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Measuring the focal length of the convex lens - Method 2: Theory: When the object distance is twice the focal length of the lens, the image distance is also twice the focal length. The transverse magnification is then -1. This means that the image is the same size as the object, but is inverted. Use Equation 1/di + 1/do = 1/f to show that if do = 2f then di = 2f also. Reduce the ray density so you can see what's going on. Using your approximate focal length from Method 1 as a guide, move the lens and the object so that the image is approximately 4f from the light source, with the lens halfway between them. Adjust the positions of both the object and the lens to meet three criteria simultaneously. The object distance must equal the image distance, the object height must equal the image height, and the image must be clear. Measure and record your values for object distance and image distance and use them to calculate the focal length of your lens. Object distance = Image distance = Calculated focal length = _ Refer to part a. 5. and draw an accurate ray diagram depicting how the image is formed in this case - you may need to place here.

          Measuring the focal length of the convex lens - Method 2:  
Theory: When the object distance is twice the focal length of the lens, the image distance is also twice the focal length. The transverse magnification is then -1. This means that the image is the same size as the object, but is inverted.  
Use Equation 1/di + 1/do = 1/f to show that if do = 2f then di = 2f also.  
Reduce the ray density so you can see what's going on. Using your approximate focal length from Method 1 as a guide, move the lens and the object so that the image is approximately 4f from the light source, with the lens halfway between them.  
Adjust the positions of both the object and the lens to meet three criteria simultaneously. The object distance must equal the image distance, the object height must equal the image height, and the image must be clear.  
Measure and record your values for object distance and image distance and use them to calculate the focal length of your lens.  
Object distance = __________________  
Image distance = __________________  
Calculated focal length = _________________  
Refer to part a. 5. and draw an accurate ray diagram depicting how the image is formed in this case - you may need to place here.

Added by John R.

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