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.
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.
4) Other posts on converting lens:
What is focal length and how to identify
Finding focal length f of the lens (using a mirror and pin practical experiment)
Different converging lens ray diagram questions (must know)
Different ways to have a sharp image formed on the screen
Which distance is the focal lens of the converging lens? Olevel question
Evan's Space 