Dispersion Without Deviation Explained

Dispersion involves the separation of white light into its constituent colors because the refractive index of a medium varies with wavelength. "Without deviation" signifies that the overall direction of the light beam remains unchanged, even though colors separate.

Analyzing Dispersion Without Deviation

In optical setups designed for dispersion without deviation, such as specific prism combinations (e.g., direct vision prisms):

• The separation of colors (dispersion) occurs due to different wavelengths refracting differently.
• The net effect is zero deviation, meaning the emergent beam is parallel to the incident beam.

Statement Completion and Justification

Completing the statement "In dispersion without deviation..." requires identifying which component aligns with the incident ray's direction. The correct option is:

Option C: yellow coloured ray is parallel to the incident ray

Reasoning: In common experimental setups engineered for dispersion without net deviation, the yellow light (often representing the mean wavelength) is typically designed to emerge parallel to the incident ray. While other colors like violet and red deviate slightly, they do so symmetrically around the yellow ray, ensuring the overall beam experiences no net angular shift. This makes the yellow ray the reference for zero deviation in these specific cases.

Why other options are incorrect:

• Option 1 is wrong because if all emergent rays were parallel to the incident ray, there would be no dispersion (separation of colors).
• Option 2 is incorrect as it implies a net deviation if the parallel emergent rays are not aligned with the incident ray.
• Option 4 is less precise; while red light might be parallel in certain specific theoretical cases, yellow light is the standard reference for zero deviation in practical direct vision prisms.