If we have planned for picnic or trekking,we never forget to capture those beautiful moments. For this we eagerly wait for the person with the instrument shown in fig. i.e. camera. It may be in mobile or special purpose camera, all uses concave lenses for capture the object whose photograph is to be taken. If one cannot see distinct objects, ophthalmic doctors will prescribe concave lens to him/her.
So let’s learn about concave lens in detail.
See in 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 less than that of outer edge, i.e. it is internally bulged called as concave lens.When object is kept close and in front of concave lens it produces virtual image of that object. The image may be magnified or diminished depends upon the distance between lens and object. Similar to mirrors lens can also rotate the image of object by 1800; hence it may be inverted also. But concave lens always produces virtual image.
When a parallel beam of light passes through this lens, all rays distributed away from each other, which looks as if all the refracted rays are started from a same point, and moving away from each other in all possible directions. This is called as divergence of the rays. Refer the diagram shown below.
The point where all refracted rays supposed to meets is called as Principal focus of lens. Position for the principal focus of concave lens appears towards left side; hence power of concave lens is taken as negative. Hence concave lens is widely used for divergence of refracted ray.
Let’s learn some key points for images formed by concave lens as below.
- Image formed by concave lens is always appearing in front of the mirror.
- As the image can’t be located on screen, it is always a virtual image.
- Image does not rotated by any angle; hence it appears upright, i.e. erected.
Now go through the following applications of concave lens.
- Peep hole of door: This is safety device kept in door. If any unknown person/or any threat is observed through peep hole, we can be alert before opening the door.
- Spectacles: Person those are suffering from the disease in which he/she can see nearby objects clearly but unable to see distinct objects clearly is known as nearsightedness. For correcting the vision of such person doctors prescribe concave lens of suitable power.
- Camera lens: To focus the single/multi object concave lenses are used so that we can capture clear and perfect photograph.
- Telescope/microscope: To bring distinct object in our visible range, or to magnify minute (micro) objects we use concave lens.
Q. What do you mean by concave lens?
Ans: The lens for which thickness at middle is less than that of outer edge, i.e. it is internally bulged called as concave lens.
Q. What types of images can be produced by concave lens?
Ans: Concave lens can produce virtual and erected image.
Q. What do you mean by principal focus of concave lens?
Ans: Whena parallel beam of light passes through concave lens, all rays distributed away from each other, which looks as if all the refracted rays are started from a same point. This point is called as principal focus.
Q. Why concave lens is also called as diverging lens?
Ans: When parallel beam of light falls on concave lens, after refraction all rays moving away but appears to be distributed from same point behind mirror, due to this property concave lens is also known as diverging lens.
Q. What is power of concave lens?
Ans: Concave lens has negative power.
Q. List some important applications of concave lens.
Ans: Concave lens can be used in 1) Camera, 2) Peep hole of door, 3) Telescope etc.