Chapter 16: Reflection of Light
Solved textbook questions with step-by-step explanations.
Grade 8 Q&A: Chapter 16: Reflection of Light
Welcome to the Questions and Answers section for Grade 8 Science, Chapter 16: "Reflection of Light." This chapter explores the fundamental phenomenon of light reflection, including its laws, image formation by plane and spherical mirrors (concave and convex), and their diverse applications in daily life.
Important Questions and Answers
Q1: Define 'Reflection of Light'.
Answer: Reflection of light is the phenomenon of bouncing back of light rays from a surface after striking it.
Q2: State the two Laws of Reflection.
Answer:
- The incident ray, the reflected ray, and the normal to the surface at the point of incidence all lie in the same plane.
- The angle of incidence (∠i) is always equal to the angle of reflection (∠r). (∠i = ∠r)
Q3: Differentiate between 'Regular Reflection' and 'Diffused Reflection'.
Answer:
- Regular Reflection: Occurs from a smooth, polished surface (e.g., plane mirror). Parallel incident rays reflect as parallel reflected rays, forming clear images.
- Diffused Reflection: Occurs from a rough or irregular surface (e.g., wall, paper). Parallel incident rays reflect in different directions, not forming clear images.
Q4: List four characteristics of the image formed by a Plane Mirror.
Answer: The image formed by a plane mirror is:
- Virtual: Cannot be obtained on a screen.
- Erect (Upright): Not inverted.
- Laterally Inverted: Left appears right, and vice-versa.
- Same Size: Size of image is same as object.
- Same Distance: Image distance behind mirror equals object distance in front.
Q5: Define 'Concave Mirror' and 'Convex Mirror'.
Answer:
- Concave Mirror: A spherical mirror whose inner (concave) surface is the reflecting surface. It is a converging mirror.
- Convex Mirror: A spherical mirror whose outer (convex) surface is the reflecting surface. It is a diverging mirror.
Q6: Define the following terms related to spherical mirrors: Pole, Center of Curvature, Principal Axis.
Answer:
- Pole (P): The geometrical center of the reflecting surface of a spherical mirror.
- Center of Curvature (C): The center of the hollow sphere of which the mirror is a part.
- Principal Axis: The straight line passing through the Pole (P) and the Center of Curvature (C) of the spherical mirror.
Q7: Define 'Principal Focus (F)' for a Concave Mirror and a Convex Mirror.
Answer:
- Concave Mirror: It is a point on the principal axis where all parallel rays of light converge after reflection.
- Convex Mirror: It is a point on the principal axis from which parallel rays of light appear to diverge after reflection.
Q8: What is 'Focal Length (f)'? How is it related to the Radius of Curvature (R)?
Answer: Focal length (f) is the distance between the Pole (P) and the Principal Focus (F) of a spherical mirror. It is related to the Radius of Curvature (R) by the formula: f = R/2 (Focal length is half of the radius of curvature).
Q9: List two uses of a Concave Mirror.
Answer:
- Used as shaving mirrors to see a magnified, erect image of the face.
- Used by dentists to see magnified images of teeth.
- Used in headlights of cars/scooters, searchlights, and torchlights to get a powerful parallel beam of light.
- Used in solar furnaces to concentrate sunlight.
Q10: List two uses of a Convex Mirror.
Answer:
- Used as rear-view mirrors (side mirrors) in vehicles to see traffic behind, as they provide a wider field of view and always form erect, diminished images.
- Used as security mirrors in shops to monitor a large area.
- Used as street light reflectors to spread light over a wider area.
Q11: What type of image is always formed by a Convex Mirror, regardless of object position?
Answer: A Convex Mirror always forms a virtual, erect, and diminished image, regardless of the object's position.
Q12: When does a Concave Mirror form a virtual and magnified image?
Answer: A Concave Mirror forms a virtual and magnified image only when the object is placed between its Principal Focus (F) and Pole (P).
Q13: What happens to a ray of light parallel to the principal axis after reflection from a concave mirror?
Answer: A ray of light parallel to the principal axis, after reflection from a concave mirror, passes through its Principal Focus (F).
Q14: What happens to a ray of light passing through the center of curvature of a spherical mirror after reflection?
Answer: A ray of light passing through the center of curvature (C) of a spherical mirror, after reflection, is reflected back along the same path.
Q15: What is 'Lateral Inversion'? Give an example.
Answer: Lateral inversion is the phenomenon where the left side of an object appears as the right side of its image, and vice-versa, when viewed in a plane mirror. Example: The word 'AMBULANCE' is often written laterally inverted on the front of ambulances so that drivers in front can read it correctly in their rear-view mirrors.
Q16: Differentiate between a 'Real Image' and a 'Virtual Image'.
Answer:
- Real Image: Formed when reflected (or refracted) rays actually meet at a point. Can be obtained on a screen. Always inverted.
- Virtual Image: Formed when reflected (or refracted) rays appear to meet at a point (they do not actually meet). Cannot be obtained on a screen. Always erect.
Q17: Why do convex mirrors provide a wider field of view?
Answer: Convex mirrors are diverging mirrors, meaning they spread out the reflected light rays. This divergence allows them to gather light from a wider area, providing a larger field of view compared to plane or concave mirrors. This property makes them ideal for rear-view mirrors in vehicles and security mirrors.
Q18: Where should an object be placed in front of a concave mirror to get a real, inverted, and same-sized image?
Answer: To get a real, inverted, and same-sized image from a concave mirror, the object should be placed at the Center of Curvature (C).
Q19: What type of mirror is used by dentists and why?
Answer: Dentists use concave mirrors. They use them because when a tooth is placed between the pole and principal focus of the concave mirror, it forms a magnified and erect image of the tooth, allowing the dentist to examine it clearly.
Q20: How does light travel? Does it need a medium?
Answer: Light travels in straight lines (rectilinear propagation). Unlike sound, light is an electromagnetic wave and does not need a medium to travel; it can travel through a vacuum.
Q21: If the angle of incidence is 30°, what will be the angle of reflection?
Answer: According to the second law of reflection, the angle of incidence is equal to the angle of reflection. So, if the angle of incidence is 30°, the angle of reflection will also be 30°.
Q22: What is the relationship between the radius of curvature (R) and focal length (f) of a spherical mirror?
Answer: The focal length (f) of a spherical mirror is half of its radius of curvature (R). The relationship is f = R/2.
Q23: Why are solar furnaces designed with concave mirrors?
Answer: Solar furnaces use large concave mirrors because concave mirrors are converging mirrors. They can collect a large amount of parallel sunlight and converge (focus) it to a single point (their principal focus), generating a very high temperature at that point for heating purposes.
Q24: What type of image is formed when an object is placed at infinity in front of a concave mirror?
Answer: When an object is placed at infinity in front of a concave mirror, the image is formed at the Principal Focus (F), and it is real, inverted, and highly diminished (point size).
Q25: Why is the word 'AMBULANCE' written laterally inverted on the front of ambulances?
Answer: The word 'AMBULANCE' is written laterally inverted on the front of ambulances so that drivers of vehicles in front, when looking at their rear-view mirrors (which are convex mirrors and produce laterally inverted images), can read the word correctly as 'AMBULANCE' and give way to the ambulance.
Exercise Solutions (From Screenshot Page 115)
Q1: Fill in the blanks.
- The bouncing back of light rays from a surface is called reflection.
- The angle of incidence is always equal to the angle of reflection.
- A smooth, polished surface causes regular reflection.
- A rough surface causes diffused reflection.
- The image formed by a plane mirror is always virtual and erect.
- The image formed by a plane mirror is laterally inverted.
- A spherical mirror whose inner surface is reflecting is a concave mirror.
- A spherical mirror whose outer surface is reflecting is a convex mirror.
- A concave mirror is a converging mirror.
- A convex mirror is a diverging mirror.
- The center of the reflecting surface of a spherical mirror is called the pole.
- The center of the hollow sphere of which the mirror is a part is called the center of curvature.
- The distance between the pole and the principal focus is called the focal length.
- Focal length is half of the radius of curvature.
- Dentists use concave mirrors.
- Rear-view mirrors in vehicles are convex mirrors.
Q2: Match the pairs.
(Note: As an AI, I cannot create interactive matching. I will provide the correct pairs.)
- Regular reflection - Plane mirror
- Diffused reflection - Rough surface
- Angle of incidence - Angle of reflection
- Virtual image - Cannot be obtained on screen
- Real image - Can be obtained on screen
- Concave mirror - Converging mirror
- Convex mirror - Diverging mirror
- Shaving mirror - Concave mirror
- Rear-view mirror - Convex mirror
- Solar furnace - Concave mirror
Q3: Give scientific reasons.
- We can see objects due to diffused reflection.
Reason: Diffused reflection occurs when light strikes a rough or irregular surface and scatters in various directions. This scattering of light allows the light from an object to reach our eyes from different angles, regardless of our viewing position. If only regular reflection occurred, we would only see clear images when our eyes are in a specific position, making most objects invisible from many angles. Thus, diffused reflection enables us to see non-luminous objects around us from almost any direction. - A concave mirror is used as a shaving mirror.
Reason: A concave mirror is used as a shaving mirror because when an object (the face) is placed between its pole (P) and principal focus (F), the concave mirror forms a virtual, erect, and magnified image. This magnified image allows a person to see a larger, clearer view of their face, making shaving easier and more precise. - A convex mirror is used as a rear-view mirror in vehicles.
Reason: A convex mirror is used as a rear-view mirror in vehicles because it always forms a virtual, erect, and diminished image of objects. More importantly, due to its diverging nature, it provides a much wider field of view compared to a plane mirror of the same size. This wider field of view allows the driver to see a larger area of traffic behind them, enhancing safety. - Solar furnaces use concave mirrors.
Reason: Solar furnaces are designed to generate high temperatures by concentrating sunlight. Concave mirrors are converging mirrors, meaning they can collect a large amount of parallel light rays (from the sun) and converge them to a single point, their principal focus. By placing the object to be heated at this focal point, a very high concentration of solar energy is achieved, leading to extremely high temperatures suitable for various applications like melting metals.
Q4: Answer the following questions.
- Explain the laws of reflection.
Answer: The reflection of light from any surface, whether smooth or rough, obeys two fundamental laws:- First Law of Reflection: The incident ray (the light ray falling on the surface), the reflected ray (the light ray bouncing back from the surface), and the normal (an imaginary line perpendicular to the surface at the point of incidence) all lie in the same plane.
- Second Law of Reflection: The angle of incidence (the angle between the incident ray and the normal) is always equal to the angle of reflection (the angle between the reflected ray and the normal). This is mathematically expressed as ∠i = ∠r.
- Explain the image formation by a plane mirror.
Answer: When an object is placed in front of a plane mirror, the image formed has the following characteristics:- Virtual: The image cannot be obtained on a screen because the reflected rays only appear to meet behind the mirror; they do not actually converge.
- Erect (Upright): The image is upright, meaning it is not inverted vertically.
- Laterally Inverted: The left side of the object appears as the right side of the image, and vice-versa.
- Same Size: The size of the image is exactly the same as the size of the object.
- Same Distance: The distance of the image formed behind the mirror is equal to the distance of the object in front of the mirror.
- Explain the types of spherical mirrors and their terminology.
Answer: Spherical mirrors are mirrors whose reflecting surface is a part of a hollow sphere. There are two types:- Concave Mirror: Its inner (concave) surface is the reflecting surface. It is a converging mirror as it converges parallel rays of light after reflection.
- Convex Mirror: Its outer (convex) surface is the reflecting surface. It is a diverging mirror as it diverges parallel rays of light after reflection.
- Pole (P): The center of the mirror's reflecting surface.
- Center of Curvature (C): The center of the sphere from which the mirror is cut.
- Radius of Curvature (R): The distance between P and C.
- Principal Axis: The straight line passing through P and C.
- Principal Focus (F): For a concave mirror, it's where parallel rays converge; for a convex mirror, it's where parallel rays appear to diverge from.
- Focal Length (f): The distance between P and F. It is half of the radius of curvature (f = R/2).
- Explain the uses of concave and convex mirrors.
Answer: Uses of Concave Mirrors:- Shaving mirrors/Dentists' mirrors: To get a magnified, erect image when the object is placed between P and F.
- Headlights of vehicles/Searchlights/Torchlights: The light source is placed at the focus to produce a powerful parallel beam of light.
- Solar furnaces: To concentrate sunlight at the focus to generate high temperatures.
- Rear-view mirrors in vehicles: They always form virtual, erect, and diminished images and provide a wider field of view, allowing drivers to see more traffic behind them.
- Shop security mirrors: Used in shops to monitor a large area from a single point.
- Street light reflectors: To spread light over a wider area.
Q5: Differentiate between.
- Real Image and Virtual Image
Feature Real Image Virtual Image Formation Formed when reflected/refracted rays actually meet Formed when reflected/refracted rays appear to meet Screen Can be obtained on a screen Cannot be obtained on a screen Orientation Always inverted (upside down) Always erect (upright) Example Image formed by concave mirror (object beyond F) Image formed by plane mirror or convex mirror - Concave Mirror and Convex Mirror
Feature Concave Mirror Convex Mirror Reflecting Surface Inner (bulges inwards) Outer (bulges outwards) Nature Converging mirror (brings parallel rays together) Diverging mirror (spreads parallel rays apart) Image Formed Can form real/virtual, inverted/erect, magnified/diminished images Always forms virtual, erect, and diminished images Uses Shaving mirrors, headlights, solar furnaces Rear-view mirrors, security mirrors
References
- Maharashtra State Board Science and Technology Standard Eight Textbook (Specific Edition/Year) - Chapter 16: Reflection of Light.
- Maharashtra State Board 8th Standard Science Syllabus.
- Balbharati Science and Technology Textbook.