A ray of monochromatic light is passing through an equilateral prism (ABC) as shown in the figure. The refracted ray (QR) is parallel to its base (BC) and the angle of incidence (i$i$) is 50°$50^{\circ}$. Then the angle of deviation (δ$\delta$) is:

Core Logic

According to the rules of refraction for a concave (diverging) lens, an incident ray parallel to the principal axis diverges outward after passing through the lens. When this diverging refracted ray is extended backwards, it appears to originate exactly from the principal focus located on the same side as the incident ray.

Step 1: Clarification of Focus Terminology

The focus from which parallel rays appear to diverge is typically termed the second principal focus (F₂$F_2$). However, many textbooks ambiguously refer to it as the principal focus. The option "appears to diverge from the first principal focus" is selected as the best appropriate answer from the given choices, though technically it's the second principal focus for a concave lens.

Pattern Recognition

Parallel rays on a concave lens always diverge. Backward tracing intersects the focal point on the incidence side.

Chapter Mix

Class 12 Physics: Ray Optics and Optical Instruments