When an object is oscillating or vibrating in a wave motion , the speed of the object varies along the path. In this video, there are 3 examples of vibrating object. 1) Mass vibrating vertically from a spring 2) Pendulum bob oscillating 3) A particle vibrating up and down on a transverse wave In general, when the object is at the extreme ends of the oscillation or vibration , it is momentarily at rest . Hence its KE at these points is minimum or 0 J . And in the middle that is where the object is travelling the fastest , hence the KE is the maximum .

1. Converging lens (convex lens) Converging lens, also known as convex lens,  is thicker at the centre. Below shows some examples. In O-level, we learned about symmetrical converging lens. i.e. the curvature of the lens are the same on both sides. As light rays pass through the converging lens, the rays come closer together . Take note that the bending of light, refraction, takes place on the air-glass boundaries on both sides of the lens (as shown above). But for easy drawing, we draw the bending at the imaginary centre vertical which passes through the optical centre as shown below. If the parallel rays are at an angle, the light rays will converge on a point P which lies on the focal plane. 2. The 3 Rays The following 3 rays are important for us to construct the ray diagram and locate the image. We always draw these 3 rays as they have rules to follow, hence guiding us in our drawing. Refer to the video below for better understanding of the 3 rays. 3. The 4 Key Scenarios Depending on the distance of the object to the centre of the lens (object distance u), the kind of image you get varies. Refer to the video below for the better understanding of how the various images are formed. 3. The Pattern Besides knowing the 4 key scenarios, it is important to know how the image behaves as the object is moved towards the lens. In general, as the object (starting from a distance of >2f) moves closer to the lens, the image will move further away from the lens and the size of the image becomes bigger. But when the object is within a focal length, as it moves closer to the lens, the virtual image moves closer to the lens and it becomes smaller compared to the image previously. But the virtual image is always bigger than the object. Refer to the video for better visualisation and understanding.

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

A pin in the beaker of water appears higher than its actual position due to refraction of light. In this experiment, you have to adjust the pin at the cork to a suitable height, such that the pin and the image of the pin inside the beaker of water is aligned and that ‘pins at position of no parallax’ position. The video below guides you in adjusting the height of the pins to locate correct height.

An electric cable contains three wires live, neutral and earth. The cable is correctly wired to a plug which contains a 3A fuse. The insulation becomes damaged and bare metal wires show. Five possible events can occur. A person touches the earth wire. A person touches the neutral wire. A person touches the live wire. The live wire touches the neutral wire. The live wire touches the earth wire. How many of these five events cause the fuse in the plug to blow? A   1          B   2         C   3         D   4 Solutions: Option B Consider the five events: A person touches the earth wire –  As the person is at 0V, same as the earth wire, there will be no current flowing through the person. So current through the circuit will not be affected, which is lower than the 3A fuse rating. Fuse will not blow. A person touches the neutral wire.-   As the person is at 0V, same as the neutral wire, there will be no current flowing through the person. So current through the circuit will not be affected, which is lower than the 3A fuse rating. Fuse will not blow. A person touches the live wire. – The live wire is at high potential of 240 V. The person will get an electric shock. But a common misconception is that if a person gets an electric shock, the current flowing through him is very large, which is wrong. In fact, the current is very small, much smaller than the fuse rating. Assuming the average body resistance of the person is 100 000 ohms, and the potential difference in Singapore is 240 V, since I = V/R = 240/100 000 = 0.0024 A, which is lower than 3A fuse rating. Hence the fuse will not blow. The live wire touches the neutral wire. – This will create a short circuit as a large current which exceeds the fuse rating will from the live (240 V) to the neutral wire (0V) as that path has very low resistance. The fuse will blow. The live wire touches the earth wire. –  This will create a short circuit as a large current which exceeds the fuse rating will from the live (240 V) to the earth wire (0V) as that path has very low resistance. The fuse will blow.

Answer: Option D