• Tags: prism-combination, achromatic, direct-vision
• Difficulty: Advanced

Achromatic combination (deviation without dispersion): Two prisms with ω_{1}$A_{1}$(n_{1}-1) + ω_{2}$A_{2}$(n_{2}-1) = 0. The prisms must have different dispersive powers and are arranged so their dispersions cancel.

Direct vision combination (dispersion without deviation): Net deviation = 0, so (n_{1}-1)$A_{1}$ + (n_{2}-1)$A_{2}$ = 0 (for mean wavelength). One prism is crown glass, the other is flint glass, placed inverted relative to each other.

JEE tip: These are commonly asked as 2-mark derivation or assertion-reason questions.