Lever (moment) System and Hydraulic System

Why the fuse and switch must be on the live wire?

A common mistake is to state that the reason for switch to be on the live wire is “so that the switch can turn on/off the appliance”. This explanation is wrong.

In general, in the event where the fuse is blown or the switch is open, it disconnect the appliance from the high potential of the live wire. So the appliance will no become ‘live’ and it is save to touch even though there is a fault.

Refer to the video for the reason why switch and fuse must be on the live wire.

Refer to the notes given

Converging Lens Overview

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 PatternBesides 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.