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Science 8

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Light and optical systems

Ten questions of mixed difficulty, covering Light and optical systems. Print it, or work through it on screen — the answer key starts on its own page.

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Light and optical systems

Science 8 · maddyhelps.com

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  1. A store hangs a mirror that bulges outward high in a corner, so staff can watch several aisles at once. Why is a convex mirror used?

    1. a) It spreads reflected light, giving a wide view with small images
    2. b) It flips everything upside down, which makes movement stand out
    3. c) It reflects only light from moving objects, not the still shelves
    4. d) It focuses reflected light, making the far aisles look bigger

  2. A pinhole camera is a box with a tiny hole in one end and a paper screen at the other. The image of a tree on the screen is upside down. Why?

    1. a) Light reflects off the inside walls of the box before reaching the screen
    2. b) The pinhole acts like a lens, bending the light so that the image flips
    3. c) Rays travel straight, so light from the treetop lands at the bottom
    4. d) The whole image squeezes through the hole and turns over on the way

  3. A straw standing in a glass of water looks broken where it enters the water. What causes this?

    1. a) The straw really bends where it goes into the water
    2. b) Light bends as it passes from water into air
    3. c) Light bounces off the surface of the water
    4. d) Light speeds up as it passes into the water

  4. A camera focuses on near and far objects by moving its lens closer to or farther from the sensor. How does the human eye focus?

    1. a) Muscles move the lens forward and back
    2. b) Muscles change the shape of the lens
    3. c) The retina moves closer to the lens
    4. d) The pupil widens to focus on near things

  5. A ray of light strikes a flat mirror with an angle of incidence of 35°. What is the angle of reflection?

    1. a) 145°
    2. b) 70°
    3. c) 55°
    4. d) 35°

  6. A bathroom window of frosted glass lets daylight into the room, but nobody can see clearly through it. The frosted glass is

    1. a) transparent
    2. b) translucent
    3. c) reflective
    4. d) opaque

  7. A ray of light hits a flat mirror, making an angle of 25° with the mirror's surface. What is the angle of reflection?

    1. a) 130°
    2. b) 25°
    3. c) 65°
    4. d) 50°

  8. A beam of white light passes through a glass prism and comes out as a band of colours, from red to violet. What does this show?

    1. a) The prism absorbs white light and gives off coloured light
    2. b) White light is a mix of colours, each bent differently
    3. c) The glass adds colour to the light as the light passes through
    4. d) Each colour reflects off a different face of the prism

  9. A student holds a convex lens between a bright window and a white wall, and moves it until a sharp image of the window appears on the wall. Compared with the real window, the image is

    1. a) upside down and smaller
    2. b) upright and smaller
    3. c) upright and the same size
    4. d) upside down and larger

  10. Some ancient thinkers believed the eye sends out rays that travel to an object, and that this is how we see it. Which observation is the strongest evidence against that idea?

    1. a) You cannot see anything in a completely dark room
    2. b) Objects far away look smaller than objects nearby
    3. c) Your eyes feel tired after a long time reading
    4. d) Two people can look at the same tree at once

Answer key · Light and optical systems

Science 8 · maddyhelps.com

  1. a) It spreads reflected light, giving a wide view with small images — A convex mirror curves outward, so it spreads reflected rays apart and gathers light from a much wider area into your view, though everything looks smaller. A concave mirror, which curves inward, brings rays together; that is how a makeup mirror gives a larger image when you are close to it.
  2. c) Rays travel straight, so light from the treetop lands at the bottom — Light from the top of the tree travels in a straight line down through the hole and lands at the bottom of the screen, while light from the bottom lands at the top, so the image is flipped. A pinhole has no glass and bends nothing; the image is built point by point from straight rays, not carried through the hole as a whole picture.
  3. b) Light bends as it passes from water into air — Light from the underwater part of the straw changes speed as it crosses from water into air, so it bends, and your eye traces it back along a straight line to the wrong place. This bending is refraction; light slows down in water and speeds up again as it comes out, not the other way round.
  4. b) Muscles change the shape of the lens — The eye's lens is flexible: tiny muscles let it become rounder for near objects and flatter for distant ones, so it never has to move. The pupil controls how much light gets in, like a camera's aperture, not what is in focus.
  5. d) 35° — The law of reflection says the angle of reflection equals the angle of incidence, both measured from the normal, the line at 90° to the mirror. 55° comes from measuring from the mirror's surface instead, and 70° is the whole angle between the two rays.
  6. b) translucent — Translucent materials let light through but scatter it, so shapes behind them look blurred. Clear window glass is transparent, letting light through without scattering it, and an opaque material, such as a wooden door, lets no light through at all.
  7. c) 65° — The law of reflection measures angles from the normal, not from the mirror, so the angle of incidence is 90° − 25° = 65°, and the angle of reflection equals it. 25° is the angle to the surface, the usual trap, and 130° is the whole angle between the incoming and reflected rays.
  8. b) White light is a mix of colours, each bent differently — Each colour slows by a slightly different amount in glass, so each bends by a different amount and the colours fan out, violet bending most and red least. The prism adds nothing: a second prism turned the other way can combine the colours back into white light.
  9. a) upside down and smaller — A convex lens bends the rays from each point of the window back together, and rays from the top of the window cross over to the bottom of the image, so it is upside down. The window is much farther from the lens than the wall is, so the image is also smaller; it is a real image, one that can be caught on a screen.
  10. a) You cannot see anything in a completely dark room — If the eyes sent out their own rays, you could see in the dark with your eyes open. You cannot, because seeing needs light from a source that bounces off the object and into your eye; far objects looking small fits either idea, so it cannot decide between them.