CBSE Class 10 Science (2026–27)
Chapter 9: Light – Reflection and Refraction
20 Important Questions and Answers
The chapter covers reflection by spherical mirrors, refraction through lenses, mirror and lens formulae, magnification, refractive index, and applications of mirrors and lenses. These topics are part of the CBSE Class 10 Science syllabus.
Q1. State the laws of reflection of light.
Answer:
The laws of reflection describe how light behaves when it strikes a reflecting surface. The first law states that the angle of incidence is always equal to the angle of reflection. The second law states that the incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane. These laws apply to plane mirrors as well as spherical mirrors. Reflection enables us to see objects because light reflected from them enters our eyes. These laws form the basis for understanding image formation in mirrors and many optical instruments used in daily life.
Q2. Differentiate between a concave mirror and a convex mirror.
Answer:
A concave mirror has its reflecting surface curved inward toward the centre of the sphere, while a convex mirror has its reflecting surface curved outward. A concave mirror can form both real and virtual images depending on the position of the object. It is used in shaving mirrors, headlights, and solar furnaces. A convex mirror always forms a virtual, erect, and diminished image irrespective of the object’s position. Due to its wider field of view, it is commonly used as a rear-view mirror in vehicles. Thus, both mirrors have distinct properties and practical applications.
Q3. What is the principal focus of a concave mirror?
Answer:
The principal focus of a concave mirror is the point on its principal axis where rays of light parallel to the principal axis converge after reflection. It is represented by the letter F. Since reflected rays actually meet at this point, it is called a real focus. The distance between the pole and the principal focus is known as the focal length. The concept of principal focus is important in ray diagrams and image formation by mirrors. The focusing property of concave mirrors makes them useful in torches, vehicle headlights, and reflecting telescopes.
Q4. Why are convex mirrors used as rear-view mirrors in vehicles?
Answer:
Convex mirrors are used as rear-view mirrors because they provide a wider field of view compared to plane or concave mirrors. They always form virtual, erect, and diminished images of objects behind the vehicle. This allows drivers to see a larger area of traffic and surroundings, improving safety while driving. Since the images are smaller, more vehicles and objects can be observed at the same time. The wider visibility helps drivers monitor traffic conditions effectively and reduces blind spots. Therefore, convex mirrors are ideal for use in automobiles.
Q5. Explain the mirror formula and its significance.
Answer:
The mirror formula establishes a relationship between 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}{v}+\frac{1}{u}=\frac{1}{f}”}}
This formula is valid for both concave and convex mirrors when the proper sign convention is followed. It helps determine the position of an image formed by a mirror without drawing ray diagrams. The formula is widely used in numerical problems related to image formation. Understanding this relationship is essential for solving board examination questions and studying optical instruments based on mirrors.
Q6. What is magnification produced by a mirror?
Answer:
Magnification is the ratio of the height of the image to the height of the object. It indicates whether the image formed is enlarged or diminished. For mirrors, magnification is given by:
m=-\frac{v}{u}
A positive magnification indicates that the image is virtual and erect, whereas a negative magnification indicates a real and inverted image. If the magnitude of magnification is greater than one, the image is enlarged. If it is less than one, the image is diminished. Magnification helps in analyzing image characteristics formed by spherical mirrors.
Q7. What is refraction of light?
Answer:
Refraction is the bending of light when it passes obliquely from one transparent medium to another having a different optical density. This bending occurs because the speed of light changes when it enters a new medium. Light bends towards the normal when it travels from a rarer medium to a denser medium and away from the normal when moving from a denser to a rarer medium. Refraction is responsible for many everyday phenomena, such as the apparent bending of a pencil in water and the functioning of lenses in spectacles and cameras.
Q8. State the laws of refraction of light.
Answer:
Refraction follows two important laws. The first law states that the incident ray, refracted ray, and the normal at the point of incidence lie in the same plane. The second law, known as 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. This constant is called the refractive index of the medium. These laws explain how light changes direction while passing through different media and form the basis for understanding lenses and optical devices.
Q9. Define refractive index.
Answer:
The refractive index of a medium is a measure of how much the medium slows down light compared to its speed in vacuum. It is represented by the symbol ‘n’. A medium with a higher refractive index is optically denser and bends light more strongly. The refractive index helps determine the extent of refraction when light passes between two media. It is an important quantity in optics and is used in designing lenses, microscopes, telescopes, and other optical instruments. Understanding refractive index helps explain why light bends differently in different materials.
Q10. Differentiate between a convex lens and a concave lens.
Answer:
A convex lens is thicker at the centre and thinner at the edges. It converges parallel rays of light to a point and is therefore called a converging lens. A concave lens is thinner at the centre and thicker at the edges. It diverges parallel rays of light and is known as a diverging lens. Convex lenses can form both real and virtual images, whereas concave lenses always form virtual, erect, and diminished images. Convex lenses are used in cameras and magnifying glasses, while concave lenses are used to correct myopia.
Q11. What is the principal focus of a convex lens?
Answer:
The principal focus of a convex lens is the point on the principal axis where rays of light parallel to the principal axis converge after refraction through the lens. It is represented by F. A convex lens has two principal foci, one on each side of the lens. The distance between the optical centre and the principal focus is called the focal length. The principal focus is important for constructing ray diagrams and determining image positions. The converging nature of a convex lens makes it useful in microscopes, cameras, and telescopes.
Q12. State the lens formula.
Answer:
The lens formula gives the relationship between object distance (u), image distance (v), and focal length (f) of a spherical lens. It is expressed as:
\frac{1}{v}-\frac{1}{u}=\frac{1}{f}
This formula is valid for both convex and concave lenses when sign conventions are correctly applied. It is useful in finding the image position, focal length, or object distance in numerical problems. The lens formula simplifies calculations and helps students understand the behavior of light through lenses. It is one of the most important formulas in this chapter for board examinations.
Q13. What is power of a lens?
Answer:
The power of a lens indicates its ability to converge or diverge light rays. It is defined as the reciprocal of the focal length measured in metres. Its SI unit is dioptre (D). Mathematically,
P=\frac{1}{f}
A convex lens has positive power, while a concave lens has negative power. Lenses with shorter focal lengths have greater power. Power is used by opticians while prescribing spectacles. It helps determine the strength of a lens required to correct vision defects. Understanding lens power is important in optics and practical applications involving lenses.
Q14. What is magnification produced by a lens?
Answer:
Magnification produced by a lens is the ratio of the height of the image to the height of the object. It helps determine whether the image is enlarged or diminished. For a lens, magnification is given by:
[
m=\frac{v}{u}
]
A positive magnification indicates an erect image, while a negative magnification indicates an inverted image. Magnification plays an important role in optical instruments such as microscopes, telescopes, and cameras. By calculating magnification, one can determine the size and orientation of the image formed by a lens. It is a commonly asked concept in board examinations.
Q15. Why is a concave mirror used as a shaving mirror?
Answer:
A concave mirror is used as a shaving mirror because it can produce a magnified, erect, and virtual image when the face is placed between the pole and the principal focus of the mirror. The enlarged image allows a person to see facial details clearly, making shaving or applying makeup easier and more accurate. The mirror provides better visibility compared to a plane mirror. This property is also utilized by dentists for examining teeth. Therefore, the ability to form enlarged images makes concave mirrors highly useful for personal grooming purposes.
Q16. Explain why a pencil appears bent when partially immersed in water.
Answer:
A pencil appears bent when partially immersed in water because of the refraction of light. Light rays coming from the submerged portion of the pencil travel from water, a denser medium, into air, a rarer medium. As the rays pass from water to air, they bend away from the normal. Due to this bending, the submerged portion appears raised from its actual position. The eye traces the refracted rays backward in a straight line, creating an apparent image at a different location. This optical illusion makes the pencil appear bent at the water surface.
Q17. What are the uses of concave mirrors?
Answer:
Concave mirrors have several practical applications due to their converging nature. They are used as shaving and makeup mirrors because they produce enlarged images. Dentists use them to examine teeth clearly. Concave mirrors are also used as reflectors in torches, searchlights, and vehicle headlights because they can produce a strong parallel beam of light. In solar furnaces, they concentrate sunlight at a single point to generate heat. Reflecting telescopes also employ concave mirrors for collecting and focusing light from distant objects. Thus, concave mirrors are widely used in scientific and everyday applications.
Q18. What are the uses of convex mirrors?
Answer:
Convex mirrors are widely used because they provide a broad field of view. Their most common application is as rear-view mirrors in vehicles, where they help drivers observe traffic behind them. Convex mirrors are also used at road intersections and sharp turns to improve visibility and reduce accidents. In shops, malls, and security systems, they are used for surveillance because they allow a large area to be monitored at once. Since they always produce virtual, erect, and diminished images, they are ideal wherever a wider view of the surroundings is required.
Q19. What is the Cartesian sign convention?
Answer:
The Cartesian sign convention is a standard method used in optics for assigning signs to distances and heights while solving numerical problems. All distances are measured from the pole of a mirror or the optical centre of a lens. Distances measured in the direction of incident light are taken as positive, while those measured opposite to the direction of incident light are taken as negative. Heights measured above the principal axis are positive, and those measured below it are negative. This convention ensures uniformity and accuracy in applying mirror and lens formulas.
Q20. Why is the chapter “Light – Reflection and Refraction” important in daily life?
Answer:
The concepts of reflection and refraction are closely related to many devices used in everyday life. Mirrors help in personal grooming, vehicle safety, and scientific instruments. Lenses are essential in spectacles, cameras, microscopes, telescopes, and mobile phone cameras. Refraction explains phenomena such as the apparent bending of objects in water and image formation by lenses. Understanding these concepts helps us appreciate how optical instruments work. The chapter also develops problem-solving skills through formulas, ray diagrams, and numerical applications, making it one of the most important and scoring chapters in CBSE Class 10 Science.