Assertion (A): Without any dispersion, sunlight reaches us as pure white light rather than its constituent colours. 

Reason (R): Dispersion occurs as a result of changes in the refractive indices for various wavelengths. However, in a vacuum, the speed of light is wavelength-independent, making it a non-dispersive medium.

The reason is the correct explanation of the assertion. 

For assertion 

● Dispersion only occurs after the sunlight enters the atmosphere at a slow rate 

● So, sunlight reaches us without dispersion in the form of white light and not as its components. 

● Therefore, the assertion is true. 

For Reason 

● This happens as before entering the atmosphere, the sunlight passes through the vacuum. 

● In a vacuum, the light doesn't disperse 

● Here, all colours travel at the same speed. 

Hence option (a) Both A and R are true, and R is the correct explanation of the assertion is correct.