Correct Option (d)
The phenomenon observed when a compact disc (CD) exhibits rainbow-like colours in sunlight is primarily explained by a combination of refraction, diffraction, and transmission. Sunlight, which is white light, is composed of various colours. When this light strikes the surface of a CD, it undergoes the following processes:
- Refraction: Light enters the transparent protective layer of the CD and bends due to the change in medium.
- Diffraction: The data pits on the CD surface act as a diffraction grating. As light reflects off these microscopic grooves, it is diffracted, meaning it spreads out and separates into its constituent wavelengths (colours). Different wavelengths are diffracted at slightly different angles.
- Transmission: While reflection is dominant for the visible colours, some light also transmits through the protective layer before encountering the reflective layer and subsequently being diffracted.
The interplay of these three phenomena leads to the separation of white light into its spectral components, creating the iridescent, rainbow-like appearance.
Incorrect Options:
- Option (a) reflection and diffraction: While reflection and diffraction are crucial components, the initial bending of light as it enters the CD's protective layer (refraction) and its passage through this layer (transmission) are also integral to the complete phenomenon. Without considering refraction and transmission, the explanation is incomplete.
- Option (b) reflection and transmission: This option omits the critical role of diffraction. Diffraction by the microscopic grooves on the CD surface is the primary mechanism responsible for separating white light into its constituent colours, which creates the rainbow effect. Reflection and transmission alone would not produce the spectral separation.
- Option (c) diffraction and transmission: This option overlooks the phenomenon of refraction. Light first refracts as it passes from air into the CD's transparent layer before interacting with the reflective surface and undergoing diffraction. While diffraction is key for colour separation, refraction is an initial step in the light's interaction with the CD.