Optical Instruments

Optical instruments are the devices which process light wave to enhance an image for more clear view. Use of an optical instruments, such as a magnifying lens or any complicated device like microscope or telescope usually makes things bigger and helps us to see in a more detailed manner. The use of converging lenses makes things appear larger and on the other hand, diverging lenses always gets you smaller images.

While using a converging lens, it’s important to remember that, if an object is at a larger distance then the image is diminished and will be very nearer to the focal point. While the object keeps on moving in the direction of the lens, the image moves beyond the focal point and enlarges. When object is placed at 2F, which is two times the focal distance from the lens, the image and object becomes of the same size. When the object moves from 2F towards the focal point (F), it’s image keeps moving out of the lens and enlarges till it goes to infinity when the object reaches the focal point, F. As the object moves closer to the lens, the image moves in the direction of the lens from negative infinity and gets smaller when the object gets closer to lens.

Applications of Optical Instruments:

Multiple Lenses:

There are many devices like telescopes and microscopes which uses multiple lenses to create images.Analyzing any system having multiple lenses shows that it works in stages where each lens forms an image of the object. The original object will work as the object for the first lens and creates an image. This new image will be the object for the second lens and so on. You can see this with these examples.


A microscope has two converging lens. This is because it is easier to get higher magnification with two lenses rather than just one. Use of one lens can magnify 5 times more and using a second will magnify 7 times, and you will get an overall magnification of 35 which is not possible in one lens. It’s an easy procedure than to get magnification by a factor of 35 with a single lens. A ray diagram of microscope arrangement is given below. Here you can see the image is the object for the second lens and the image formed by the second lens is the image that you would see when you looked through the microscope.



As we use telescope to view a object that is at very distant place, therefore, a telescope needs at least two lenses. The first lens forms a diminished image which is nearer to its focal point. This device is designed in a way so that real and inverted image formed by the first lens is just nearer to the second lens than its focal length. WIth the help of a magnifying glass, we gets a enlarged image which is virtual. The final image then inverted with respect to the object. This doesn’t really matter with the astronomical telescope.While observing an object which is on earth, we usually prefer a straight image which is obtained using a third lens.


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