Angle of Refraction Definition Steps:

• Locate surface XY as the air-glass interface where light enters the block.
• Identify the normal perpendicular to XY at the incidence point.
• Trace the refracted ray inside the glass after bending toward the normal.
• Label the angle between this refracted ray and the normal as the refraction angle.

Why D is correct:

• D marks the angle in the glass medium between the refracted ray and normal, per Snell's law definition.

Why the others are wrong:

• A marks the incident angle in air from the incident ray to normal.
• B marks an angle inside the block unrelated to XY refraction.
• C marks the emergence angle at the exit surface, not XY.

Final answer: D