CBSE Class 12 Physics (2026–27)
Chapter 9: Ray Optics and Optical Instruments
20 Important Questions and Answers (2–3 Marks, 100–120 Words Each)
Q1. State the laws of reflection of light. Why is reflection important in optical instruments?
Answer:
Reflection of light follows two fundamental laws. First, the incident ray, reflected ray, and the normal at the point of incidence all lie in the same plane. Second, the angle of incidence is equal to the angle of reflection. Reflection is an important phenomenon because many optical instruments such as mirrors, periscopes, reflecting telescopes, and kaleidoscopes work on this principle. Plane mirrors form virtual images, while curved mirrors can produce magnified or diminished images depending on object position. Reflection helps in directing light rays, producing clear images, and increasing the efficiency of optical devices used in scientific observations and everyday applications.
Q2. Explain the mirror formula and its sign convention.
Answer:
The mirror formula relates the object distance (u), image distance (v), and focal length (f) of a spherical mirror. It is expressed as:
genui{“math_block_widget_always_prefetch_v2”:{“content”:”\frac{1}{f}=\frac{1}{v}+\frac{1}{u}”}}
According to the Cartesian sign convention, all distances are measured from the pole of the mirror. Distances measured in the direction of incident light are positive, while those measured opposite to it are negative. Heights above the principal axis are positive, and those below are negative. This sign convention helps in solving numerical problems consistently. The mirror formula is useful in determining image position, focal length, and object distance for both concave and convex mirrors.
Q3. What is magnification produced by a mirror? Write its expression.
Answer:
Magnification is the ratio of the height of the image to the height of the object. It indicates how much larger or smaller an image is compared to the object. For spherical mirrors, magnification is given by:
[
m=\frac{h_i}{h_o}=-\frac{v}{u}
]
where (h_i) is image height, (h_o) is object height, (v) is image distance, and (u) is object distance. Positive magnification indicates an erect image, while negative magnification indicates an inverted image. Magnification greater than one means the image is enlarged, whereas magnification less than one means it is diminished. This concept is widely used in designing mirrors for vehicles, telescopes, and cosmetic applications.
Q4. Differentiate between real and virtual images.
Answer:
A real image is formed when light rays actually meet after reflection or refraction. It can be obtained on a screen and is usually inverted. Examples include images formed by a concave mirror and convex lens when the object is beyond the focal point. A virtual image is formed when light rays only appear to meet at a point. It cannot be obtained on a screen and is generally erect. Examples include images formed by a plane mirror, convex mirror, and magnifying glass. Real images involve actual convergence of rays, whereas virtual images involve apparent convergence. Both types are important in optical instruments.
Q5. What is refraction of light? State Snell’s law.
Answer:
Refraction is the change in the direction of light when it passes obliquely from one transparent medium to another due to a change in its speed. The phenomenon is responsible for the apparent bending of objects seen through water and the working of lenses. Snell’s law states that for a given pair of media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction remains constant. Mathematically,
[
n=\frac{\sin i}{\sin r}
]
where (n) is the refractive index. This law helps in understanding image formation through lenses and optical fibers and is fundamental to geometrical optics.
Q6. Define refractive index. What factors affect it?
Answer:
The refractive index of a medium is the ratio of the speed of light in vacuum to its speed in that medium. It is represented by:
[
n=\frac{c}{v}
]
where (c) is the speed of light in vacuum and (v) is its speed in the medium. A higher refractive index indicates that light travels more slowly in the medium. The refractive index depends on the nature of the medium, wavelength of light, temperature, and pressure. Generally, denser materials have higher refractive indices. It plays a crucial role in determining how much light bends while entering a medium and is essential in lens design and optical communication systems.
Q7. What is total internal reflection? State its conditions.
Answer:
Total internal reflection is the phenomenon in which a light ray travelling from a denser medium to a rarer medium is completely reflected back into the denser medium instead of refracting. Two conditions are necessary for total internal reflection. First, light must travel from an optically denser medium to an optically rarer medium. Second, the angle of incidence must be greater than the critical angle of the medium pair. This phenomenon is used in optical fibers, prisms, endoscopes, and binoculars. Total internal reflection allows efficient transmission of light with minimal loss and forms the basis of modern communication technology.
Q8. Define critical angle. How is it related to refractive index?
Answer:
The critical angle is the angle of incidence in a denser medium for which the angle of refraction in the rarer medium becomes 90°. Beyond this angle, total internal reflection occurs. The critical angle depends on the refractive indices of the two media. For a denser medium with refractive index (n) and air as the rarer medium, the relation is:
[
\sin C=\frac{1}{n}
]
where (C) is the critical angle. Materials with higher refractive indices have smaller critical angles. The concept of critical angle is important in understanding optical fibers, prism-based instruments, and light-guiding systems used in communication networks.
Q9. Explain the principle and working of an optical fibre.
Answer:
An optical fibre is a thin transparent strand of glass or plastic that transmits light signals over long distances. It works on the principle of total internal reflection. The fibre consists of a core surrounded by cladding with a lower refractive index. Light entering the core at a suitable angle undergoes repeated total internal reflections and remains confined within the fibre. This allows efficient transmission with very little loss of energy. Optical fibres are widely used in telecommunications, medical endoscopy, internet networks, and industrial imaging. They provide high-speed communication, immunity to electromagnetic interference, and reliable signal transmission.
Q10. State the lens formula and explain its significance.
Answer:
The lens formula relates object distance, image distance, and focal length of a thin lens. It is expressed as:
\frac{1}{f}=\frac{1}{v}-\frac{1}{u}
This formula is valid for both convex and concave lenses when proper sign conventions are followed. It helps determine the position of the image formed by a lens for a given object position. The formula is widely used in solving numerical problems and designing optical devices such as cameras, microscopes, telescopes, and spectacles. Understanding the lens formula is essential for analyzing image formation and the optical behavior of lenses.
Q11. Define power of a lens. What is its SI unit?
Answer:
The power of a lens measures its ability to converge or diverge light rays. It is defined as the reciprocal of the focal length expressed in metres. Mathematically,
[P=\frac{1}{f}]
where (P) is power and (f) is focal length. The SI unit of power is dioptre (D). A convex lens has positive power because it converges light, while a concave lens has negative power because it diverges light. Lenses with shorter focal lengths have greater power. The concept of power is important in correcting vision defects and prescribing spectacles and contact lenses.
Q12. Distinguish between convex and concave lenses.
Answer:
A convex lens is thicker at the centre and thinner at the edges. It converges parallel light rays to a focus and is therefore called a converging lens. A concave lens is thinner at the centre and thicker at the edges. It diverges parallel light rays and is called a diverging lens. Convex lenses can form both real and virtual images depending on object position, whereas concave lenses always form virtual, erect, and diminished images. Convex lenses are used in cameras, microscopes, and telescopes, while concave lenses are commonly used to correct myopia and in optical instruments requiring light divergence.
Q13. What is dispersion of light? Explain with a prism.
Answer:
Dispersion is the splitting of white light into its constituent colours when it passes through a prism. Different colours have different wavelengths and hence different refractive indices in glass. As a result, each colour bends by a different amount. Violet light deviates the most, while red light deviates the least. The sequence of colours produced is known as the visible spectrum: Violet, Indigo, Blue, Green, Yellow, Orange, and Red (VIBGYOR). Dispersion demonstrates that white light is a combination of many colours. This phenomenon explains the formation of rainbows and is important in spectroscopy and optical analysis.
Q14. What is the angle of deviation in a prism?
Answer:
The angle of deviation is the angle between the direction of the incident ray and the emergent ray after passing through a prism. It measures how much the prism bends light. The angle of deviation depends on the prism angle, refractive index of the prism material, and wavelength of light. Different colours experience different deviations because of dispersion. At a particular angle of incidence, the deviation becomes minimum, known as the angle of minimum deviation. Measurement of the angle of deviation helps determine the refractive index of prism materials and is widely used in optical experiments and spectrometers.
Q15. Explain the working of a simple microscope.
Answer:
A simple microscope consists of a single convex lens of short focal length. It is used to observe small objects by producing a magnified virtual image. The object is placed between the optical centre and the focal point of the lens. The lens forms an erect, enlarged, and virtual image on the same side as the object. The magnification depends on the focal length of the lens; shorter focal lengths provide greater magnification. Simple microscopes are used by watchmakers, jewellers, and biologists for examining small details. They represent the simplest form of optical magnifying instruments.
Q16. Describe the construction and working of a compound microscope.
Answer:
A compound microscope consists of two convex lenses: an objective lens and an eyepiece. The objective lens has a short focal length and forms a real, inverted, and magnified image of the object. This image acts as the object for the eyepiece. The eyepiece further magnifies the image and produces a highly enlarged virtual image. The microscope provides much greater magnification than a simple microscope because it uses two stages of magnification. Compound microscopes are extensively used in biology, medicine, and research laboratories for studying microorganisms, cells, tissues, and other minute structures invisible to the naked eye.
Q17. What is an astronomical telescope? State its use.
Answer:
An astronomical telescope is an optical instrument used to observe distant celestial objects such as stars, planets, and galaxies. It consists of two convex lenses: an objective lens of large focal length and aperture, and an eyepiece of short focal length. The objective collects light from distant objects and forms a real image near its focus. The eyepiece magnifies this image to produce a final enlarged virtual image. The telescope provides high angular magnification, enabling detailed observation of distant objects. Astronomical telescopes are important tools in astronomy, space research, and the study of the universe.
Q18. What are the defects of vision? Name the common defects.
Answer:
Defects of vision occur when the eye cannot focus light correctly on the retina. As a result, objects may appear blurred. The common defects are myopia (short-sightedness), hypermetropia (long-sightedness), presbyopia (age-related defect), and astigmatism. Myopia causes difficulty in viewing distant objects, while hypermetropia affects near vision. Presbyopia occurs due to reduced flexibility of the eye lens with age. Astigmatism results from irregular curvature of the cornea or lens. These defects can usually be corrected using suitable lenses. Understanding vision defects is important in medical optics and the design of corrective eyewear.
Q19. Explain myopia and its correction.
Answer:
Myopia, or short-sightedness, is a defect in which a person can see nearby objects clearly but distant objects appear blurred. It occurs when the eyeball becomes elongated or the eye lens has excessive converging power. As a result, images of distant objects form in front of the retina instead of on it. Myopia is corrected using a concave lens of suitable focal length. The concave lens diverges incoming light rays slightly before they enter the eye, enabling the image to form on the retina. Corrective spectacles and contact lenses effectively restore normal distant vision.
Q20. Explain hypermetropia and its correction.
Answer:
Hypermetropia, or long-sightedness, is a defect in which a person can see distant objects clearly but has difficulty seeing nearby objects. It occurs when the eyeball is shorter than normal or the eye lens has insufficient converging power. Consequently, images of nearby objects form behind the retina. Hypermetropia is corrected using a convex lens of suitable focal length. The convex lens converges incoming light rays before they enter the eye, helping the image form exactly on the retina. Corrective lenses improve near vision and are commonly prescribed for reading and close-up work.