High School Physics Materials Optical Instruments The eye is a very important sense of sight. We can see this beautiful world with our eyes. The eye is an optical instrument because it contains the eye lens which is used to receive light reflected by the objects we see. In this case, the eye can see an object if there is light and the object can reflect light. When it is dark, our eyes cannot see objects. This is due to the absence of light entering the eye from objects that reflect it or from a light source.

a. Optical Instruments

As an optical instrument, the parts of the eye work based on the properties of light. Look at the pictures of the following parts of the eye!

  1. The cornea is the outermost layer of the eye which is strong and translucent. The function of the cornea is to receive and transmit light.
  2. Aqueous humor is the fluid between the cornea and the lens of the eye.
  3. Crystalline lens, the eye lens that plays an important role in adjusting the location of the image so that it falls right on the yellow spot. The eyepiece is made of clear and supple material. The lens of the eye is used to form images of objects. The eye lens is a convex lens.
  4. Iris, the membrane that forms a circular slit in the middle. The iris gives color to the eye and functions to adjust the size of the pupil to limit the amount of light entering.
  5. The pupil, the slit formed by the iris, serves as the entry point for light.
  6. Eye muscles, muscles that support the crystalline lens and regulate the size of the lens.
  7. Vitreous humour, the clear fluid that fills the eye socket.
  8. Retina, the layer on the back of the eyeball where the image is formed. The retina is where the image formed by the lens of the eye falls.
  9. Yellow spots, curvature of the retina which is the most sensitive part of the retina.
  10. The optic nerve, which transmits light from the retina to the brain.
Normal eyes can see clearly everything that is at a distance of 25 cm in front of the eye to infinity. When the eye sees an object that is close, the eye’s lens contracts to become more convex. Meanwhile, when looking at objects in the distance, the eye lens relaxes so that the eye lens becomes increasingly flat. This is done so that the image of the object falls right on the area around the yellow spot on the retina.

The ability of the lens of the eye to contract and relax is called the power of accommodation of the eye. If the eye sees objects that are getting closer, the power of accommodation increases. Conversely, if you look at objects that are farther away, the power of accommodation will be smaller. The power of accommodation causes the eye to have a near point (punctum proximum) and a far point (punctum remotum). The near point of the eye is the closest point that the eye can clearly see with maximum accommodation. The far point is the farthest point that the eye can clearly see without accommodation.

b. Optical Instrument Glasses

Glasses are one of the tools that can be used to treat eye defects. Glasses consist of a concave lens or a convex lens, and a frame or frame in which the lens is located, as you can see in the Figure below:

The function of the glasses is to adjust the image of objects that cannot be seen clearly by the eye to fall at the near point or at the far point of the eye, depending on the type of eye defect. In middle school, you learned that if an object is placed in front of a lens, an image will be formed by that lens. How close the image is to the lens, depends on the location of the object and the focal length of the lens.

This relationship can be written mathematically as follows:

(1-1)

with :

S = distance of object to lens (m),

S’ = image distance to lens (m), and

f = focal length of the lens (m).

In addition, you have also studied the power or power of the lens. The power or power of the lens is the ability of the lens to focus the incident light parallel to the lens. The relationship between lens power and lens power satisfies the equation:

P = 1 / f (1-2)

with :

P = power or power of the lens (diopters), and

f = focal length of the lens (m).

a. Concave Lens Glasses for Myopia

As discussed earlier, myopic eyes cannot see clearly distant objects or their far point is limited to a certain distance. The eyeglass lenses used by people with myopia must form images of distant objects (S .). ~ ) right at the far point of the eye or S’ = –PR, where PR stands for punctum remotum, which means far point. The negative sign on S’ is given because the image formed by the eyeglass lens is in front of the lens or is virtual. If you put the values ​​of S and S’ into Equation (1–1), you get:

(1-3)


Equation (1–3) shows that the focal length of the eyeglass lens is negative from the far point of the myopic eye. The negative sign indicates that the myopic eye needs to be overcome by negative lenses (concave or divergent).

If Equation (1–3) is inserted into Equation (1–2), we get:

(1-4)

where PR is expressed in m (meters) and P in diopters.

c. Camera Optical Tools

The camera is an optical device that resembles an eye. The basic elements of a lens are a convex lens, a slit diaphragm, and a film (sensitive plate). The convex lens serves to form the image of objects, the diaphragm slit serves to regulate the intensity of incoming light, and the film serves to capture the image formed by the lens. Films are made of materials that contain chemicals that are sensitive to light (changes when light hits the material). In the eye, these three basic elements resemble the eye’s lens (convex lens), iris (slit diaphragm), and retina (film).

The working principle of the camera in general is as follows. The object to be photographed must be in front of the lens. When the diaphragm is opened, light passing through the object enters through the aperture of the diaphragm into the eyepiece. The lens of the eye will form an image of the object. In order for the image of the object to fall on the film clearly, the location of the lens must be shifted toward or away from the film. Shifting the lens on the camera, such as adjusting the focal length of the lens on the eye (accommodation).

d. Lup Optical Instruments

A loupe or magnifying glass (or some people call it a solar lens) is a convex lens that is used to see small objects so that they appear clearer and bigger. The use of a loupe as a magnifying glass stems from the fact that objects of the same size will look different to the eye when the distance to the eye is different. The closer to the eye, the larger the object can be seen. Conversely, the farther into the eye, the smaller the object can be seen. For example, a pencil when viewed at a distance of 25 cm will appear twice as large as when viewed at a distance of 50 cm. This happens because the eye’s point of view on an object that is at a distance of 25 cm is twice that of an object that is 50 cm away.

Even though the closest object that can be clearly seen is 25 cm (for normal eyes), the loupe allows you to place the object closer than 25 cm, even if it is smaller than the focal length of the loupe. This is because when you observe an object using a loupe, what you see is the image of the object, not the object. When the object is closer to the eye, the angle of view of the eye will become larger so that the object looks bigger. The ratio of the angle of view of the eye when using the loupe and the angle of view of the eye when not using the loupe is called the magnification of the angle of the eye.

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