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Magnifying Glass

posted Feb 2, 2015, 7:22 PM by   [ updated May 6, 2018, 1:59 PM by Upali Salpadoru ]

Fig.1. Terry is using a magnifying glass.

Terry’s uncle gave him a magnifying glass. He observed tiny bugs and pond-weeds and many other things. He observed the instrument more carefully and noted that the glass was not plane. It was curving upwards. 
Such surfaces are called 'convex surfaces'
So the lens of this instrument was a double convex glass.

When Ali and Nelly joined him they decided to carry out more experiments.

Passing sun light through the magnifier.

The rays can become so intense that some materials can catch fire; it may even burn your hand if held at the focus.

Fig.2  Sunlight passing through a magnifying glass.

They held the glass perpendicular to the rays of the sun and observed how they emerged. This they did by holding a white paper screen at varying distances. It was observed that the patch of light on the screen became smaller as it was moved away from the lens The brightness of the patch also changed. The lighted region seemed to be a cone Some result are shown in the  form of a graph below. You can move the screen further away from the lens and complete the graph. The reading will not be tha same for all magnifying glasses

Fig.3  Measured readings from the experiment no 1.

Experiment 2

Kindling a fire

As you may know, sunlight consists of a parallel beam of rays. When these have passed through the magnifying glass they bend to form a converging beam. The point at which the rays meet is called the focus of the lens. The distance from the centre of the lens to the focus is called the focal length. If you can fix the lens to a stand and allow sunlight to pass you can get sufficient heat to start a fire.

 More wonders

Ali and Nelly found that a magnifying glass can do more wonders than just magnifying a letter. The first success was almost accidental. Ali stood near a window inside a room. When Nelly held the lens near an opposite wall , an upside down image appeared. They were a bit puzzled at first. After much deliberation they came to the conclusion that the light rays would have crossed at the lens. Their thought is illustrated here.


Fig, 4. Nelly got an upside down image of Ali on the wall.

It appeared that the light had crossed at the magnifying glass. How could this happen?  But actually what happened is explained here.

Some light reflected from the body of  Ali go through the lens. By considering only two rays, it is possible to get the size and the position of the image. In the diagram above the lens is shown by a vertical dotted line. The black line drawn through the middle of the lens is called the principal axis. The blue line above represents a light ray emerging parallel to the  principal axis. All rays parallel to the principal axis pass through the focus.; denoted by ‘F’.  The rays that pass the mid point of the lens go undeviating. (without changing direction)  

It’s very interesting to obtain images in a practical way. Light a candle on a bench. Mount the lens on a suitable stand to remain in a vertical position. Get a white screen or use a wall.
The experiments have to be carried out at night to get the best results.

Start taking the distance between the flame and the screen to be about five or six times the focal length of the lens. Focal length is the distance from the lens to the point where the sunlight gets focused. Most of the time. The image will be blurred. At certain positions, it will become really sharp. When this happens, note down the distances and the size of the image.
Now Ali and Nelly are trying to make a slide projector using this magnifying glass. Like to beat them? Yes why not try.

Making a cup of Tea.

  Place a polythene paper on a circular frame such as a hoopla ring. (A bottomless basin would be ideal.) 

  Tie the corners of the sheet to the stand with some thread so that the sheet can bear the weight of water.

  Add some water to the sheet to give a convex shape on the lower side.

  Find out the place where the light rays converge. Place the object to be heated there.


This diagram shows how a light ray bends (refracts) by the curved lower surface of water.

Focal length is the distance from lens to the place where sunlight gets focused.
Light rays parallel to the principal axis go through the focus.
Light rays passing through the mid point of the lens go undeviating.

        For more click- Light-Lenses.