💡 Chapter 14: Light – Constructed Response
Prepared by Muhammad Tayyab, Subject Specialist Physics, Govt Christian High School Daska. Based on PECTAA 2026 syllabus (National Curriculum 2023).
📖 What's Inside: This chapter covers refraction, prism dispersion, total internal reflection, optical fibres, and magnifying glass through constructed response questions. Each question is presented with the exact exam-ready answer as per the official PECTAA 2026 Physics curriculum.
📚 Related Resources – Chapter 14: Light
Light covers reflection, refraction, total internal reflection, optical fibres, lenses, prisms, dispersion, and optical instruments.
📖 Constructed Response Questions & Answers (PECTAA 2026)
14.1 When a pencil is partially dipped in water, it appears bent or broken. What principle of light explains this phenomenon, and how does it relate to changes in speed?
This phenomenon is explained by the refraction of light. Refraction occurs because light travels at different speeds in different materials.
When light rays from the underwater part of the pencil pass from water (an optically denser medium) to air (a rarer medium), they speed up and bend away from the normal, making the pencil appear bent or broken at the water's surface.
This phenomenon is explained by the refraction of light. Refraction occurs because light travels at different speeds in different materials.
When light rays from the underwater part of the pencil pass from water (an optically denser medium) to air (a rarer medium), they speed up and bend away from the normal, making the pencil appear bent or broken at the water's surface.
14.2 Why can a prism separate white light into its component colours while a plane glass slab cannot, even though both cause refraction?
A prism can separate white light into its component colours because it has two non-parallel (inclined) surfaces. Different colours (wavelengths) refract by different amounts, so they spread out and form a spectrum. A plane glass slab has parallel surfaces. Although light is refracted, the colours recombine when they emerge, so no dispersion is observed.
A prism can separate white light into its component colours because it has two non-parallel (inclined) surfaces. Different colours (wavelengths) refract by different amounts, so they spread out and form a spectrum. A plane glass slab has parallel surfaces. Although light is refracted, the colours recombine when they emerge, so no dispersion is observed.
14.3 Why does light undergo total internal reflection when travelling from water to air at a certain angle but not the other way around?
Total internal reflection occurs only when light travels from a denser medium to a rarer medium. When light travels from water to air, it bends away from the normal. If the angle of incidence is greater than the critical angle (48.8°), the light is completely reflected back into the water. When light travels from air to water, it bends towards the normal and always enters the water, so total internal reflection cannot occur.
Total internal reflection occurs only when light travels from a denser medium to a rarer medium. When light travels from water to air, it bends away from the normal. If the angle of incidence is greater than the critical angle (48.8°), the light is completely reflected back into the water. When light travels from air to water, it bends towards the normal and always enters the water, so total internal reflection cannot occur.
14.4 Optical fibres use total internal reflection for signal transmission. What would happen if the cladding had a higher refractive index than the core, and why?
If the cladding had a higher refractive index than the core, total internal reflection would not occur, and the optical fibre would not function properly. Total internal reflection requires the core to have a higher refractive index than the cladding. If the cladding had a higher refractive index, light would refract into the cladding instead of reflecting back into the core, causing signal loss.
If the cladding had a higher refractive index than the core, total internal reflection would not occur, and the optical fibre would not function properly. Total internal reflection requires the core to have a higher refractive index than the cladding. If the cladding had a higher refractive index, light would refract into the cladding instead of reflecting back into the core, causing signal loss.
14.5 A person uses a magnifying glass to read small print. What must be the relationship between the object distance and the focal length of the lens for the image to be upright and magnified?
For a magnifying glass to produce an upright and magnified (virtual) image, the object must be placed between the lens and its focal point. In other words, the object distance must be less than the focal length (p < f).
For a magnifying glass to produce an upright and magnified (virtual) image, the object must be placed between the lens and its focal point. In other words, the object distance must be less than the focal length (p < f).
📐 Key Formulas – Light
Refractive Index: \( n = \frac{c}{v} \)
Snell's Law: \( n_1 \sin i = n_2 \sin r \)
Critical Angle: \( C = \sin^{-1} \left( \frac{1}{n} \right) \)
Mirror / Lens Formula: \( \frac{1}{f} = \frac{1}{p} + \frac{1}{q} \)
Magnification: \( M = \frac{h_i}{h_o} = -\frac{q}{p} \)
Power of Lens: \( P = \frac{1}{f} \)
Unit 10Thermal Physics
Unit 11Transfer of Thermal Energy
Unit 12Waves
Unit 13Sound
Unit 14✨ Light
Unit 15Electrostatics
Unit 16Electricity
Unit 17Electromagnetism
Unit 18Electromagnetic Induction & EM Waves
Unit 19Electronics
Unit 20Atomic and Nuclear Physics
Unit 21Space and Environment
📖 Complete syllabus coverage for Class 10 Physics (PECTAA 2026) – Units 10 to 21
💡 Exam Tip:
For constructed response questions, provide clear explanations with scientific reasoning. Use appropriate terminology, mention relevant principles, and connect concepts to real-world applications. These questions follow the PECTAA 2026 pattern and are prepared by Subject Specialist Muhammad Tayyab.
Created by Hira Science Academy | Aligned with PECTAA 2026 Syllabus