Focal Length of Lenses 1. Focal Length of Lenses. Objective: To measure the
focal length of convex (positive or convergent) lenses and concave (negative.
Focal Length of Lenses Objective: To measure the focal length of convex (positive or convergent) lenses and concave (negative or divergent) lenses. Theory: A lens is a transparent object with two refracting surfaces whose central axes coincide. Diverging lenses are concave lenses where the rays diverge. Converging lenses are convex lenses that converge to the central axis. For lenses the real image forms on the opposite side of the lens whilst the virtual image is on the same side as the object. Diverging lenses do not form real images. If we have an object distance of d, and an image distance of i, then the focal length, f is related by the formula below.
d
a f
f b i
1 1 1 For the evaluation of the focal length of the convex concave compound lens, 1 1 1 1 1 1 Focal Length of Lenses 1
Ray Diagrams
Converging lens with object behind focus
Converging lens with object in front of focus
Converging lens with object at focus
No Image
Diverging lens with object behind focus
For a thin lens or mirror, the magnification of the image may be calculated by the ratio image distance to object distance. Magnification is also defined as the ratio of image height to object objec height.
If more than one lens is involved in the system then it is the product of the magnifications of the lenses that become the total magnification.
Apparatus: Optical bench, lens holders, screen, illuminate illuminated d object (lamp), convex lenses, and concave lenses
Focal Length of Lenses 2
Procedure: Convex Lens 1. 2. 3. 4. 5. 6. 7. 8.
Place the lamp and screen on the optical bench about 40 cm apart. Place the convex lens in between the lamp and the screen. Adjust the lens position until a sharp image is formed on the screen. Record object distance d, image distance I, object height ho, and image height hi, (size of the inverted arrow on the screen). Find the two positions of the lens for which sharp images are produced. Use the lens equation to calculate focal length f. Calculate m from hi/ho and from i/d. Calculate the percent error and use the hi/ho for the actual value. Repeat the procedure for another distance between the lamp and the screen. Concave Lens
1. 2. 3. 4. 5.
Place the lamp and screen on the optical bench about 40 cm apart. Place the convex lens in between the lamp and the screen. Place the concave lens between the convex lens and the screen Adjust the lens position until a sharp image is formed on the screen. Record object distance d2, image distance i2, object height ho, and image height hi, (size of the inverted arrow on the screen). 6. Calculate m from hi/ho and from i/d. Calculate the percent error and use the hi/ho for the actual value.
