Page 18 - 8-Sci-10 LENSES
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10. LENSES eLearn.Punjab
KEY POINTS
• A lens is a piece of any transparent material (like glass) with two faces, of which at least
one is curved.
• A lens which is thick at the middle and thinner at the edges is called convex lens and the
lens which is thin at the middle and thicker at the edges is called concave lens.
• The centre of the sphere of which any surface of the lens is a part , is known as its
centre of curvature.
• Parallel rays after refraction through a convex lens converge at a point F which is called
the principal focus of the lens.
• Parallel rays of light after refraction through a concave lens diverge out. They appear to
come from a point F, which is called the principal focus of the lens.
• The distance between the optical centre and the principal focus is known as focal length
of the lens.
• The image that can be obtained on the screen is known as real image.
• A convex lens forms real image on the screen while a concave lens always forms a
virtual image.
• In short-sightedness, a person can see near objects clearly but distant objects appear
blurred. This defect is corrected by using suitable concave lens.
• In long-sightedness, a person can see distant object clearly but near objects appear
blurred. This defect is corrected by using convex lens of suitable focal length.
QUESTIONS
10.2 Give short answers.
(i) Describe the paths of three rays which form image after passing through a convex lens.
(ii) Write the names of three instruments in which convex lens is used.
(iii) Define focal length.
(iv) Can image be obtained on the screen by a concave lens? Explain your answer briefly.
(v) How is the focal length affected when the lens of eye becomes thicker?
(vi) Upon what factor does the amount of light entering in a camera depend?
(vii) How long our eye takes to acquire dark adaption at its maximum?
(viii) Define short-sightedness and long-sightedness.
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