1-A single bi-convex lens (a lens with two convexsurfaces) made of glass (index
ID: 1678083 • Letter: 1
Question
1-A single bi-convex lens (a lens with two convexsurfaces) made of glass (index of refraction n = 1.55) has surfaceswith radii of curvature r1 = 12.0 cm andr2 = -12.0 cm. What is the focal length of the lensin air?2-If an object is placed at p = 38.9 cm from the lens,where is the image? (Use plus sign for a real image, and minus signfor a virtual image.)
3-If the object has a height of h = 1.30 cm, how largeis the image? (Use plus sign for an upright image, and minus signfor an inverted image.)
Explanation / Answer
Given index of refraction n = 1.55
R1= 12 cm
R2 = -12 cm
We know from lens maker formula ( 1/ f) = ( n-1 ) [ ( 1/R1)– ( 1/ R2 ) ]
= 0.55 [ 0.1666 ]
= 0.09166
Focal length f = 10.9 cm
(b). Object distance u = 38.9 cm
From the relation ( 1/ v ) – ( 1/ u ) = ( 1/ f)
(1/ v ) = ( 1/ f) + ( 1/ u )
= 0.1173
Image distance v = 8.519cm
(3) height of object O = 1.3cm
Height of image I = ( v / u ) * O
= 0.284 cm
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