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Wonders of Physics

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Finding focal length f of the lens

A thin convex lens is placed on a plane mirror and an object pin is then moved along the axis of the lens until an image is seen to coincide with the object pin when viewed from above. What is then the distance between the pin and the lens? (Take f as the focal length of the lens)


A     0.5 f            B     1.0 f          C      1.5 f          D      2.0 f

Answer: Option B

A common mistake is to assume this is the scenario (2nd scenario) where the object is at 2F and the image formed is at 2F, hence the image is the same size as the object, inverted and real. But this is not the case.

Refer to the video tutorial for the explanation.

This set up of the lens with the mirror is using the concept that when parallel light rays (parallel to principal axis) enter the lens, the rays will converge to a point after passing through the lens. This point is called the focal point, F. The distance between F and the optical center of lens is the focal length f. Refer to the diagram below.

Capture           IMG-0284a

When you adjust the object (pin) until both the object and the image coincide even when you move your eye forward or backward perpendicular to the axis, the distance between the optical center and the object (pin) is the focal length f.

At this position, as the rays from the object pass through the lens, due to refraction, the rays converge and becomes parallel to the axis. Due to the mirror, the parallel rays will be reflected back to the lens and then converge to a point that coincides with the object.

Refer to the video on how to get that position.

Before you start the experiment to find the focal length f, there is a fast and easy way to estimate the f. Refer to the video below.

Below is another image of another set-up but with the same concept.

focal length





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Identifying what lens, focal length and image from 2 rays – PP2010P1Q23 and SP2014P1Q11

These 2 questions are actually the same. Q23 is from 2010 Pure Physics P1 while Q11 is from 2014 Sci Physics P1. Take a look at these 2 questions. If you are not sure, view the video below for the explanation.

Capture1 PP 2010

Answer to Q23: Option A

Capture2 2014SP

Answer to Q11: Option D

If you do not know how to answer these 2 questions, view this video and also refer to the lens summary below.


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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: