Rules For Image Formation In Lenses


 
 
Concept Explanation
 

Rules For Image Formation In Lenses

Only two rays from the same point of an object are needed for image formation and the point where the rays after refraction through the lens intersect or appear to intersect is the image of the object. If they actually intersect each other the image is real and if they appear to intersect the image is said to be virtual. In order to locate the image formed by a lens graphically following rules are adopted

Rays used for ray diagram of Image formation:

1.  We choose the ray that is incident parallel to the principal axis. In case of a convex lens, the transmitted ray will pass through the focus, and in case of a concave lens, the transmitted ray’s backward projection will pass through the focus.

2.  We choose the ray that passes through the optical centre of the lens. This ray is transmitted without any deviation.

3. A ray passing through first focus or directed towards it, after refraction from the lens becomes parallel to the principal axis (by definition of F).

The point of intersection of any of two of these transmitted rays is the image of the point object.

In the case of lens refraction occurs at both the surfaces, Once at the surface when the ray enters the lens and then at the surface when the ray comes out of the lens. While tracing the rays, we do not show the ray bending at each surface of the lens. Instead, we show the incident rays getting bent at the central, vertical line of the lens because we are using a thin lens.

Sample Questions
(More Questions for each concept available in Login)
Question : 1

Only _____________rays from the same point of an object are needed for image formation

Right Option : B
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Explanation
Question : 2

While finding the position of an image in case of a lens we can use a ray passing through first focus or directed towards it, after refraction from the lens it becomes parallel to the _____________________ .

Right Option : B
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Explanation
Question : 3

Which of the following are correct ?

(a) While identifying the position of the image formation in case of lens we can choose the ray that passes through the optical centre of the lens. 

(b) In the case of lens refraction occurs at both the surfaces once when the ray enters the lens and then at the surface when the ray comes out of the lens.

Right Option : C
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Explanation
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