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Different Rays –
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(i) A ray of light from the object, parallel to the principal axis, after refraction from a convex lens, passes through the principal focus on the other side of the lens.
- In case of a concave lens, the ray appears to diverge from the principal focus located on the same side of the lens.
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- (ii) A ray of light passing through a principal focus, after refraction from a convex lens, will emerge parallel to the principal axis.
- A ray of light appearing to meet at the principal focus of a concave lens, after refraction, will emerge parallel to the principal axis.
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- (iii) A ray of light passing through the optical centre of a lens will emerge without any deviation.
- The ray diagrams for the image formation in a convex lens for a few positions of the object.
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Spherical Lens Image Formation and Optical Variations:
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Details
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- Lenses form images by refracting light.
- The nature, position and relative size of the image formed by a convex and concave lens for various positions of the object is summarized in the table.
 [Nature, position and relative size of the image formed by a convex lens for various]
[The position, size and the nature of the image formed by a convex lens for various positions of the object]
- In conclusion it can be said that a concave lens will always give a virtual, erect and diminished image, irrespective of the position of the object.
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Spherical Lens Sign Convention: Optical Measurements and Focal Insights
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- For lenses, the sign convention is similar to that used for mirrors.
- All the rules for signs of distances are applied except that all measurements are taken from the optical centre of the lens.
- According to the convention, the focal length of a convex lens is positive and that of a concave lens is negative.
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Spherical Lens Formula: Understanding Relationships in Optics:
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- This formula gives the relationship between object distance (u), image-distance (v) and the focal length (f).
- The lens formula is expressed as 1/v – 1/u = 1/f
- The lens formula given above is general and is valid in all situations for any spherical lens.
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Spherical Lens Magnification: Optical Enlargement and Relationships
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- The magnification produced by a lens, similar to that for spherical mirrors, is defined as the ratio of the height of the image and the height of the object.
- Magnification is represented by the letter m.
- If h is the height of the object and h′ is the height of the image given by a lens, then the magnification produced by the lens is given by,
- Magnification produced by a lens is also related to the object-distance u, and the image-distance v.
- This relationship is given by: Magnification (m) = h′/h = v/u
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Spherical Lens Power Dynamics: Optical Convergence and Divergence
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- The ability of a lens to converge or diverge light rays depends on its focal length.
- Example: A convex lens of short focal length bends the light rays through large angles, by focussing them closer to the optical centre.
- Similarly, concave lenses of very short focal length cause higher divergence than the one with longer focal length.
- The degree of convergence or divergence of light rays achieved by a lens is expressed in terms of its power.
- The power of a lens is defined as the reciprocal of its focal length.
- It is represented by the letter P.
- The power P of a lens of focal length f is given by:
- The SI unit of power of a lens is ‘dioptre’.
- It is denoted by the letter D.
- If ‘f’ is expressed in metres, then, the power is expressed in dioptres.
- Thus, 1 dioptre is the power of a lens whose focal length is 1 metre.
1D = 1m-1.
- The power of a convex lens is positive and that of a concave lens is negative.
- Opticians prescribe corrective lenses indicating their powers.
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