Page 177 - Science Class 08

v Have you to squeeze your eyes to see? v Help those who have any eye-disorder by
Have you bring your book nearer the eyes advising them to do the needful.
or far from them while reading? If yes, get v Never go out in the hot sun without
your eye-sight tested and follow the wearing sun-glasses. The extra glaze can
doctor’s advice. harm the eyes a lot.

Convex lens : A lens made up of two equal arcs meeting each other face to face
Concave lens : A lens made up of two arcs meeting each other back to back
Converge : Come from different directions and tend to meet at a common point

Diverge : To proceed in different directions after emerging from a common point
Microscope : Optical device that helps us to view very small things appear in a larger view
Telescope : Optical device that helps us to view very distant things making them appear
nearer

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i A lens is made up of a glass slab in the middle having prisms on both sides.
i Lenses are of two chief types — convex and concave.
i Compound lenses are combination of one or both these types of lenses.

i Convex lenses cause parallel light-rays to converge when they pass through them.
i Concave lenses cause parallel light-rays diverge when they pass through them.
i Images formed by convex lenses differ according to the varying distance of the objects from
their optical centres.
i Images formed by concave lenses are smaller, virtual and erect. They are located between
the focus and the optical centre and on the same side as the object.

i Real images can be obtained on screens while unreal images cannot be obtained on screens.
i Optical devices involve the use of sight, light and lenses.

i An optical microscope is used to see very small things in enlarged sizes.
i An optical telescope is used to see distant things as if located very near.
i A camera is an optical device used to take photographs.
i A magnifying glass is a device that can show things in their enlarged forms and also can
focus light-ray to start fires.

i The human eye has a natural lens that carries the sense of sight.
i Three common eye-disorders are — myopia, hypermetropia and astigmatism.

i Myopia needs the use of concave glasses or lenses to be corrected.
i Hypermetropia needs the use of convex glasses or lenses to be corrected.
i Astigmatism needs the use of cylindrical glasses or lenses to be corrected.

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