A plano convex lens of refractive index 1.5 and radius of curvature 30...
KEY CONCEPT : The focal length(F) of the final mirror
The combination acts as a converging mirror. For the object to be of the same size of mirror, u = 2F = 20 cm
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A plano convex lens of refractive index 1.5 and radius of curvature 30...
To determine the distance at which an object must be placed in order to form a real image of the same size using a plano-convex lens that has been silvered at the curved surface, we can use the lens formula and magnification formula.
Given data:
Refractive index (n) = 1.5
Radius of curvature (R) = 30 cm
1. Lens formula:
The lens formula relates the object distance (u), image distance (v), and focal length (f) of a lens. It is given by:
1/f = 1/v - 1/u
In this case, since the lens is plano-convex, the focal length (f) can be calculated using the following formula:
f = R/2
Substituting the given values, we find:
f = 30/2 = 15 cm
2. Magnification formula:
The magnification (m) of a lens is given by the ratio of the height of the image (h') to the height of the object (h). It is given by:
m = -v/u
In this case, since we want the image to be of the same size as the object, the magnification will be 1. Therefore:
m = 1
3. Calculating the object distance:
Using the magnification formula, we can write:
m = -v/u
Since m = 1, we have:
1 = -v/u
Simplifying, we find:
u = -v
This means the object distance (u) is equal to the negative of the image distance (v).
4. Applying the lens formula:
Substituting the value of f and the relationship between u and v into the lens formula, we have:
1/f = 1/v - 1/u
1/15 = 1/v - 1/-v
Simplifying, we find:
1/15 = (2v + v)/(v^2)
15v^2 = 3v + v^2
14v^2 - 3v = 0
Solving this quadratic equation, we find two possible values for v: v = 0 or v = 3/14 cm.
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5. Calculating the object distance:
Since u = -v, we find:
u = -(-3/14) = 3/14 cm
Converting to centimeters, we have:
u = 3/14 * 100 = 21.43 cm
Therefore, the object must be placed at a distance of approximately 21.43 cm from the lens in order to form a real image of the same size.
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