Advertisements
Advertisements
Question
Define the terms principal foci and focal lengths as applied to a convex lens, and show them with the help of proper diagrams.
Solution
A convex lens has two principal foci:
(i) first focus, and
(ii) second focus.
First focus: First focus of a convex lens is a point F1 on the principal axis of the lens such that the rays of light starting from it after refraction from the lens become parallel to the principal axis. This is shown in figure. The distance of the first focal point Fj from the optical centre O is called first focal length. In the figure, it has been labelled by f1 (i.e., OF1 = f1)
Second focus: Second focus of a convex lens is a point F2 on the principal axis of the lens such that the rays of light incident parallel to the principal axis of the lens after refraction from the lens passes through it. This is shown in figure. The distance of the second focal point F2 from the optical centre O is called the second focal length. In the figure, it has been labelled by f2 (i.e., OF2 = f2).
APPEARS IN
RELATED QUESTIONS
Where should an object be placed so that a real and inverted image of the same size as the object is obtained using a convex lens?
Draw a labelled ray diagram to show the angle of incidence and the angle of refraction for a refracted ray of light.
Draw a ray diagram to show the formation of a real magnified image by a convex lens. (In your sketch the position of object and image with respect to the principal focus of lens should be shown clearly).
Describe with the help of a ray diagram the nature, size and position of the image formed when an object is placed at infinity (considerable distance) in front of a convex lens. State three characteristics of the image so formed.
Name one simple optical instrument in which the above arrangement of convex lens is used.
A student did an experiment with a convex lens. He put an object at different distances 25 cm, 30 cm, 40 cm, 60 cm and 120 cm from the lens. In each case he measured the distance of the image from the lens. His results were 100 cm, 24 cm, 60 cm, 30 cm and 40 cm, respectively. Unfortunately his results are written in wrong order.
Which of the object distances gives the biggest image?
A lens forms an inverted image of an object. Name the kind of lens.
Which lens can produce a real and inverted image of an object?
: Object near the lens : : ______ : 
Distinguish between Concave lens and Convex lens.
