Ray Optics and Optical Instruments appeared 2 times across 1 year — 4.4% of Physics.
This question is from Refraction at Spherical Surfaces and by Lenses.
In a concave lens, a ray of light emanating from the object parallel to the principal axis of the lens after refraction:
A.emerges parallel to the principal axis.$\text{emerges parallel to the principal axis.}$
B.appears to diverge from the first principal focus.$\text{appears to diverge from the first principal focus.}$
C.passes through 2F, which is the radius of curvature of the lens.$\text{passes through 2F, which is the radius of curvature of the lens.}$
D.passes through the second principal focus.$\text{passes through the second principal focus.}$
Solution & Explanation
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.
Ray diagram showing divergence from principal focus in a concave lens Q19
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
Keywords:#concave lens#NEET 2026 Code 12 Q19#Ray Optics and Optical Instruments NEET 2026#Refraction at Spherical Surfaces and by Lenses NEET 2026
More Ray Optics and Optical Instruments Previous-Year Questions
Q23neet_2026_03_may_morningRefraction Through a Prism
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:
Equilateral prism ABC with ray refracting parallel to the base BC.
A.40°$40^{\circ}$
B.45°$45^{\circ}$
C.55°$55^{\circ}$
D.35°$35^{\circ}$
Solution
Related Formula
i + e = A + δ$$i + e = A + \delta$$At minimum deviation: i = e and δ = δm$$\text{At minimum deviation: } i = e \text{ and } \delta = \delta_m$$
Core Logic
For a ray refracting through a prism, if the internal refracted ray is parallel to the base of an isosceles or equilateral prism, the prism is in the position of minimum deviation.
Therefore, the angle of emergence (e$e$) equals the angle of incidence (i$i$).
Given:
Angle of incidence i = 50°$i = 50^{\circ}$
Equilateral prism angle A = 60°$A = 60^{\circ}$
Thus, e = 50°$e = 50^{\circ}$
Step 1: Calculate Angle of Deviation
Using the prism formula:
i + e = A + δ$$i + e = A + \delta$$50° + 50° = 60° + δ$$50^{\circ} + 50^{\circ} = 60^{\circ} + \delta$$100° - 60° = δ$$100^{\circ} - 60^{\circ} = \delta$$δ = 40°$$\delta = 40^{\circ}$$
Pattern Recognition
"Ray parallel to base" is the secret trigger word for the minimum deviation condition (i=e$i=e$).
Chapter Mix
Class 12 Physics: Ray Optics and Optical Instruments
More Ray Optics and Optical Instruments Questions — neet_2026_03_may_morning
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