QNR 13. Lenses are objects made of transparent materials such as glass or clear plastic that has curved surfaces. There are two main kinds of lenses. Diverging lenses are thicker at their edges than in their centres and they make light rays passing through them spread out. Converging lenses are thicker in the middle than at their edges and were the earliest kind of lens made. The earliest examples of these date back two thousand years. They have been used in spectacles to help people with poor vision see better since at least the tenth century. These days, as well as being used in spectacles, converging lenses are used in many other devices. Magnifying glasses, microscopes and some types of telescopes use converging lenses to make small things appear much larger or to make distant objects appear much closer. Converging lenses magnify by bending the rays of light that pass through them to meet at a point. This point is called the focus. The thicker that a converging lens is in its centre, the more it magnifies and the closer the focus is to the lens.
The magnifying power of a single converging lens such as that used in a camera or magnifying glass is equal to the length of the enlarged image divided by the length of the original object. For example, if a 2 cm long beetle appears to be 6 cm long when viewed through a magnifying glass, the glass magnifying power is 3 (written as 3x). Similarly, a 10x lens would make an object look 10 times longer. It will also look 10 times wider.
The diagram below shows the sizes of objects that can be observed effectively using the human eye, a light microscope and an electron microscope.
(i) If the smallest of the viruses appears to have a diameter of 2 mm when observed through an electron microscope, the microscopes magnification power is
(ii) Which of the following diagrams show side views of converging lenses?
1) A and C
2) B and D
3) B and C
4) B, C and D
(iii) When seen through a 5x magnifying glass, a small leaf appears to have an area of 25 mm2. What is the actual area of the leaf?
1) 1 mm2
2) 5 mm2
3) 20 mm2
4) 125 mm2
(iv) The human eye contains a lens that allows light rays entering the pupil to be focused on the retina which is located at the back of the eye.
The lens is surrounded by muscles that can change the thickness of the lens. One of the diagrams below correctly shows the way light rays are focussed when the thickness of the lens is changed.
Which is the correct diagram?
Ans (i) As mentioned in the passage:
Magnifying power = Length of the enlarged image/Length of the original object
Here, length of the enlarged image = diameter of the smallest of virus as it appears under the microscope= 2mm.
And, length of the original object = size of the smallest of virus = 10 nm = 10-5 mm.
Magnifying power = 2/ 10-5 =200,000x . Option (4)
(ii) Converging lens are thicker in the middle than near the edges. Ans- Option (2) B and D.
(iii) Since both the length and breadth would be magnified, we need to consider that while solving this.
Let initial length and breadth be ‘l’ and ‘b’. Initial area, A= lxb sq. units.
Under magnification, length and breadth would be ‘5l’ and ‘5b’; Magnified area= ‘5l’ * ‘5b’ = 25 lb sq. units
Here, 25 lb = 25 => lb= 1.
Actual area of the leaf = 1 sq units. = Option (1).
(iv) Thicker the lens, closer is the focal point.
Option 1. Lens is thin, the focus should shift beyond the retina. This is not the case.
Option 2. Lens is thick, the focus should shift closer to the lens. This is also not correct.
Option 3. Lens is thick, the focus should shift closer to the lens. This is correct. Let us check the last option.
Option 4. Lens is thin, the focus should shift beyond the retina. This is not the case.
Ans- option (3) is correct.