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Question
How to draw ray diagrams of a convex lens?
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Solution
When an object is placed in front of a lens, light rays coming from the object fall on the lens and get refracted. The refracted rays produce an image at a point where they intersect or appear to intersect each other. The formation of images by lenses is usually shown by a ray diagram. To construct a ray diagram we need at least two rays whose path after refraction through the lens is known. Any two of the following rays are usually considered for constructing ray diagrams.
1. A ray of light passing through the Optical Center of the lens travels straight without suffering any deviation. This holds good only in the case of a thin lens.
2. An incident ray parallel to the principal axis after refraction passes through the focus.
3. An incident ray passing through the focus of a lens emerge parallel to the principal axis after refraction.
When the object is at infinity, the rays coming from it are parallel to each other. Let one of the parallel rays pass through the focus F1 and the other ray pass through the optical center. The ray which passes through F1 becomes parallel to the principal axis after refraction and the ray which passes through the optical center does not suffer any deviation.
The table below given at a glance the position, size and nature of the image formed by a convex lens corresponding to the different positions of the object and also its application.
Position of thePosition of theNature of theSize of theApplicationobjectimageimageimageBetween O and F1on the same sideErect and virtualMagnifiedMagnifying lens (simple microscope),of the lenseye piece of many instrumentsAt 2F1At 2F1Inverted and realSame sizePhotocopying cameraBetween F and 2F1Beyond 2F2Inverted and realMagnifiedProjects, objectivesmicroscopeAt F1At infinitInverted and realMagnifiedThreatre spot lightsBeyond 2F1Between F2 and 2F2Inverted and realDiminishedPhotocopying (reduction camera)At infinityAt F2Inverted and realDiminished
At infinity i.e. u=∞At focus i.e, v=fm<1 diminishedVirtualErectAnywhere betweenBetween opticalm<1VirtualErectinfinity and optical centrecentre and focusdiminished
1. A ray of light passing through the Optical Center of the lens travels straight without suffering any deviation. This holds good only in the case of a thin lens.
2. An incident ray parallel to the principal axis after refraction passes through the focus.
3. An incident ray passing through the focus of a lens emerge parallel to the principal axis after refraction.
When the object is at infinity, the rays coming from it are parallel to each other. Let one of the parallel rays pass through the focus F1 and the other ray pass through the optical center. The ray which passes through F1 becomes parallel to the principal axis after refraction and the ray which passes through the optical center does not suffer any deviation.
The table below given at a glance the position, size and nature of the image formed by a convex lens corresponding to the different positions of the object and also its application.
Position of thePosition of theNature of theSize of theApplicationobjectimageimageimageBetween O and F1on the same sideErect and virtualMagnifiedMagnifying lens (simple microscope),of the lenseye piece of many instrumentsAt 2F1At 2F1Inverted and realSame sizePhotocopying cameraBetween F and 2F1Beyond 2F2Inverted and realMagnifiedProjects, objectivesmicroscopeAt F1At infinitInverted and realMagnifiedThreatre spot lightsBeyond 2F1Between F2 and 2F2Inverted and realDiminishedPhotocopying (reduction camera)At infinityAt F2Inverted and realDiminished
At infinity i.e. u=∞At focus i.e, v=fm<1 diminishedVirtualErectAnywhere betweenBetween opticalm<1VirtualErectinfinity and optical centrecentre and focusdiminished
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