Consider the following statements about wavefronts:
(A) A plane wave passing through a thin prism comes out tilted, because its lower part, near the base, crosses more glass.
(B) A plane wave passing through a thin convex lens comes out as a diverging spherical wave.
(C) A plane wave reflected by a concave mirror becomes a spherical wave converging to the focus.
(D) When a convex lens forms a real image, light along the central ray takes less time than light near the edge.
Which of the above statements are incorrect?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (B) and (D) only
Light moves more slowly in glass. So the part of a wavefront that crosses more glass falls behind the rest, and the wavefront changes shape.
(A) A plane wave passing through a thin prism comes out tilted, because its lower part crosses more glass. — Correct statement. The thick part of the prism delays the lower part of the wavefront. So the emerging wavefront is tilted.
(B) A plane wave passing through a thin convex lens comes out as a diverging spherical wave. — Incorrect statement. The central part crosses the thickest glass and is delayed most. The wavefront gets a dip at the centre and converges to the focus.
(C) A plane wave reflected by a concave mirror becomes a spherical wave converging to the focus. — Correct statement. After reflection the wavefront curves inwards and meets at the focal point F.
(D) When a convex lens forms a real image, light along the central ray takes less time than light near the edge. — Incorrect statement. The central ray has a shorter path but moves slowly through more glass. The time from object to image is the same along every ray.
NCERT (2026-27):
Refraction and Reflection of Plane Waves Using Huygens Principle – “… Clearly, since the speed of light waves is less in glass, the lower portion of the incoming wavefront (which travels through the greatest thickness of glass) will get delayed resulting in a tilt in the emerging wavefront …”
Refraction and Reflection of Plane Waves Using Huygens Principle – “… The emerging wavefront has a depression at the centre and therefore the wavefront becomes spherical and converges to the point F which is known as the focus. …”
Refraction and Reflection of Plane Waves Using Huygens Principle – “… a plane wave is incident on a concave mirror and on reflection we have a spherical wave converging to the focal point F. …”
Refraction and Reflection of Plane Waves Using Huygens Principle – “… For example, when a convex lens focusses light to form a real image, although the ray going through the centre traverses a shorter path, but because of the slower speed in glass, the time taken is the same as for rays travelling near the edge of the lens.”