Note three polymers that may be candidates for eyeglass lenses, and then tabulate values of the properties noted in part (b) for these three materials.
The correct answer and explanation is :
Eyeglass lenses require materials that are transparent, durable, lightweight, and resistant to scratching and impact. The polymers used in the production of eyeglass lenses must also have specific optical properties such as high refractive index, low dispersion, and resistance to yellowing over time. Three common polymers that are often considered for eyeglass lenses are:
- Polycarbonate (PC)
- Trivex (TM)
- CR-39 (Plastic)
Now, let’s look at a table summarizing the important properties of these materials for eyeglass lenses:
| Property | Polycarbonate (PC) | Trivex (TM) | CR-39 (Plastic) |
|---|---|---|---|
| Refractive Index | 1.58 | 1.53 | 1.49 |
| Density (g/cm³) | 1.2 | 1.11 | 1.32 |
| Abbe Number (Optical Dispersion) | 29 | 43 | 58 |
| Impact Resistance | High | High | Moderate |
| UV Protection | Yes | Yes | Yes |
| Scratch Resistance | Moderate to Low | High | Moderate |
| Light Transmission | High (88-92%) | High (88-92%) | High (88-92%) |
| Cost | Moderate | High | Low |
Explanation of the Properties:
- Refractive Index:
This property indicates how much light is bent or refracted when it passes through the lens material. A higher refractive index allows the lens to be thinner and lighter while still providing adequate vision correction. Polycarbonate has a higher refractive index than CR-39, making it ideal for high-prescription lenses. - Density:
This is a measure of how heavy the material is. Lighter lenses are more comfortable to wear, especially for long periods. Trivex is the lightest among the three, making it a great option for comfort. - Abbe Number:
The Abbe number measures the dispersion of light through the material and impacts the clarity of vision. A lower number means more chromatic aberration (color fringing). CR-39 has the highest Abbe number, offering clearer vision with less color distortion. - Impact Resistance:
Eyeglass lenses must be able to withstand impacts, especially in sports or for children. Polycarbonate and Trivex are both highly impact-resistant, with Trivex having a slightly edge in terms of durability and comfort due to its lightweight nature. - UV Protection:
All three materials provide UV protection, which is crucial to shield the eyes from harmful ultraviolet radiation. - Scratch Resistance:
Scratch resistance is important to maintain the lens clarity over time. While Trivex has the highest natural scratch resistance, polycarbonate and CR-39 may require special coatings for enhanced protection. - Light Transmission:
This refers to how much light passes through the lens. A high light transmission is important for good visual clarity. All three materials offer similar transmission rates, ensuring good optical quality. - Cost:
Cost is an important factor. Polycarbonate and CR-39 are more affordable, while Trivex tends to be more expensive due to its advanced properties.
In conclusion, the choice of polymer for eyeglass lenses depends on the specific needs of the wearer, such as optical performance, durability, weight, and cost considerations. Polycarbonate and Trivex are preferred for high-impact applications, while CR-39 is often selected for standard prescription glasses due to its excellent optical clarity and affordability.