Ever wondered what the lens in your eyes is like? How you can easily see large objects? What is the capacity of magnification of our eye lens? How our eye can produce exact and sharp image of the object? You must have noticed that person who cannot see nearby objects clearly uses spectacles whose glasses are thicker at the centre and thin at corner. Visit the jewellery shop and check which type of lens that jeweller uses to check quality of ornaments. It is nothing but the convex lens. So let’s learn about convex lens in detail.
See the diagram below. What did you observed?
As we can see middle portion of transparent material (i.e. lens) is thinner than its outer portion.
The lens for which thickness at middle is more than that of outer edge, i.e. it is externally bulged called as convex lens. Convex lens can produce real and inverted image for different positions of object from lens. The image may be magnified or diminished. Similar to mirrors lens can also rotate the image of object by 1800; hence it may be inverted also.
When a parallel beam of light pass through convex lens, all refracted rays get collected at the same point. This is called as convergence of the rays.
The point where all refracted rays supposed to meets is called as Principal focus of lens. Position for the principal focus of convex lens appears towards right side; hence power of convex lens is taken as positive. Try a simple activity given below to learn convergence of refracted rays using convex lens. (Strictly, under supervision of your teacher or parents!)
- Take a convex lens; hold it in the path of sun light.
- Now place a paper below and close to the lens.
- You will see that paper start burning after some time.
- What did you understand from the given activity?
You can conclude that, a beam of light is concentrated at a same point so that intensity of sun light increases and so the heat, which burns the paper. Refer the following ray diagram for more clarification.
Now take candle, keep it in front of lens. And observe the image of flame of candle at different distances from convex lens by keeping following points in mind.
- Image formed by convex lens is appears behind lens, almost for all positions.
- As the image can be located on screen, it is always a realand inverted image.
- Convex lens produces virtual and erected image only if object is kept very close to lens. (i.e. How we hold magnifying glass to see bigger image of object.)
Refer the diagrams given below for more detailed explanation.
Now go through the following applications of convex lens.
- Magnifying glass: A single convex lens used to see magnified (big) image of object. In this case object is held very close to the lens, so that virtual, erected and magnified image can be seen.
- Spectacles: Person those are suffering from the disease in which he/she can see distinct objects clearly but unable to see nearby objects clearly is known as farsightedness. For correcting the vision of such person doctors prescribe convex lens of suitable power.
- Compound microscope: In a laboratory compound microscope is used to bring distinct object in our visible range, or to magnify minute (micro) objects we use more than one convex lens of different power.
Q. What do you mean by convex lens?
Ans: The lens for which thickness at middle is more than that of outer edge, i.e. it is internally bulged called as concave lens.
Q. What types of images can be produced by convex lens?
Ans: Concave lens can produce real and inverted image.
Q. What do you mean by principal focus of convex lens?
Ans: The point where all refracted rays supposed to meets is called as Principal focus of lens.
Q. Why convex lens is also called as converging lens?
Ans: When a parallel beam of light pass through convex lens, all refracted rays get collected at the same point, due to this convex lens is also called as converging lens.
Q. What is power of convex lens?
Ans: Concave lens has positive power.
Q. List some important applications of concave lens.
Ans: Concave lens can be used in 1) magnifying glass, 2) Spectacles, 3) Microscope etc.