Important Questions: Light: Mirrors and Lenses | Science Curiosity Class 8 - New NCERT PDF Download

Q1. What are spherical mirrors? Differentiate between concave and convex mirrors with examples of their uses in daily life.
Ans: Spherical mirrors are mirrors shaped like a part of a hollow sphere. A concave mirror has a reflecting surface that curves inward, like the inside of a spoon, and can form enlarged or inverted images depending on the object’s distance. A convex mirror has a reflecting surface that curves outward, like the back of a spoon, and always forms erect, diminished images. Concave mirrors are used in dental mirrors to provide an enlarged view of teeth, while convex mirrors are used as side-view mirrors in vehicles to offer a wider view of traffic.

Q2. Describe how you can distinguish between a concave mirror and a convex mirror by observing their side views.
Ans: A concave mirror can be identified by its reflecting surface curving inward, resembling a cave or the inner surface of a bowl. A convex mirror has its reflecting surface curving outward, bulging like the outer surface of a sphere.

Q3. How does the image formed by a concave mirror change when an object is moved from close to the mirror to farther away? Explain with reference to size and orientation.
Ans: When an object is close to a concave mirror, the image is erect, enlarged, and virtual. As the object moves farther away, the image becomes inverted, initially enlarged, and then diminishes in size, becoming real.

Q4. Why does a convex mirror always form an erect and diminished image, regardless of the object’s position?
Ans: A convex mirror always forms an erect and diminished image because its outward-curved surface causes reflected light rays to diverge, making the image appear smaller and upright regardless of the object’s distance.

Q5. State the two laws of reflection and explain how they apply to both plane and spherical mirrors.
Ans: The two laws of reflection are: (1) The angle of incidence is equal to the angle of reflection. (2) The incident ray, the reflected ray, and the normal at the point of incidence lie in the same plane. These laws apply to plane and spherical mirrors, ensuring that each ray reflects predictably, though the curved surfaces of spherical mirrors cause parallel rays to converge (concave) or diverge (convex).

Q6. How can you experimentally demonstrate that the angle of incidence equals the angle of reflection using a plane mirror? Describe the setup and observations.
Ans: Use a plane mirror, a torch, a comb with a slit to create a thin beam of light, and a white paper sheet. Place the mirror upright on the paper, direct the light beam onto the mirror, and observe the reflected beam. Draw the incident and reflected rays, mark the normal (a line perpendicular to the mirror at the point of incidence), and measure the angles of incidence and reflection. Repeating this with different angles shows that the angle of incidence always equals the angle of reflection.

Q7. What happens when multiple parallel beams of light fall on a concave mirror compared to a convex mirror? Explain the terms “converge” and “diverge” in this context.
Ans: When multiple parallel beams of light fall on a concave mirror, they converge, meaning they come together at a point after reflection due to the inward-curved surface. On a convex mirror, the beams diverge, meaning they spread apart after reflection due to the outward-curved surface. “Converge” refers to light rays meeting at a point, while “diverge” refers to rays spreading out.

Q8. How can a concave mirror be used to concentrate sunlight and ignite a piece of paper? Describe the activity and its significance.
Ans: Hold a concave mirror facing the sun to reflect and focus sunlight onto a thin sheet of paper. Adjust the paper’s distance until a sharp, bright spot forms. Holding it steady for a few minutes concentrates enough heat to ignite the paper, producing smoke. This demonstrates the concave mirror’s ability to converge light, which is significant for applications like solar concentrators used for heating or generating electricity.

Q9. What is a lens? Differentiate between a convex lens and a concave lens in terms of their shape and effect on light.
Ans: A lens is a transparent material, usually glass or plastic, with curved surfaces that refract light. A convex lens is thicker at the center than at the edges and converges light beams, forming real or virtual images depending on the object’s position. A concave lens is thinner at the center and diverges light beams, always forming erect, diminished, virtual images.

Q.10 How does the image formed by a convex lens change as an object is moved farther from it? Compare this with the image formed by a concave lens.
Ans: For a convex lens, when an object is close, the image is erect, enlarged, and virtual; as the object moves farther, the image becomes inverted, initially enlarged, then diminishes, and becomes real. For a concave lens, the image is always erect, diminished, and virtual, with the size decreasing slightly as the object moves farther.

Q.11 Explain how a convex lens can converge light beams and why it is called a converging lens. How does a concave lens differ in its effect on light?
Ans: A convex lens converges light beams by bending them inward to meet at a point, earning it the name “converging lens” due to its thicker center. A concave lens diverges light beams, spreading them outward due to its thinner center, making it a “diverging lens.”

Q.12 A ray of light is incident on a plane mirror at an angle of 30° with the mirror surface. What is the angle of reflection, and how do you calculate it using the laws of reflection?
Ans: The angle of incidence is measured relative to the normal, which is perpendicular to the mirror surface. If the ray makes a 30° angle with the mirror, the angle of incidence is 90° − 30° = 60°. By the first law of reflection, the angle of reflection equals the angle of incidence, so the angle of reflection is 60°.

Q.13 Why are convex mirrors preferred for side-view mirrors in vehicles? Explain how their image formation properties contribute to safety.
Ans: Convex mirrors are preferred for side-view mirrors because they always form erect, diminished images, providing a wider field of view compared to plane mirrors. This allows drivers to see more of the traffic behind and around them, reducing blind spots and enhancing safety by helping prevent collisions.

Q.14 Why convex mirrors are used in vehicles? 
Ans: Convex mirrors are used in vehicles because the image formed by convex mirror is always erect and smaller in size. 

Q.15 Explain the two laws of reflection. 
Ans: There are two laws of reflection: the angle of incidence is equal to the angle of reflection and incident rays, reflected rays and normal rays drawn at the point of incidence to the reflecting surface lies in the same plane.