(a) Draw ray diagrams showing the image formation by a convex lens when an object is placed

(i) at twice the focal length of the lens


(ii) at infinity


(b) The image of a candle flame formed by a lens is obtained on a screen placed on the other side of the lens. If the image is three times the size of the flame and the distance between lens and image is 80 cm, at what distance has the candle been placed from the lens? What is the nature of the image and the lens?


OR


Write laws of refraction of light. Explain the same with the help of ray diagram, when a ray of light passes through a rectangular glass slab

(a)

i.



ii.



(b)SINCE image is formed on other side of the lens the lens used is a convex lens. The magnification for a convex lens is negative.


M = -3; v = + 80 cm; u = ?


m = v/u


So u = v/m


= 80/(-3)


u = 26.6 cm


The nature of image is real, inverted, enlarged.


OR


Laws of refraction of light:


a. The incident ray, the normal to the refracting surface at the point of incidence and the refracted ray all lie in one plane.


b. For the light of a given colour and for a given pair of media, the ratio of sine of angle of incidence i to the sine of angle of refraction r is a constant.


sin i/ sin r = constant



ABCD is a rectangular glass slab. As shown in figure light ray is incident obliquely at an angle of incidence i. Since light rays travel from air to glass i.e. rarer to denser medium, the light ray bends towards the normal NN’. The refracted ray is then incident on opposite surface CD. Then light travels from glass to air i.e. denser to rarer medium, it bends away from normal MM’. The emergent ray is parallel to the incident ray.


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