Chapter 10 Light Reflection and Refraction Solutions
Question - 21 : - We wish to obtain an erect image of an object, using a concave mirror of focal length 15 cm.
Answer - 21 : - What should be the range of distance of the object from the mirror? What is the nature of the image? Is the image larger or smaller than the object? Draw a ray diagram to show the image formation in this case.
Answer
Range of the distance of the object = 0 to 15 cm from the pole of the mirror.
Nature of the image = virtual, erect, and larger than the object.
Question - 22 : - Name the type of mirror used in the following situations.
Answer - 22 : -
(a) Headlights of a car
(b) Side/rear-view mirror of a vehicle
(c) Solar furnace
Support your answer with reason.
Answer-
(a) Concave Mirror: Because concave mirrors can produce a powerful parallel beam of light when the light source is placed at their principal focus.
(b) Convex Mirror: Because of its largest field of view.
(c) Concave Mirror: Because it concentrates the parallel rays of the sun at a principal focus.
Question - 23 : - One-half of a convex lens is covered with a black paper.
Answer - 23 : - Will this lens produce a complete image of the object? Verify your answer experimentally. Explain your observations.
Answer
Yes, it will produce a complete image of the object, as shown in the figure. This can be verified experimentally by observing the image of a distant object like a tree on a screen when the lower half of the lens is covered with a black paper. However, the intensity or brightness of the image will reduce.
Question - 24 : - An object 5 cm in length is held 25 cm away from a converging lens of focal length 10 cm.
Answer - 24 : - Draw the ray diagram and find the position, size and the nature of the image formed.
Answer
Height of the Object, h0 = 5 cm
Distance of the object from converging lens, u = -25 cm
Focal length of a converging lens, f = 10 cm
Using lens formula,
Thus, the image is inverted and formed at a distance of 16.7 cm behind the lens and measures 3.3 cm. The ray diagram is shown below.
Question - 25 : - A concave lens of focal length 15 cm forms an image 10 cm from the lens. How far is the object placed from the lens? Draw the ray diagram.
Answer - 25 : -
Focal length of concave lens (OF1), f = – 15 cm
Image distance, v= – 10 cm
According to the lens formula,
The negative value of u indicates that the object is placed 30 cm in front of the lens. This is shown in the following ray diagram.
Question - 26 : - An object is placed at a distance of 10 cm from a convex mirror of focal length 15 cm. Find the position and nature of the image.
Answer - 26 : -
Focal length of convex mirror (f) = +15 cm
Object distance (u) = – 10 cm
According to the mirror formula,
The image is located at a distance of 6 cm from the mirror on the other side of the mirror.
The positive and a value of less than 1 of magnification indicates that the image formed is virtual and erect and diminished.
Question - 27 : - The magnification produced by a plane mirror is +1. What does this mean?
Answer - 27 : - The positive sign means an image formed by a plane mirror is virtual and erect. Since the magnification is 1 it means that the size of the image is equal to the size of the object.
Question - 28 : - An object 5 cm is placed at a distance of 20 cm in front of a convex mirror of radius of curvature 30 cm. Find the position, nature and size of the image.
Answer - 28 : -
Object distance (u) = – 20 cm
Object height (h) = 5 cm
Radius of curvature (R) = 30 cm
Radius of curvature = 2 × Focal length
R = 2f
f = 15 cm
According to the mirror formula,
The positive value of image height indicates that the image formed is erect.
Hence, the image formed is erect, virtual, and smaller in size.
Question - 29 : - An object of size 7.0 cm is placed at 27 cm in front of a concave mirror of focal length 18 cm.
Answer - 29 : - At what distance from the mirror should a screen be placed so that a sharply focused image can be obtained? Find the size and nature of the image.
Answer-
Object distance (u) = – 27 cm
Object height (h) = 7 cm
Focal length (f) = – 18 cm
According to the mirror formula,
The negative value of image height indicates that the image formed is inverted.
Question - 30 : - Find the focal length of a lens of power -2.0 D. What type of lens is this?
Answer - 30 : -
Power of lens (P) = 1/f
P = -2D
f = -1/2 = -0.5 m
A concave lens has a negative focal length. Therefore, it is a concave lens.