Optics

To determine which statement about red light is true, we need to consider the concepts of refractive index and light deviation.

1. Refractive Index: The refractive index of a material indicates how much it slows down light compared to a vacuum. A lower refractive index means the light travels faster through the material, while a higher refractive index indicates the light travels slower.

2. Deviation: In optics, deviation refers to the change in direction of a light ray when it passes through a medium with a different refractive index. The amount of deviation depends on the refractive index; lower refractive index results in lesser deviation and vice versa.

For red light:

• Red light has a longer wavelength compared to other colors in the visible spectrum. This usually results in a lower refractive index when it passes through most media like glass or water.
• Because of its lower refractive index, red light undergoes less deviation than other colors like violet or blue, which have higher refractive indices and experience greater bending.

Based on this understanding, the correct statement is: It has low refractive index and suffers low deviation.

The human eye lens has the ability to change its focal length to focus on objects at various distances. This ability to adjust is known as accommodation. The minimum focal length of the eye lens when focusing on a nearby object is crucial for clear vision. The typical range of focal lengths for a healthy human eye lens extends from about 2.7 cm to 2.5 cm when observing distant objects, and can be reduced significantly when focusing on closer objects. However, the minimum focal length corresponding to the standard near point, which is the closest distance for clear vision without strain, is approximately 25 cm.

Therefore, the correct answer is 25 cm as it represents the minimum focal length of the eye-lens for a healthy human eye when focusing on nearby objects, such as reading a book held comfortably.

Step 1: Understand the concept of white light and its components.
White light is composed of different colours, each corresponding to a different wavelength. When white light is separated into its constituent colours, a band of these colours is observed.
Step 2: Analyze the given options.

1. Refraction:
   This is the bending of light as it passes from one medium to another with a different refractive index. While refraction can lead to the separation of colours (as in a prism), refraction itself is not the name for the band of colours.
2. Dispersion:
   This is the phenomenon of the splitting of white light into its constituent colours due to the variation of refractive index with wavelength. Dispersion is the process that creates the band of colours.
3. Scattering:
   This is the phenomenon where light is deflected in different directions by particles in a medium. Scattering can affect the colour of light we see (e.g., the blue sky), but it is not the name for the band of separated colours.
4. Spectrum:
   This is the band of colours obtained when white light is dispersed. It shows the component wavelengths of the light beam arranged in order of their wavelengths or frequencies. For visible light, this band consists of colours from violet to red.

Step 3: Identify the term for the band of coloured components of light.
The band of the coloured components of a light beam, obtained through dispersion, is called its spectrum.