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Theory questions on concave and convex lenses

  • A ray of light parallel to the principal axis after refraction through a concave lens appears to come from:
    • Answer: focal point
  • A lens can be considered to be made up of a glass slab and ___?
    • Answer: 2 prisms
  • The formula for calculating magnification is?
    • Answer: image height/object height
  • A lens that can form an erect, magnified and virtual image is?
    • Answer: convex
  • A lens that always forms an erect, diminished and virtual image is?
    • Answer: concave
  • How is power of lens related to its focal length?
    • Answer: reciprocal of
  • If one half of a convex lens is covered with black paper, will it produce a complete image?
    • Answer: Yes, less intense
  • If a convex lens (ref. index 1.5) is placed in water(ref. index 1.33) its focal length will?
    • Answer: increase
  • If thickness of a convex lens increases its focal length will?
    • Answer: decrease
  • If the focal length of a lens is negative, the lens is?
    • Answer: concave
  • If the focal length of a lens is positive, the lens is?
    • Answer: convex
  • If magnification of a lens is positive then the image formed is?
    • Answer: erect and virtual
  • If magnification of a lens is negative then the image formed is?
    • Answer: inverted and real
  • If the focal length of a lens is decreased its power will:
    • Answer: increase
  • A ray of light passing through a point on the principal axis goes undeviated. The point is:
    • Answer: optic centre
  • The point at which a convex lens converges all incident rays is called?
    • Answer: focal point
  • The lens present in a human eye is?
    • Answer: convex
  • Which lens is used as a magnifying lens?
    • Answer: convex
  • Which lens is used in a simple microscope?
    • Answer: convex
  • Peephole in the door of house uses which lens?
    • Answer: concave
  • Convex lens diverges a beam of light.
    • Answer: False
  • Concave lens is also known as converging lens.
    • Answer: False
  • The power of a lens is +1.0 D. Its focal length and type is?
    • Answer: +1m, convex
  • The power of a lens is +2.0 D. Its focal length and type is?
    • Answer: +0.5m, convex
  • The power of a lens is -2.0 D. Its focal length and type is?
    • Answer: – 0.5 m concave
  • The power of a concave lens of focal length 20 cm is?
    • Answer: -5D
  • The power of a convex lens of focal length 25 cm is?
    • Answer: +4D
  • The power of a concave lens of focal length 25 cm is?
    • Answer: -4D
  • Which lens is used to correct myopia?
    • Answer: concave
  • Which lens is used to correct hypermetropia?
    • Answer: convex
  • The lens used to correct an eye defect where the image is formed in front of the retina is?
    • Answer: concave
  • The lens used to correct an eye defect where the image is formed behind the retina is?
    • Answer: convex
  • The formula for calculating magnification is?
    • Answer: image dist./object dist.
  • Concave lens has a virtual focus.
    • Answer: True
  • Which lens is used to correct short-sightedness?
    • Answer: concave
  • Which lens is used to correct farsightedness?
    • Answer: convex

Numerical questions

  • When an object was placed 15 cm in front of a convex lens its image was formed 15 cm behind it. The focal length of the lens is?
    • Answer: f = +7.5 cm, ( u = -15, v = +15 )
  • When an object was placed 20 cm in front of a convex lens its image was formed 20 cm behind it. The focal length of the lens is?
    • Answer: f = +10 cm, ( u = -20, v = +20)
  • When an object was placed 10 cm in front of a convex lens its image was formed 10 cm behind it. The focal length of the lens is?
    • Answer: f = +5 cm, ( u = -10, v = +10 )
  • When an object was placed 30 cm in front of a convex lens its image was formed 30 cm behind it. The focal length of the lens is?
    • Answer: f = +15 cm, ( u = -30, v = +30 )
  • A 2 cm tall object is placed 15 cm in front of a convex lens of focal length 10 cm. Find the position of image.
    • Answer: v = +30 cm, ( u = -15, f = +10 )
  • If an object is placed at a distance of 12 cm in front of a convex lens of focal length of 8 cm, find the position of the image.
    • Answer: v = +24 cm, (u = -12, f = +8)
  • An object placed 10 cm in front of converging lens forms a real image 15 cm behind it. What is the focal length of the lens?
    • Answer: f = +6 cm, (u = -10, v = +15)
  • An object placed 4.8 cm in front of a converging lens forms a real image 8 cm behind it. What is the focal length of the lens?
    • Answer: f = +3 cm; (u = -4.8, v = +8)
  • A convex lens of focal length 10 cm is placed 15 cm in front of a screen. Find object position to get it’s real image on screen?
    • Answer: u = -30 cm, (v = +15, f = +10 )
  • A convex lens of focal length 4 cm is placed 6 cm in front of a screen. Find object position to get it’s real image on screen?
    • Answer: u = -12 cm, (v = +6 , f = +4 )
  • If a real inverted image is formed 12 cm behind a convex lens with a focal length of 8 cm. Find the position of the object.
    • Answer: u = -24 cm, (v = +12, f = +8 )
  • A convex lens of focal length 20 cm is placed 30 cm in front of a screen. Find object position to get it’s real image on screen?
    • Answer: u = -60 cm, (v = +30, f = +20 )
  • An object is placed 10 cm from a convex lens of focal length 20 cm. Find the position of the image.
    • Answer: v = -20 cm, (u = -10 , f = +20)
  • An object is placed 5 cm from a convex lens of focal length 10 cm. Find the position of the image.
    • Answer: v = -10 cm, (u = -5 , f =  +10)
  •  An object is placed 2.5 cm from a convex lens of focal length 5 cm. Find the position of the image.
    • Answer: v = -5 cm; (u = -2.5 , f = +5)
  • An object is placed 4 cm from a convex lens of focal length 6 cm. Find the position of the image.
    • Answer: v = -12 cm; (u = -4 , f = +6)
  • An object is placed 10 cm from a convex lens of focal length 30 cm. Find the position of the image.
    • Answer: v = -15 cm; (u = -10 , f = +30)
  • An object is placed 7.5 cm from a convex lens of focal length 10 cm. Find the position of the image.
    • Answer: v = -30 cm; (u = -7.5, f = +10)
  • An object is placed 4.5 cm from a convex lens of focal length 6 cm. Find the position of the image.
    • Answer: v = -18 cm; (u = -4.5, f = +6 )
  • An object is placed 15 cm from a convex lens of focal length 20 cm. Find the position of the image.
    • Answer: v = -60 cm; (u = -15 , f = +20)
  • A concave lens has a focal length of 20 cm. At what distance should an object be placed to form an image 15 cm in front of the lens?
    • Answer: u = -60 cm; (v = -15 , f = -20)
  • A concave lens has a focal length of 15 cm. At what distance should an object be placed to form an image 10 cm in front of the lens?
    • Answer: u = -30 cm; (v = -10 , f = -15)
  • An object 60 cm from a concave lens gives a virtual image 20 cm in front of the lens. What is the focal length of the lens?
    • Answer: f = -30 cm; (u = -60, v = -20 )
  • An object is placed 15 cm in front of a concave lens of focal length 10 cm. Find the position of the image.
    • Answer: v = -6 cm; (u = -15, f = -10 )
  • An object 3 cm from a concave lens gives a virtual image 2 cm in front of the lens. What is the focal length of the lens?
    • Answer: f = -6 cm; (u = -3, v = -2 )
  • An object 60 cm from a concave lens gives a virtual image 24 cm in front of the lens. What is the focal length of the lens?
    • Answer: f = -40 cm; (u = -60, v = -24)
  • An object 20 cm from a concave lens gives a virtual image 10 cm in front of the lens. What is the focal length of the lens?
    • Answer: f= -20 cm; (u = -20, v = -10)
  • An object 30 cm from a concave lens gives a virtual image 15 cm in front of the lens. What is the focal length of the lens?
    • Answer: f = -30 cm; (u = -30, v = -15 )