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Wonders of Physics

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Different Lens Ray Diagram questions

Drawing ray diagrams for converging lens come in many forms. But the basic concepts needed are the 3 rays (which have rules to follow) to locate the image. The following ray diagrams for the 4 scenarios must be learned well, together with the respective image characteristics and applications.



With the basic concepts learned, when questions are asked in different ways, you should be able to draw the ray diagrams. Refer to some different ray diagram questions below and their video tutorials.














Lens J from evantoh on Vimeo.

Click the followings for other lens ray diagram questions. https://evantoh23.wordpress.com/2010/11/09/20101109converging-lens-important-concepts/


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OLevel SP P1 Q12 2010 focal length of lens using distant object

Which distance is equal to the focal length of a lens?

(A) the distance between a distant object and the 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

Solutions: Option C

Focal length f is the distance between the focal point and the centre of the lens (optical centre).

Note that only when parallel rays of light enter a converging lens, the rays will converge to a point. That point is considered to be focal point F (principal focus). The distance between focal point F and the optical centre is the focal length f. Refer to below.

As none of the options is similar to the above definition, you have to consider that the rays from a distant (far away) object are considered parallel. Hence the sharp image formed on the screen is considered the forcal point F of the lens and the distance between the image and the optical centre is the focal length f.

Distant object

If it is a close object, the rays entering are not considered parallel. Hence even if the rays converged to a point, that point is NOT focal point F and the distance between this converged point and the optical centre is NOT focal length f.

close object


Click the following posts for other lens concepts:



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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.