An object 20cm high is placed 45cm from the lens of focal length 15cm? calculate the image distance and the size of the image and the nature of the image? If a concvex lens and a concave lens is used.

1 Answer
Anjali G
May 13, 2018

Image distance #d_i = 22.5 " cm"#

The image size is #h_i = -10 " cm"#, meaning that the image is inverted.

Explanation:

Given:
#h_o = 20 " cm"#

#d_o = 45 " cm"#

#f = 15 " cm"#

To calculate image distance, use the thin lens equation:

#(1/d_o) + (1/d_i) = 1/f#

#(1/45) + (1/d_i) = 1/15#

#1/(d_i) = 2/45#

#d_i = 22.5 " cm"#

To calculate image size, use the magnification equation:

#m = frac{h_i}{h_o} = frac{-d_i}{d_o}#

#frac{h_i}{20} = frac{-22.5}{45}

#h_i = (20)*(-1/2)#

#h_i = -10 " cm"#

If a convex lens is used, I think the image would be real. However, if a concave lens were used, I think the image would be virtual because it is a diverging lens.

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