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. 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 using pin and mirror. Another similar question below. The answer is Option B

The video below shows a typical lens experiment. (reference to O-Level SciPhy 2015). I will briefly go through the set-up, main steps and how to get the 1st set of readings. In the next video, it highlights the various types of lens practical which you might have in the school lab. e.g. different kind of crossed-wire, a beaker of water as a converging lens and different kind of images formed. Key points : 1) Make sure the object (illuminated crossed-wire), lens and screen are aligned properly. 2) Source of Error: identifying the sharpest image Improvement: (i) Repeat the experiment a few times for the same independent variable to identify the sharpest image. (ii) Move the lens (or screen) forward and backward about the sharp image, until the sharpest image is determined. 3) In general, the focal lens of the lens used in the lab is usually 10 cm or 15 cm . Most lens experiment requires you to find the focal length. There is also a easy way to quickly determine the focal length before starting the experiment.

Vernier caliper and micrometer screw gauge are used to measure length. Vernier caliper is used in general for measuring length between  0 to 15 cm . Micrometer screw gauge is used in general for measuring length between  0 to 2.5 mm . The accuracy (precision) of – ruler (0.1 cm) – vernier caliper (0.01 cm) – micrometer screw gauge (0.01 mm)

Answer: Option B

Answer : Option A

Answer: Option A

Answer: Option B

Answer: Option B

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. Example A Example B Example C Example D Example E Example F Example G Example H Example I Example J

Keep in mind that you can utilize the two main concepts of a speed-time graph. As object is under free-fall, the acceleration (the gradient of the graph) is 10 m/s2. The area undereath the speed-time graph is the distance. So the area under the graph from P to Q is 120 m. With this two equation, you can solve all unknowns. Solutions: Option A

Which of the following light rays behaves correctly when it passes through the converging lens? Solutions: Option C Recall these basic concepts on the 3 rays