Case Study Based Questions: Light: Mirrors and Lenses | Science Class 8 PDF Download

Case Study - 1

During a school science experiment, Priya explores the properties of light using curved mirrors and lenses. She uses a shiny, bowl-shaped mirror to reflect light from a torch onto a white sheet, observing that parallel light beams come together at a single point. When she places a small toy near the same mirror, she notices the image is large and upright when the toy is close, but it flips upside down when moved farther away. Next, Priya tries a different mirror that bulges outward and finds that the toy’s image is always small and upright, no matter where she places it. She then uses a transparent, curved object that is thicker in the middle to project an image of a distant tree onto a piece of paper, noticing that the image is small and inverted. Priya also experiments with a thin, curved glass piece that spreads out light beams, creating a smaller, upright image of the toy. Through these experiments, Priya learns how curved surfaces manipulate light to create different images, which are used in devices like telescopes and vehicle mirrors.

Questions:

1. What type of mirror is Priya using when parallel light beams come together at a single point?
   (a) Convex mirror
   (b) Plane mirror
   (c) Concave mirror
   (d) None of these
   Ans: (c)
   Sol: A concave mirror, which curves inward, reflects parallel light beams to converge at a single point, known as the focus, due to its converging properties.

2. What happens to the image in the bowl-shaped mirror when Priya moves the toy farther away?
   (a) It remains erect and enlarged
   (b) It becomes inverted and may vary in size
   (c) It remains erect and diminished
   (d) It disappears
   Ans: (b)
   Sol: In a concave mirror, when the object (toy) is moved farther away, the image changes from erect and enlarged (when close) to inverted, with size varying depending on the distance (initially enlarged, then diminished).

3. Why does the outward-bulging mirror always produce a small, upright image of the toy?
   (a) It converges light to a single point
   (b) It diverges light, forming a diminished virtual image
   (c) It reflects light like a plane mirror
   (d) It scatters light in all directions
   Ans: (b)
   Sol: The outward-bulging mirror is a convex mirror, which diverges light rays, always forming an erect, diminished, virtual image regardless of the object’s position.

4. What type of transparent object is Priya using to project a small, inverted image of a distant tree?
   (a) Concave lens
   (b) Convex lens
   (c) Plane glass
   (d) Cylindrical lens
   Ans: (b)
   Sol: A convex lens, thicker in the middle, converges light from a distant object (like a tree) to form a small, inverted, real image on a paper, as used in devices like telescopes.

5. What is the effect of the thin, curved glass piece that spreads out light beams in Priya’s experiment?
   (a) It forms an erect, enlarged image
   (b) It forms an inverted, real image
   (c) It forms an erect, diminished, virtual image
   (d) It reflects light like a mirror
   Ans: (c)
   Sol: The thin, curved glass piece is a concave lens, which diverges light beams and always forms an erect, diminished, virtual image of the object.

Case Study - 2

At a science fair, a student named Arjun sets up an exhibit to demonstrate the properties of mirrors and lenses. He uses a shiny metallic spoon to show how curved surfaces reflect light differently from flat ones. When he holds the inner side of the spoon close to his face, he notices his image appears inverted, but when he uses the outer side, the image is erect and smaller. Arjun then sets up two mirrors—one curved inward and the other curved outward—along with a small toy car as an object. He observes that the inward-curved mirror produces an enlarged, erect image when the car is close, but an inverted image when the car is moved farther away. The outward-curved mirror always shows a smaller, erect image. Next, Arjun uses a transparent glass piece with a curved surface, thicker in the middle, to focus sunlight onto a piece of paper, creating a bright spot. He also demonstrates that this glass piece can make text appear larger when held close to a book. Through these setups, Arjun explains how curved surfaces bend light differently from flat surfaces, creating varied images used in everyday applications like vehicle mirrors and magnifying glasses.

Questions:

1. What type of mirror is Arjun using when he observes an inverted image on the inner side of the spoon?
   (a) Plane mirror
   (b) Convex mirror
   (c) Concave mirror
   (d) None of these
   Ans: (c)
   Sol: The inner side of the spoon curves inward, forming a concave mirror. Concave mirrors produce inverted images when the object is placed beyond a certain distance, as observed by Arjun.

2. Which statement is true about the image formed by the outward-curved mirror in Arjun’s setup?
   (a) It is always inverted and enlarged
   (b) It is always erect and diminished
   (c) It is always erect and enlarged
   (d) It depends on the object’s distance
   Ans: (b)
   Sol: The outward-curved mirror is a convex mirror, which always forms an erect and diminished image, regardless of the object’s position, due to its diverging reflection properties.

3. What happens to parallel light beams when they strike the inward-curved mirror in Arjun’s exhibit?
   (a) They diverge after reflection
   (b) They remain parallel after reflection
   (c) They converge after reflection
   (d) They scatter in all directions
   Ans: (c)
   Sol: An inward-curved (concave) mirror causes parallel light beams to converge after reflection, focusing them at a point due to its inward-curving shape.

4. What type of lens is Arjun using when he focuses sunlight to create a bright spot on the paper?
   (a) Concave lens
   (b) Convex lens
   (c) Plane glass
   (d) Cylindrical lens
   Ans: (b)
   Sol: A convex lens, which is thicker in the middle, converges light beams to a point, allowing Arjun to focus sunlight into a bright spot on the paper.

5. Why does the curved glass piece make text appear larger when held close to the book?
   (a) It scatters light in all directions
   (b) It converges light to form an erect, enlarged image
   (c) It diverges light to form a diminished image
   (d) It reflects light like a mirror
   Ans: (b)
   Sol: The curved glass piece is a convex lens. When held close to the book, it forms an erect, enlarged, virtual image of the text, making it appear larger, as seen in magnifying glasses.