Light: Reflection and Refraction
NCERT Exemplar Problems
Short Answer Part 2
Question 25: A convex lens of focal length 20 cm can produce a magnified virtual as well as real image. Is this a correct statement? If yes, where shall the object be placed in each case for obtaining these images?
Answer: The statement is correct.
A convex lens forms enlarged and real image in following two conditions:
- When object is placed between F and 2F, thus in this condition the object will be placed anywhere between 20 cm to 40 cm
- When object is placed at F, i.e. at focal length, a highly magnified real image is formed at infinity, thus to meet this condition object will be placed at 20cm
(ii)A convex lens forms enlarged and virtual image when the object is placed between focus and pole. Thus in this case object will be placed anywhere at a distance which is less than 20cm from lens.
Question 26: Sudha finds out that the sharp image of the window pane of her science laboratory is formed at a distance of 15 cm from the lens. She now tries to focus the building visible to her outside the window instead of the window pane without disturbing the lens. In which direction will she move the screen to obtain a sharp image of the building? What is the approximate focal length of this lens?
Answer: Sudha will move the screen towards lens.
The approximate focal length of the lens = 15 cm
Exaplanation: When object is at the infinity, the image formed at the focus of a convex lens, but this image is highly diminished and of point size.
Thus, assuming that the window pane is beyond 2F of the lens, and building is slightly far from window, which is also beyond 2F. In this condition image is formed between F and 2F at the other side of lens.
The position of formation of image will be decreases towards focal length of the lens with the increase of distance of object from lens.
Since, building is slightly far from window, thus Sudha has to move the screen towards lens.
Since, building and window both are beyond 2F which can be considered as infinity and a sharp image of window is formed at screen. It is possible only when screen is placed about the focal length. Thus, the focal length of the lens is approximately 15 cm.
Question 27: How are power and focal length of a lens related? You are provided with two lenses of focal length 20 cm and 40 cm respectively. Which lens will you use to obtain more convergent light?
Answer: Power of lens is indirectly proportional to the focal length of lens, i.e. power increases with decrease in focal length. Since, focal length is indirectly proportional to the power, i.e. conversing power, thus, lens having focal length equal to 20cm will be used to obtain more convergent light.
Question 28: Under what condition in an arrangement of two plane mirrors, incident ray and reflected ray will always be parallel to each other, whatever may be angle of incidence. Show the same with the help of diagram.
Answer: When two plane mirrors are placed at right angle to each other then the incident and reflected rays will always be parallel to each other. This can be shown by the following diagram.
Question 29: Draw a ray diagram showing the path of rays of light when it enters with oblique incidence (i) from air into water; (ii) from water into air.
Answer:
Note: All figures on this page are taken from NCERT Exemplar Problems Class 10 Science Book