- Distinguish between nearsightedness farsightedness and astigmatism. Use a ray diagram to show how the image is formed in each condition. square

Let's break down the differences between nearsightedness (myopia), farsightedness (hyperopia), and astigmatism, and illustrate them with ray diagrams.<br /><br />**1. Nearsightedness (Myopia)**<br /><br />* **Problem:** The eye is too long or the cornea is too curved. Light rays from distant objects converge *before* reaching the retina, resulting in a blurry image. Near objects are seen clearly.<br /><br />* **Ray Diagram:**<br /><br />```<br /> Object (distant)<br /> |<br /> |<br /> \/<br /> ( ) Cornea & Lens (too strong)<br /> /\<br /> |<br /> | Focal point (in front of retina)<br /> |<br /> (---) Blurred image on retina<br /> Retina<br />```<br /><br />* **Correction:** A concave (diverging) lens is used to spread the light rays slightly before they enter the eye, allowing them to focus correctly on the retina.<br /><br /><br />**2. Farsightedness (Hyperopia)**<br /><br />* **Problem:** The eye is too short or the cornea is not curved enough. Light rays from nearby objects would converge *behind* the retina if they could, resulting in a blurry image. Distant objects are usually seen clearly.<br /><br />* **Ray Diagram:**<br /><br />```<br /> Object (near)<br /> |<br /> |<br /> \/<br /> ( ) Cornea & Lens (too weak)<br /> /\<br /> |<br /> |<br /> |<br /> (---) Retina<br /> |<br /> | Focal point (behind retina)<br /> \/<br /> Blurred image (if projected)<br />```<br /><br />* **Correction:** A convex (converging) lens is used to converge the light rays slightly before they enter the eye, allowing them to focus correctly on the retina.<br /><br /><br />**3. Astigmatism**<br /><br />* **Problem:** The cornea or lens has an irregular shape, more like a football than a basketball. This causes light rays to converge at different points, resulting in distorted or blurry vision at all distances. Think of it as having different refractive powers in different meridians of the eye.<br /><br />* **Ray Diagram:** It's difficult to represent astigmatism with a simple 2D ray diagram. Imagine the above diagrams, but with the cornea or lens shaped so that some rays focus correctly while others focus in front of or behind the retina, creating multiple focal points. For example, vertical lines might be in focus while horizontal lines are blurry, or vice-versa.<br /><br />* **Correction:** Toric lenses are used to correct astigmatism. These lenses have different refractive powers in different meridians, compensating for the irregular shape of the cornea or lens. They can be combined with concave or convex lenses to correct nearsightedness or farsightedness, respectively, if those conditions are also present.<br /><br /><br />**In Summary:**<br /><br />| Feature | Nearsightedness | Farsightedness | Astigmatism |<br />|----------------|-----------------|-----------------|-----------------|<br />| **Problem** | Eye too long/cornea too curved | Eye too short/cornea not curved enough | Irregular cornea/lens |<br />| **Focus** | In front of retina | Behind retina | Multiple focal points |<br />| **Close Vision**| Clear | Blurry | Blurry/Distorted |<br />| **Distant Vision**| Blurry | Clear | Blurry/Distorted |<br />| **Correction** | Concave lens | Convex lens | Toric lens |<br />