1. The optical bench was set up with the lamp at the edge of the table facing parallel to the long edge of the table. The meter bar was set up in front of it, also parallel to the long edge of the table. A lens holder was clamped approximately 20 cm in front of the lamp and the white screen approximately 20 cm beyond the lens holder. 2. The focal lengths of two convex lenses (A and B) were measured using the very distant object method with a lamp projecting light from about 3 m away. This light source stood in for light projected from the sun, or light from an infinite distance, whose light rays were parallel. 3. The lens with the shortest focal length, B, was placed into the lens holder. The lens holder was positioned at object distances 20 cm, 30 cm, 40 cm, and 50 cm and the value of s’ was measured for images clearly focused on the screen. 4. Both lenses A and B were taped together around the edges and the very distant object method was used to measure its focal length. It was then placed in the lens holder at 0.50 m in front of the lens and its s and s’ values recorded. 5. Lenses B and C were taped together along the edges and the very distant object method was used to measure its focal length. It was then placed in the lens holder at 0.50 m in front of the lens...
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