ICSE-X-Physics
05: Refraction through a Lens
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- Qstn #12State the condition when a lens is called an equi-convex or equi-concaveAns : 12 A lens is called an equiconvex or equiconcave when radii of curvature of the two surfaces of lens are equal.
- Qstn #13Define the term principal foci of a convex lens and illustrate your answer with the aid of proper diagrams.Ans : 13 A light ray can pass through a lens from either direction. Therefore, a lens has two principal foci.
For a convex lens, the first focal point is a point F1 on the principal axis of the lens such that the rays of light starting from it or passing through it, after refraction through lens, become parallel to the principal axis of the lens.
The second focal point for a convex lens is a point F2 on the principal axis such that the rays of light incident parallel to the principal axis, after refraction from the lens, pass through it.
- Qstn #14Define the term principal foci of a concave lens and show them with the help of proper diagrams.Ans : 14 A light ray can pass through a lens from either direction. Therefore, a lens has two principal foci. For a concave lens, the first focal point is a point Fı on the principal axis of the lens such that the incident rays of light appearing to meet at it, after refraction from the lens become parallel to the principal axis of the lens.
The second focal point for a concave lens is a point F2 on the principal axis of the lens such that the rays of light incident parallel to the principal axis, after refraction from the lens, appear to be diverging from this point.
- Qstn #15Draw a diagram to represent the second focus of a concave lens.Ans : 15
Concave lens representing second focus
- Qstn #16Draw a diagram to represent the second focus of a convex lens.Ans : 16
Convex lens representing second focus
- Qstn #17A ray of light, after refraction through a concave lens emerges parallel to the principal axis.Ans : 17
- #17-adraw a ray diagram to show the incident ray and its corresponding emergent ray.Ans :
- #17-bThe incident ray when produces meets the principal axis at a point. Name the point.Ans : The point where incident ray when produced meets the principal axis is called first focus.
- Qstn #18A ray of light after refraction through a convex lens emerges parallel to principal axis.Ans : 18
- #18-aDraw a ray diagram to show it.Ans :
- #18-bThe incident ray passes through a point on the principal axis. Name the point.Ans : The point where incident ray passes through a point on the principal axis is called first focus.
- Qstn #19A beam of light incident on a convex lens parallel to its principal axis converges at a point on the principal axis. Name the point. Draw a ray diagram to show it.Ans : 19 : Such a point will be second focus.
- Qstn #20A beam of light incident on a thin concave lens parallel to its principal axis diverges and appears to come from a point on the principal axis. Name the point. Draw a ray diagram to show it.Ans : 20 It appears to come from 'Second Focus'.
- Qstn #21Define the term focal length of a lens.Ans : 21 The distance from the optical centre O of the lens to its second focal point is called the focal length of the lens.
- Qstn #22What do you mean by focal plane of a lens?Ans : 22 A plane passing through the focal point and normal to the principal axis of the lens is called the first focal plane.