Light Exercise Solutions | KalviGavani

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This page contains complete solutions for the Samacheer Kalvi Class 8 Science Chapter 3 (Light) exercise. Work through each section in order -- start with MCQs to test recall, then progress through fill in the blanks, true or false, and detailed answers. Use the Key Concepts table for a quick refresher before attempting questions, and check the Common Mistakes and Exam Checklist sections before your exam.

Key Concepts (Quick Recap)

I. Choose the Best Answer (MCQs)

Q1. Mirrors having a curved reflecting surface are called as

• (a) Plane mirrors (b) Spherical mirrors (c) Simple mirrors (d) None of the above

Q2. The spherical mirror with a reflecting surface curved inward is called

• (a) Convex mirror (b) Concave mirror (c) Curved mirror (d) None of the above

Q3. The centre of a sphere of which the reflecting surface of a spherical mirror is a part is called

• (a) Pole (b) Centre of curvature (c) Radius of curvature (d) Aperture

Q4. The spherical mirror used as a rear view mirror in the vehicle is

• (a) Concave mirror (b) Convex mirror (c) Plane mirror (d) None of the above

Q5. The imaginary line passing through the centre of curvature and pole of a spherical mirror is called

• (a) Centre of curvature (b) Pole (c) Principal axis (d) Radius of curvature

Q6. The distance from the pole to the focus is called

• (a) Pole length (b) Focal length (c) Principal axis (d) None of the above

Q7. Focal length is equal to half of the

• (a) Centre of curvature (b) Axis (c) Radius of curvature (d) None of the above

Q8. If the focal length of a spherical mirror is 10 cm, what is the value of its radius of curvature?

• (a) 10 cm (b) 5 cm (c) 20 cm (d) 15 cm

Q9. If the image and object distance is same, then the object is placed at

• (a) Infinity (b) At F (c) Between f and P (d) At C

Q10. The refractive index of water is

• (a) 1.0 (b) 1.33 (c) 1.44 (d) 1.52

II. Fill in the Blanks

1. The spherical mirror used in a beauty parlour as make-up mirror is concave mirror. 2. Geometric centre of the spherical mirror is pole. 3. Nature of the images formed by a convex mirror is smaller, virtual and erect. 4. The mirror used by the ophthalmologist to examine the eye is concave mirror. 5. If the angle of incidence is 45 degree, then the angle of reflection is 45 degree (law of reflection). 6. Two mirrors are parallel to each other, then the number of images formed is infinite. 7. The splitting of white light into seven colours is called dispersion. 8. The refractive index of glass is approximately 1.5. 9. The distance between the pole and centre of curvature is called radius of curvature. 10. The mirror used by dentists to see a magnified image of teeth is concave mirror.

III. True or False

State whether the following statements are true or false. If false, provide the correction.

IV. Match the Following

Match A:

Match B:

V. Assertion and Reason

VI. Answer in Brief

Q1. What is called a spherical mirror? Answer: Spherical mirrors are a form of curved mirrors. When the curved mirror is part of a sphere, it is termed a spherical mirror. They are of two types: concave (curved inward) and convex (curved outward). The reflecting surface of a concave mirror bends inward like the inside of a spoon, while that of a convex mirror bends outward like the back of a spoon. Spherical mirrors follow the same laws of reflection as plane mirrors, but their curved surface causes parallel rays to either converge or diverge after reflection.

Q2. Define focal length. Answer: The distance between the pole and the principal focus of a spherical mirror is called the focal length (f). It is related to the radius of curvature by the formula f = R/2, meaning the focal length is always exactly half the radius of curvature. For a concave mirror, the focus is in front of the mirror where reflected rays actually meet, while for a convex mirror, the focus is behind the mirror where reflected rays appear to diverge from.

Q3. The radius of curvature of a spherical mirror is 25 cm. Find its focal length. Answer: Given: R = 25 cm. Using the relation f = R/2, we get f = 25/2 = 12.5 cm. This means light rays parallel to the principal axis will converge (or appear to diverge) at a point 12.5 cm from the pole of the mirror.

Q4. Give two applications of a concave and convex mirror. Answer: Concave mirrors: (1) Used as makeup/shaving mirrors for magnified images. (2) Used in torches, search lights, and headlights to direct light. (3) Used by dentists to obtain a magnified view of teeth. (4) Used in solar furnaces to concentrate sunlight for heating. Convex mirrors: (1) Used as rear-view mirrors in vehicles for wider field of view. (2) Used in hallways of buildings at sharp turns to help see around corners. (3) Used on roads at curved sections for traffic safety.

Q5. State the laws of reflection. Answer: 1. The incident ray, reflected ray, and normal at the point of incidence all lie in the same plane. This means all three rays can be drawn on a single flat surface. 2. The angle of incidence equals the angle of reflection (angle i = angle r). Both angles are always measured from the normal, never from the mirror surface itself. These laws hold true for all reflecting surfaces -- whether smooth (regular reflection) or rough (irregular reflection), and for all types of mirrors including plane, concave, and convex mirrors.

Q6. If two plane mirrors are inclined at 45 degree, find the number of images formed. Answer: Number of images = (360/theta) - 1 = (360/45) - 1 = 8 - 1 = 7 images. This formula applies when 360/theta is an even integer and the object is placed asymmetrically between the mirrors. Each mirror creates multiple reflections, and the images are arranged symmetrically around the point between the two mirrors.

Q7. Define the refractive index of a medium. Answer: The refractive index (mu) is the ratio of the speed of light in vacuum (or air) to the speed of light in that medium: mu = c/v. It indicates how much light bends when entering the medium -- a higher refractive index means greater bending and slower speed of light in that medium. For example, the refractive index of water is 1.33, which means light travels 1.33 times slower in water than in vacuum. The refractive index is always greater than or equal to 1 for any material.

Q8. State Snell's law of refraction. Answer: 1. The incident ray, refracted ray, and normal at the point of incidence all lie in the same plane. 2. The ratio sin(i)/sin(r) equals the refractive index (mu), which is a constant for a given pair of media. This is expressed as: mu = sin i / sin r. Snell's law is fundamental to understanding how lenses work, how prisms disperse light, and why objects appear shifted when viewed through water or glass. It applies whenever light passes from one transparent medium to another at an angle.

VII. Answer in Detail

Q1. Explain the images formed by a concave mirror.

The image formed by a concave mirror depends entirely on where the object is placed relative to the principal focus (F) and centre of curvature (C). When the object is beyond C, the image is real, inverted, and diminished, forming between F and C. As the object moves closer to C, the image grows larger and remains real and inverted. At C, the image is the same size as the object. Between C and F, the image becomes magnified and real. When the object is exactly at F, the reflected rays are parallel and the image is formed at infinity. When the object is between F and P, the image becomes virtual, erect, and magnified -- this is the only position where a concave mirror produces a virtual image. This principle is why concave mirrors are used as shaving or makeup mirrors: the face is placed between F and P, giving a magnified, upright image.

Q2. What is reflection? Write short notes on regular and irregular reflection. Answer: Reflection is the bouncing back of light rays when they fall on a surface. When light hits any surface, some part is absorbed and the rest is reflected. The nature of the reflected light depends on the smoothness of the surface.

Regular reflection: When parallel light rays fall on a smooth, polished surface (like a plane mirror or still water), they reflect parallel to each other. The angle of incidence equals the angle of reflection for each ray, forming a clear, well-defined image. Example: reflection from a plane mirror, a polished metal surface, or calm water. Regular reflection is what allows us to see our image in a mirror.

Irregular (diffused) reflection: When light falls on a rough or irregular surface (like a wall, a piece of paper, or a road), each tiny region reflects at different angles. Although each individual ray still follows the law of reflection, the collective effect scatters the light in all directions. The image is not clear or well-defined. Example: reflection from a wall, a newspaper, or an unpolished surface. Irregular reflection is the reason we can see objects from different angles -- light reaching them is scattered in all directions, making them visible from anywhere in the room.

Q3. Explain the working of a periscope. Answer: A periscope is an optical instrument used for viewing objects that are over, around, or through an obstacle. It is most commonly used in submarines to view objects above the water surface while remaining submerged.

Construction: A periscope consists of a long, hollow, L-shaped tube. Two plane mirrors are fixed inside the tube at the two bends, one at the top and one at the bottom. Both mirrors are positioned at exactly 45 degrees to the principal axis of the tube.

Key principle: The periscope works entirely on the principle of reflection. Two reflections at 45 degrees change the direction of light by 90 degrees each, resulting in a total change of 180 degrees -- the light enters from one direction and exits in the same direction at the other end.

Q4. What is dispersion? Explain in detail. Answer: Dispersion is the splitting of white light into its seven constituent colours when passing through a transparent medium like a prism. It occurs because different colours have different wavelengths and travel at different speeds in a medium. Since refraction depends on the speed of light, each colour bends by a different amount, resulting in separation.

Mechanism: When white light enters a glass prism, it slows down and bends towards the normal. Red light has the longest wavelength and bends the least, while violet light has the shortest wavelength and bends the most. As light exits the prism, it bends away from the normal, further increasing the separation between colours. The seven colours are Violet, Indigo, Blue, Green, Yellow, Orange, Red (VIBGYOR), arranged from most deviated to least deviated.

Rainbow formation: A rainbow is a natural example of dispersion. Sunlight enters tiny water droplets suspended in the atmosphere, undergoes refraction and internal reflection inside the droplet, and then refracts again when exiting. This double refraction disperses the white light into its component colours, creating the familiar arc of VIBGYOR in the sky.

Key point: Dispersion proves that white light is not a single colour but a mixture of seven colours. Newton's prism experiment was the first to demonstrate this conclusively.

Q5. Speed of light in air is 3 x 10^8 m/s and the refractive index of a medium is 1.5. Find the speed of light in the medium. Solution: mu = c/v 1.5 = (3 x 10^8) / v v = (3 x 10^8) / 1.5 = 2 x 10^8 m/s

This means light travels at 2 x 10^8 m/s in the medium, which is approximately two-thirds of its speed in air. The refractive index of 1.5 (typical for glass) tells us that light slows down by a factor of 1.5 when entering this medium from air. The higher the refractive index, the slower light travels in that medium and the more it bends.

Additional Practice Questions

Very Short Answers

Q1. How does light travel? Answer: Light travels in straight lines (rectilinear propagation). This is demonstrated by the formation of shadows and the fact that we cannot see around corners.

Q2. What is a rainbow? Answer: A rainbow is a large arc of seven colours seen in the sky, formed by dispersion of sunlight through water droplets acting as tiny prisms.

Q3. Name the triangular piece of glass that splits white light. Answer: Prism.

Q4. The angle between incident ray and reflected ray is 60 degree. What is the angle of incidence? Answer: angle i + angle r = 60 degree. Since angle i = angle r, angle i = 60/2 = 30 degree.

Short Answers

Q5. Distinguish between real and virtual images.

Q6. A convex rear-view mirror is preferred over a plane mirror in a car. Why? Answer: A convex mirror forms a smaller, virtual image, which allows a much larger area to be visible than a plane mirror can show. This wider field of view helps the driver see more of the road behind, including vehicles in adjacent lanes, making driving safer.

Q7. The radius of curvature of a spherical mirror is 18 cm. What is the focal length? Answer: f = R/2 = 18/2 = 9 cm.

Q8. If two mirrors are placed at an inclination of 30 degree, how many images can be seen? Answer: N = (360/30) - 1 = 12 - 1 = 11 images.

Q9. What is the speed of light in diamond if its refractive index is 2.41? Answer: v = c/mu = (3 x 10^8)/2.41 = 1.24 x 10^8 m/s.

Common Mistakes

• Measuring angles from the mirror, not the normal: Always measure angles from the normal (perpendicular at the point of incidence), not from the mirror surface.
• Confusing concave and convex mirror image properties: Concave mirrors can form both real and virtual images; convex mirrors always form virtual, erect, diminished images.
• Forgetting f = R/2: Focal length is always half the radius of curvature.
• Confusing refraction with reflection: Refraction is bending of light at a medium boundary; reflection is bouncing back.
• Saying all mirrors produce virtual images: Only convex mirrors always produce virtual images. Concave mirrors can produce real images too.
• Confusing regular and irregular reflection: Regular reflection requires smooth surfaces; irregular reflection occurs on rough surfaces.
• Using wrong sign convention in mirror formula: For concave mirrors, f and R are positive (in the New Cartesian Sign Convention, they are negative). Always confirm which sign convention the question expects before substituting values.
• ❌ Wrong approach: Substituting f = +10 cm in the New Cartesian Sign Convention for a concave mirror.
• ✅ Correct approach: In the New Cartesian Sign Convention, concave mirror f = -10 cm; in the coordinate geometry convention used by many TN Board questions, f = +10 cm. Check the convention before solving.
• Confusing refractive index with speed of light: A higher refractive index means light travels slower in that medium, not faster.
• ❌ Wrong approach: Saying diamond (mu = 2.41) lets light travel faster than glass (mu = 1.5).
• ✅ Correct approach: Diamond slows light more (v = 1.24 x 10^8 m/s) compared to glass (v = 2 x 10^8 m/s) because its refractive index is higher.

Exam Checklist

• [ ] Can you state and apply the laws of reflection?
• [ ] Can you identify concave vs convex mirrors and their uses?
• [ ] Can you draw ray diagrams for concave mirrors at different object positions?
• [ ] Can you calculate focal length from radius of curvature and vice versa?
• [ ] Can you calculate number of images from mirror inclination angle?
• [ ] Can you define refractive index and calculate speed of light in a medium?
• [ ] Can you explain dispersion and formation of rainbow?
• [ ] Can you distinguish between regular and irregular reflection?
• [ ] Can you explain the working of a periscope and kaleidoscope?
• [ ] Can you apply Snell's law to solve numerical problems?
• [ ] Can you distinguish between real and virtual images with examples?
• [ ] Can you correctly measure angles from the normal (not the mirror surface) in ray diagrams?