05: Refraction Through A Lens : Icse-x-physics

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Qstn #1
What is a lens?
Ans : 1 A lens is a transparent refracting medium bounded by two curved surfaces which are generally spherical.

Qstn #2
Name the different kinds of lenses. Draw diagrams to illustrate them.
Ans : 2 Lenses are of two types :
(i) Convex or converging lens, and
(ii) Concave or diverging lens.

Qstn #4
Which lens is converging: An equiconcave lens or an equiconvex lens.
Ans : 4 Equiconvex lens is converging.

Qstn #5
Out of the two lenses, one concave and the other convex, state which one will show the divergent action on a light beam.
Ans : 5 Concave lens will show the divergent action on a light beam.

Qstn #6
Show by a diagram the refraction of two light rays incident parallel to the principal axis on a convex lens by treating it as a combination of a glass block and two triangular glass prisms.
Ans : 6 :
As shown in the figure the convex lens has two glass prisms and one glass block. One of the glass prisms is situated above the glass block and one below the block.

Qstn #7
Show by a diagram, the refraction of two light rays incident parallel to the principal axis on a concave lens by treating it as a combination of a glass block and two triangular glass prisms.
Ans : 7

As shown in the figure the concave lens has two glass prisms and one glass block. One of the glass prisms is situated above the glass block and one below the block.

Qstn #8
How does the action of a convex lens differ from that of a concave lens on a parallel beam of light incident on them? Draw diagram to illustrate your answer.
Ans : 8 If a parallel beam of light is incident on a convex lens then the upper part of the lens bends the incident ray downwards. The lower part bens the ray upwards while the central part passes the ray undeviated.

But in case of a concave lens the upper part of the lens bends the incident ray upwards and lower part bends the ray downwards while the central part passes the ray undeviated.

Qstn #9
Define the term principal axis of a lens.
Ans : 9 It is the line joining the centers of curvature of the two surfaces of the lens.

Qstn #10
Explain optical centre of a lens with the help of proper diagram(s)
Ans : 10 It is point on the principal axis of the lens such that a ray of light passing through this point emerges parallel to its direction of incidence.
It is marked by letter O in the figure. The optical centre is thus the centre of the lens.

Qstn #11
A ray of light incident at a point on the principal axis of a convex lens passes undeviated through the lens.
Ans : 11

#11-a
What special name is given to this point on the principal axis?
Ans : This point is known as Optical centre.