CBSE Class 10 Science Light Reflection and Refraction Assignment Set A

Question. Which type of mirror should a student use if he wants to project the image of a candle flame on the walls of the school laboratory?
Answer :  To project the image of a candle flame on the walls of the school laboratory, he should use a concave mirror as only a concave mirror forms a real image (which can be obtained on a screen) for all positions of object except when the object is placed between the pole and focus of the mirror. A convex mirror and plane mirror, on the other hand, always form virtual images.

Question. Why the glass slab must not be displaced from its boundary during the experiment to trace the path of a ray of light through a glass slab?
Answer: If the glass slab gets displaced from its original position/boundary, the angle of incidence will change accordingly and experiment will not be performed correctly. Boundary helps us to keep the glass slab in a proper position.

Question. What would be the observations of a student who performed the experiment to trace the path of a ray of light through a glass slab, whose two opposite faces are not parallel to each other?
Answer: (i) The emergent ray is not parallel to the incident ray. The direction of emergent ray depends upon the inclination of non-parallel faces.
(ii) Angles of refraction at both the faces, i.e., ∠r1 and ∠r2 are not equal.

Question. It is desired to obtain the erect image of an object, using a convex lens of focal length 20 cm.
(i) What should be range of object distance from the lens?
(ii) Will the image formed be bigger or smaller?
Answer: (i) To get the virtual and erect image formed by a convex lens, object should be kept between optical centre and focus. Therefore, range of the object distance is from 0 to 20 cm.
(ii) Enlarged and erect image is formed.

Question. “A convex lens of focal length ‘F’ can form a magnified erect as well as inverted image.“ Justify this statement stating the position of the object with respect to the lens in each case for obtaining these images.
Answer: The type of image formed by a convex lens depends on the position of the object in front of the lens.
• When the object is placed between the optical centre and the focus (i.e., between O and F¢), the image formed is behind the object (on the same side). It is virtual, magnified and erect.
• When the object is placed at the focus of a convex lens, the image formed is at infinity. It is real, inverted and highly magnified.
• When the object is placed between focus and the centre of curvature (i.e., between F¢ and 2F¢), the image formed is beyond 2F. It is real, inverted and magnified.
• When the object is placed at the centre of curvature (i.e., at 2F'), the image formed is at 2F. It is real, inverted and of same size of the object.
• When the object is beyond 2F¢ (or beyond centre of curvature), the image formed is between F and 2F.
It is real, inverted and diminished.
• When the object is at infinity, the image formed is at focus. It is real, inverted and much smaller than the object (highly diminished).

Question. List two possible ways in which a concave mirror can produce a magnified image of an object placed in front of it. State the difference, if any, between these two images. 
Answer: Case 1. Concave mirror can produce an erect, magnified, virtual image when the object is placed between pole and focus of the mirror
Case 2. Concave mirror also produces an inverted, magnified, real image when the object is placed between the focus and the centre of curvature.

Question. Name the type of mirror used in the following situations:
(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 is used in the headlights of a car.
Reason. When a bulb is located at the focus of a concave mirror, the light rays get reflected from the mirror in the form of strong parallel beams. These beams travel straight over a large distance increasing visibility for safe driving.
(b) Convex mirror is used in rear/side view mirrors.
Reason. (i) It produces an erect image of the object irrespective of distance.
(ii) Image formed is diminished and gives a wide field of view of the traffic behind.
(c) Large concave mirrors are used in solar furnaces to converge the sun-rays at one point (focus) to produce heat.

Question. An object 5 cm in length is held 25 cm away from a converging lens of focal length 10 cm. Draw the ray diagram and find the position, size and the nature of the image formed.
Answer: Here h1 = 5 cm, u = –25 cm, f = +10 cm.
Using Lens formula,
1/f = 1/v - 1/u
⇒ 1/10 = 1/v - 1/-25
⇒ 1/10 - 1/25 = 1/b ⇒ 5 - 2/50 = 1/v
∴ 3/50 = 1/v ⇒ v = 50 /3 = 16.6 cm
Now v/u = h1/h2 ⇒ 50/3 / -25 = ⇒ 50/-75 = h2/5
∴ h2 = -50 x 5/75 = -10/3 = –3.3 cm 

Question. 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: Here f = –15 cm, v = –10 cm, u = ?
Using Lens formula,
1/f = 1/v - 1/u ⇒ 1/-15 = 1/-10 - 1/u 
1/u = -1/10 + 1/15 ⇒ 1/u = -3+ 2 /30 = -1/30
∴ u = –30 cm
∴ Object is placed at a distance of 30 cm from concave lens.

Question. “A concave mirror of focaal length ‘f ’ can form a magnified erect as well as an inverted image of an object placed in front of it.” Justify this statement stating the position of the object with respect to the mirror in each case for obtaining these images. 
Answer: When the object is placed at a distance less than ‘f ’ (focal length) from the mirror, the image formed is virtual, magnified and erect.
When the object is placed at a distance of ‘f ’ (focal length) from the mirror, the image is real, inverted and highly magnified.
When the object is placed at a distance more than ‘f ’ (focal length) but less than ‘2f ’, the image formed is real, inverted and magnified.
When the object is placed at the distance of ‘2f ’ from the mirror, the image is real, inverted and same size of the object.
When the object is placed at a distance more than ‘2f ’ from the mirror, the image formed is real, inverted and diminished.

Question. An object of size 7.0 cm is placed at 27 cm in front of a concave mirror of focal length 18 cm. At what distance from the mirror should a screen be placed, so that a sharp focussed image can be obtained?
Find the size and the nature of the image.
Answer: Size of object, h = +7.0 cm u = –27 cm f = –18 cm v = ? h' = ? nature = ?
Using mirror formula
1/f = 1/v + 1/u ⇒ 1/v = 1/f - 1/u 
1/v = 1/-18 - 1/-27 = -3+2/54 = -1/54 ∴ v = –54
∴ The screen should be placed at a distance of 54 cm on the object side of the mirror to obtain a sharp image.
Magnification, m = h'/h = -v/u
h'/7 = (-54)/(-27) ⇒ h' = = –2 × 7 = –14 ∴ m = h'/h = -14/7 = –2
Image size = –14 cm (–ve sign shows that image is inverted)
The image is real, inverted and enlarged.

Question. A student places a candle flame at a distance of about 60 cm from a convex lens of focal length 10 cm and focuses the image of the flame on a screen. After that he gradually moves the flame towards the lens and each time focuses the image on the screen.
(A) In which direction-toward or away from the lens, does he move the screen to focus the image?
(B) How does the size of the image change?
(C) How does the intensity of the image change as the flame moves towards the lens?
(D) Approximately for what distance between the flame and the lens, the image formed on the screen is inverted and of the same size?
Answer :  (A) As the student moves the flame towards the lens, the image is formed farther away from the focus. So, he has to move the screen away from the lens from Focus to 2F
and beyond, to focus the image. However, when object distance is less than 10 cm, no real image is formed.
(B) The size of the image gradually increases from point size to very large size.
(C) The intensity of image formed gradually decreases.
(D) Image formed on the screen will be inverted and of the same size as object when object is placed at 2F, i.e. at 20 cm (since f = 10 cm).