Direct vision prism: two prisms combined so net dispersion is zero but net deviation is not. The condition for zero dispersion is: 
An₁ = n₂
BA₁ = A₂
CNo such combination possible
D(n_v - n_r)₁ A₁ = (n_v - n_r)₂ A₂
Answer & Solution
Correct answer: D. (n_v - n_r)₁ A₁ = (n_v - n_r)₂ A₂
For two prisms (angles A₁, A₂; refractive indices n₁, n₂): net dispersion (n_v - n_r)₁ A₁ - (n_v - n_r)₂ A₂ = 0 → (n_v - n_r)₁ A₁ = (n_v - n_r)₂ A₂ (using opposite orientations). Used in optical spectroscopes. Net deviation: (n_y - 1)₁ A₁ - (n_y - 1)₂ A₂ ≠ 0.
Related questions
Light passes from air to glass ($n = 1.5$). Brewster's angle is:Polaroid sunglasses reduce glare from a wet road most effectively because:In single-slit diffraction with $\lambda = 500$ nm and slit width $a = 0.1$ mm, the angulaIn Young's double slit, $\lambda = 600$ nm, slit separation $d = 0.5$ mm, screen distance For thin lens, when image distance equals object distance (|v| = |u|), object position is:Diffraction effects are more noticeable when the obstacle/aperture size is:Speed of light in glass (n = 1.5) is approximately:Refractive index of medium 1 with respect to medium 2 (denoted ₁n₂ or n₁/n₂). If ₁n₂ = 1.5