Rule1: A ray of light which is parallel to the principal axis of a concave lens,appears to coming from its focus after refraction through the lens.
Rule2: A ray of light passing through the optical centre of a concave lens goes straight after passing through the lens.
Rule3: A ray of light going towards the focus of a concave lens,becomes parallel to its principal axis after refraction through the lens.
Formation of image by a concave lens
Case1:When an object is placed anywhere between optical centre(C) and ∞ in front of a concave lens,the image formed is
- Between optical centre(C) and focus(F)
- Virtual and erect
- Diminished(smaller than the object)
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| Example of case1 |
Case2: When an object is at ∞ from a concave lens,the image formed is
- At focus(F)
- Virtual and erect
- Highly diminished (much smaller than the object)
Use of Concave Lenses
- Concave lenses are used in spectacles to correct the defect of vision myopia(or short sightedness)
- Concave lens is used as eye-lens in Galilean telescope.
- Concave lenses are used in combination with convex lenses to make high quality lens system for optical instrument.
- Concave lens is used in wide angle spyhole in doors.