CBSE Class 10 Science 2020 Outside Delhi Set 1

Question : 24

Total: 30

(a) With the help of labelled ray diagram show the path followed by a narrow beam of monochromatic light when it passes through a glass prism.
(b) What would happen if this beam is replaced by a narrow beam of white light?
OR
(a) A person is suffering from both myopia and hypermetropia.
(i) What kind of lenses can correct this defect?
(ii) How are these lenses prepared?
(b) A person needs a lens of power +3D for correcting his near vision and - 3D for correcting his distant vision. Calculate the focal lengths of the lenses required to correct these defects.

Solution:

PE - Incident ray
EF - Refracted ray
FS - Emergent ray
∠A - Angle of the prism
∠i-Angle of incidence
∠r-Angle of refraction
∠e-Angle of emergence
∠D - Angle of deviation
(b) The prism will split the incident white light into a band of colours. The various colours seen are Violet, Indigo, Blue, Green, Yellow, Orange and Red.

OR
(a) (i) If a person suffer from both myopia and hypermetropia, such people often require bifocal lenses.
(ii) A common type of bifocal lenses consists of both concave and convex lenses. The upper portion consists of a concave lens. It facilitates distant vision. The lower part is a convex lens. It facilitates near vision.

(b) For correcting distant vision;
Power, P=−3D
Using P=(1∕f) where f= focal length of the lens.
⇒‌‌f=‌

−3D

=−0.33m
The focal length of the lens for correcting distant vision =−0.33m.
Minus sign of focal length tells us that it is a concave lens.
For correcting near vision
Power, P=+3D
Using P=(1∕f) where f= focal length of the lens.
⇒‌‌f=(1∕3)=+0.33m
The focal length of the lens for correcting distant vision =+0.33m.
Plus sign of focal length tells us that it is a convex lens.

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