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Wonders of Physics

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Where is the focal point?


Answer: Option A

Refer to the diagram (below left) which many are familiar. When parallel rays of light which are parallel to the principal axis enter the lens, the rays bend (refraction), come closer and converge to a point on the principal axis called focal point (F). The distance from the optical centre (C) to the focal point (F) is the focal length (f).

But what if the parallel rays of light entering the lens are not parallel to the principal axis but at an angle as shown on the diagram (below right)?

As you can see, the rays refracted and converge to a point P which is along the focal plane (imaginary vertical line through F and is perpendicular to the principal axis). This is similar to L1 in the question. (Refer to the first section of the video simulation below to reinforce your concept)

focal plane

How about L2 in the question?

Light is reversible so you can also treat the light rays entering from the right of the lens L2. The parallel rays of light in L2 are at an angle but there is no ray through the optical centre C.

Refer to the video below, as you can see, the parallel rays of light will likewise refract and converge to a point, which is along the focal plane too.

Hence the focal point of both lenses L1 and L2 is at F2. So the answer is Option A.


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Converging Lens – Important concepts

Which of the following light rays behaves correctly when it passes through the converging lens?


Solutions: Option C

Refer to the following diagrams and recall the basic.


From the diagram above, only when parallel beam of light passes through the converging lens and converge to a point, that point is known as focal point, F. Only then,  the distance between F and optical centre, C, is known as the focal length, f.From the diagram above, note that when light ray from object is parallel to principal axis, it will pass through F after passing the converging lens.When light ray passes optical centre, it will pass straight. When light ray passes F and then the converging lens, it will be parallel to the principal axis.


A is wrong as it should be parallel to principal axis after passing through the lens.D is wrong as if after the lens the ray is parallel to principal axis, it must pass through F initially before entering the lens.


The 3 rays shown in the previous diagram are not the only rays from object. In fact, light shines on any point on the object, light is reflected in all direction. We usually concentrate on the 3 rays from drawing of image and its properties (as mentioned above).

For the other rays likewise, all reflected light from that particular point on object will be converged to the same particular point on the image.Hence, B is wrong as did not pass through F before hitting the lens, the light will definitely not pass through F on the other side.Only C is correct.Another important concept, when a parallel beam of light strikes the lens, the light rays will converge to a point which falls along the focal place. Refer to diagram below.

Click the following posts for other lens concepts: