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Wonders of Physics

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Lens – Ray Diagram

lens image

Solutions: Option C


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N2010P1Q23 Lens


Answer is A.

Many thought it is a diverging lens due to the ray which seems to diverge out. If you refer to the sketch esp the highlighted ray in pink, you will realise that it is converging lens.

Likewise you will be able to see that object is less than focal length in order to have such ray.

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PP N2002P1Q19: Lens – to form a sharp image on screen

A lens forms a blurred image of an object on a screen.lens 2

How can the image be made sharp and in focus on the screen?

A)    By moving the object away from the lens and screen.
B)    By moving the screen away from the lens and object.
C)    By using a brighter object at the same position.
D)    By using a lens of longer focal length at the same position.

Solutions: Option D (refer to the video on why the image is blurred and how to form a sharp image)

A and B will create a more blurred image.
C – brighter object will not create a shape image.
D – using a lens of longer focal length, when light rays pass through the lens, it will converge to a point further away from the lens. Hence the sharp image will be formed on the school.

Alternatively, you can move the screen nearer to the lens such that the position of the screen is at the intersection of the rays.

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N2010 P1 MCQ Q12 Sci Physics 2010 O-Level

Which distance is equal to the focal length of a lens?
A. The distance between a distant object and its image.
B. The distance between the image of a close object and the centre of the lens.
C. The distance between the image of a distant object and the centre of the lens.
D. The distance between two principal foci.
Solution: Option C.

Recall that only reflected rays from distant object (object placed far away) are considered parallel. When these parallel light rays pass through the converging lens, they converge to a point, Focal Point (F).
Only then, the distance between the (sharp) image of distant object to the centre of the lens, optical centre (C), is known as focal length (f).

Refer to the diagrams below.


Also refer to another question on lens: http://evantoh.posterous.com/2010/11/09/converging-lens-important-concepts

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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: