Class 12 NEET Physics Optics Questions Part 1

Concept:Different colors of light have different wavelengths, and refractive index varies with wavelength (dispersion). Shorter wavelengths refract more, leading to shorter focal lengths.Explanation:1. Wavelength order: Violet has the shortest wavelength, followed by green, then red (longest).$$\lambda_v < \lambda_g < \lambda_r$$2. Refractive index and wavelength: According to dispersion, the refractive index is inversely related to wavelength. Shorter wavelengths have higher refractive indices.$$n_v > n_g > n_r$$3. Lens maker's formula: The focal length of a lens is given by:$$\frac{1}{f} = (n-1)\left(\frac{1}{R_1} - \frac{1}{R_2}\right)$$Since $(n-1)$ is larger for violet light, $\frac{1}{f_v}$ is largest, making $f_v$ the smallest.4. Focal length order:$$f_v < f_g < f_r$$5. Checking options:

• A. $f_v < f_r$ ✓ (Correct)
• B. $f_g > f_r$ ✗ (False; $f_g < f_r$)
• C. $f_v = f_g$ ✗ (False; they are different)
• D. $f_g < f_r$ ✓ (Also correct, but less specific than A)

Answer:A. $f_v < f_r$(Note: Option D is also technically correct, but A is the most direct and commonly expected answer showing the extreme case of violet vs. red.)