To know the working of spectacles.
Human eye has various transparent parts which may be thought of a convex lens of some focal length which can be changed little bit while seeing objects at different distances. For clear viewing the image should form on the retina. Most common defects of vision, myopia and hypermetropia, occur because the distance from of the retina from the eye lens is less than for greater than what is required. Spectacles make the incoming light beam more divergent or less divergent to help the image to form on the retina.
Image formation on ratina, power of lenses,diverging of light.
A convex lens, a torch bulb and few lenses of different powers.
- Fix up a convex lens of large power (say ~20D) in a stand to represent the eye lens.
- Keep the torch bulb at some distance (say 20 cm) from the lens.
- Keep the screen on the other side of the lens to get sharp image of the bulb. The screen represents retina.
- Now displace the screen slightly away from the lens. This represents a myopic eye where the eye ball is elongated or due to some other reason the image forms before the retina. You see that the image gets blurred.
- Put an additional lens from the given stock in front of the eye lens and see the image. This is your spectacle glass to correct for the myopia. By trial and error choose the best power of the lens so that the image becomes sharp. Don’t displace the retina or the eye lens in this process.
- Now remove the additional lens, focus the image on the retina by adjusting its position. Now shift the retina little bit towards the eye lens. This represents presbyopic eye where due to contraction of eyeball or otherwise, the image forms behind the retina.
- Put proper power lens in front of eye lens so that the image forms on the retina.
In myopia, the retina is farther away from the eye ball and the image forms in front of the retina. The beam is to be made little more divergent so that the image shifts away and forms on the retina. Thus concave lens of appropriate power is needed. Similarly, convex lens of appropriate power is needed for presbyopic eye.