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Wonders of Physics


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Converging Lens Overview

1. Converging lens (convex lens)
Converging lens, also known as convex lens,  is thicker at the centre. Below shows some examples.

types of converging lens

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.

refraction of light at the boundaries of the lens

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.

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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.

the 3 raysRefer 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.
IMG_1375

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

 

 

 

 

 

 

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When to use the concept PV = constant and P1V1 = P2V2 to solve?

When temperature is constant (for o-level), when a fixed mass of gas (fixed number of air molecules) is compressed in a closed system (e.g. piston), the volume V decreases and pressure P increases, and vice versa.

But when you multiply pressure and volume, PV, it is always a constant.

PV = constant

Hence we can always equate the PV of the first scenario = to the PV of the second scenario, provided there is no addition or removal of air molecules from the system.

Hence, you have P1V1 = P2V2

The followings are 4 different questions which require this concept to solve. Do revise them.

Solutions: Option D (refer to the worked solutions below)

Solutions: Option D

Solutions: A

4)

Solutions:

 

5) img_0196

Solution: Option C

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