Optical Instruments

Optical Instrument

Eyes
Eyes are convex lens because they have to create real image to hit the retina so that we will be able to see objects. To see distant object, the lens will become thinner hence to see close object, the lens will become thicker, thus enable the object to be placed in zone 3 to create a reduced image in our retina.
Our eyes are able to see object or virtual image.
To see how the light comes to our eyes, visit :
http://www.ziddu.com/download/4536270/Howlightcomestoeye.pps.html

Optical instruments can be categorized into 2 types that is connected to eyes or other medium. The one which is connected to eyes should create a virtual images because our eyes only receives virtual images. Consequently, the one which is connected to medium should create real images.

Here are the examples that created virtual images:


Concave Lens
Nearsightedness (myopia) can not see distant object, to help them, we use diverging lens because diverging lens will make distant object become closer object. Their eyeball is too long, image focuses in front of retina. It will expand the focal length so the image will be focused in retina.

Convex Lens
Farsightedness can not see near object, to help them, converging lens is needed to make a further image that can be seen by farsightedness. Their eyeball is too short, image focuses behind retina. Convex lenses shorten the focal length. It also helps presbyopia which eyeball is too short because of age, image focuses behind retina

Cylindrical lenses
It is used to help astigmatism – irregular curvature of retina so image is blurred
Cylindrical lenses reduce the irregular to focus the object

Telescope
Telescope is used for seeing distant object. Virtual image should be formed in front of the telescope so that we will be able to see the object. Telescope contains at least 2 lens, one is used for the objective lens (near to the object) and the other for eyepiece (near to our eyes). The focal length of the objective lens should be bigger than the eyepiece due to the object distance. Therefore, the power of lens of the objective lens is smaller than the eyepiece.
Normally for the objective lens, the converging lens is used to converge all the light, to make a clearer image.
When the light of the distant object comes to objective lens, the image will be formed on the focal point of the objective lens. he created image will be an object for the eyepiece.
Eyepiece can be made by using both convex or concave lens, because both lens will create virtual images. However the concave lens will diminish the object, which will make us hard to see the object.
To create virtual images in a convex lens, object should be placed below focal point, this will lead to the maximum length of telescope as an addition of focal length of objective lens and eyepiece.
To create virtual images in a concave lens, object can be placed anywhere however the results are varied but in general is reduced image.
The magnification of a telescope can be calculated as focal length of an objective lens over focal length of an eyepiece.
Meanwhile, the maximum distance of a telescope is an addition of focal length of objective lens and eyepiece.


Microscope
Microscope is used for magnifying tiny object. Virtual image should be formed in front of the microscope so that we will be able to see the object. Microscope contains at least 2 lens, one is used for the objective lens (near to the object) and the other for eyepiece (near to our eyes). The focal length of the objective lens should be smaller than the eyepiece due to the object distance. Therefore, the power of lens of the objective lens is bigger than the eyepiece.
The lens that is used for microscope is convex lens, because the main idea of microscope is to magnify object, which only can be done by convex lens.
For a maximum magnification of microscope, the object in front of objective lens should be placed in zone 2 (so it will create a real and bigger image in zone 3 for the eyepiece). Then, the object for the eyepiece should be placed in zone 1 so that a virtual and magnified image will be formed.
Magnification of a microscope can be calculated as a result of magnification of objective lens times by magnification of the eyepiece.


Magnifying Glasses
Magnifying glasses or loupe is used for magnifying object. Virtual and magnified image should be formed, that's why convex lens is used.

Examples that create real images are:
Camera
The principle of camera is the same as eye because the image should be made at the back of the lens (real image). In that case, camera should use converging lens (convex). The object should be put over 2F because film / negative in camera can handle a reduced image not a magnified image. The further the object, the focal length of the camera should be longer too (we can achieve this by reducing the power of camera lens or forwarding the lens to the object). The real image will be projected to photographic paper or camera screen which enable us to see a picture. It can be used to magnify the image too.

Diascope / projector
Created image is real and magnified.

Photocopy machine
Created image is real and same size, however it can be used to magnify and reduce objects too, depends on the position of the lens. To magnify means we place the object in zone 2, hence, to have a reduced image, we should place the object in zone 3, etc.