Identify the correct statements among the following about ray optics and optical instruments.
A. Laws of reflection and refraction are true for all surfaces and pair of media at the point of the incidence.
B. Thick lenses give coloured images due to diffraction.
C. In simple microscope, the angular size of the object equals the angular size of image.
D. Optical fibres consist of glass fibres coated with a thin layer of higher refractive index.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → A and C only
(A) Laws of reflection and refraction are true for all surfaces and pair of media at the point of the incidence. — Correct. At a curved surface they are applied using the normal at the point of incidence.
(B) Thick lenses give coloured images due to diffraction. — Incorrect. Coloured images (chromatic aberration) are due to dispersion: the focal length differs for different colours.
(C) In simple microscope, the angular size of the object equals the angular size of image. — Correct. With the eye close to the lens, an object of height \( h \) at distance \( u \) and its image of height \( h' = (v/u)h \) at distance \( v \) subtend the same angle:
\( \displaystyle \theta_{\mathrm{image}} = \frac{h'}{v} = \frac{(v/u)\,h}{v} = \frac{h}{u} = \theta_{\mathrm{object}} \)
The magnification comes from holding the object closer than the near point (25 cm).
(D) Optical fibres consist of glass fibres coated with a thin layer of higher refractive index. — Incorrect. The cladding has a lower refractive index than the core, so that light is totally internally reflected at the boundary.
NCERT (2026-27):
Points to Ponder – “1. The laws of reflection and refraction are true for all surfaces and pairs of media at the point of the incidence.”
Points to Ponder – “… 4. Thick lenses give coloured images due to dispersion.”
Points to Ponder – “… 5. For a simple microscope, the angular size of the object equals the angular size of the image.”
Summary – “… Optical fibres consist of glass fibres coated with a thin layer of material of lower refractive index.”