CBSE Class 10 Science Light Reflection and Refraction Assignment Set F

Question. Where should an object be placed in front of a convex lens to get a real image of the size of the object?
(a) At the principal focus of the lens
(b) At twice the focal length
(c) At infinity
(d) Between the optical centre of the lens and its principal focus.

Answer: B

Question. Which of the following lenses would you prefer to use while reading small letters found in a dictionary?
(a) A convex lens of focal length 50 cm
(b) A concave lens of focal length 50 cm
(c) A convex lens of focal length 5 cm
(d) A concave lens of focal length 5 cm

Answer: C

Question. A ray of light is travelling from a rarer medium to a denser medium. While entering the denser medium at the point of incidence, it
(a) goes straight into the second medium
(b) bends towards the normal
(c) bends away from the normal
(d) does not enter at all

Answer: B

Question. A student does the experiment on tracing the path of a ray of light passing through a rectangular glass slab for different angles of incidence. He can get a correct measure of the angle of incidence and the angle of emergence by following the labelling indicated in figure:

(a) I
(b) II
(c) III
(d) IV

Answer: D

Assertion and Reason Type Questions
Refractive indices of kerosene, turpentine and water are 1.44, 1.47 and 1.333 respectively. Through which of these media, light travels fast? Explain.
Speed of light in a madium = c/n = 3 x 10⁸ ms^-1 / n
Since, refractive index of water (1.333) is less than the refractive indices of kerosene and turpentine, so light travels faster in water than in kerosene and turpentine.
Refractive indices of media A, B, C and D are given below:

Directions – In the following questions a statement of Assertion is followed by a Statement of Reason. Mark the correct choice as
(a) If both Assertion and Reason are true and Reason is the correct explanation of Assertion.
(b) If both Assertion and Reason are true but Reason ·is not the correct explanation of Assertion.
(c) If Assertion is true but Reason is false.
(d) If Reason is true but Assertion is false.
(e) If both Assertion and Reason are false.

Question. Assertion: Convex mirror is preferred for rearview mirror in vehicles.
Reason: The field view of a convex mirror is lesser than that of concave mirror.

Answer: C

Question. Assertion: An object appears to be black.
Reason: All the light falling on the object is absorbed and only black light is reflected from it.

Answer: A

Question. Assertion: Lateral displacement is the distance between incident ray and the emergent ray.
Reason: The wider the glass slab, the larger the lateral displacement.

Answer: A

Question. Assertion: In the dispersion of white light by a prism, the red light bends the least.
Reason: The frequency of red light is the highest.

Answer: C

Question. Assertion: The object distance is the distance of the object from the lens.
Reason: It is measured from the object to the optical centre of the Lens.

Answer: A

Paragraph Based Questions

Question. Read and understand the following:-
The refractive indices of four media A, B, C and D are given in the following Mohan is performing an experiment with four optical media, he traced the path of light in different media A, B, C and D as below

If light travels from one medium to another, in which case will the change in speed be

Question. Minimum?
a) B & C
b) A & C
c) A & D
d) D & B

Answer: A

Question. Maximum?
a) A & B
b) C& B
c) A & D
d) B & C

Answer: C

Question. A ray of light enters air from water and experiences refraction, then
(a) ∠i = ∠r
(b) ∠i < ∠r
(c) ∠i > ∠r
(d) ∠i / ∠r = 0°.

Answer: B

Question. A beam of light is incident through the holes on side A and emerges out of the hole on the other face of the box as shown in the figure. Which of the following could be inside the box?

Answer: D

Question. When a fork is seen through lenses A and B, one by one it appears as shown in the diagrams. What is the nature of (i) lens A and (ii) lens B?

(a) Both are concave lenses of different focal length.
(b) Lens A is concave and lens B is convex.
(c) Lens A is convex and lens B is concave.
(d) Cannot be determined.

Answer: B

Short Answer Type Questions

Question. A ray of light travelling in air enters obliquely into water. Does the light ray bend towards the normal or away from the normal? Why?
Answer: The ray of light bends towards the normal. When a ray of light enters from an optically rarer medium (having low refractive index) to an optically denser medium (having high refractive index), its speed slows down and it bends towards the normal. Since water is optically denser than air, a ray of light entering from air into water will bend towards the normal.

Question. Light enters from air to glass having refractive index 1.50. What is the speed of light in the glass? The speed of light in vacuum is 3 x 108 ms-1.
Answer: Refractive index of a medium, nm = Speed of light in vacuum / Speed of light in the medium
Speed of light in vacuum, c = 3 × 10⁸ ms^-1
Refractive index of glass, n_g = 1.50
Speed of light in the glass, v = Speed of light in vacuum/ Refractive index of glass= c/n_g=3 × 10⁸/1.50 = 2x 10⁸ ms^-1.

Question.Draw a ray diagram to show the path of the reflected ray corresponding to an incident ray which is directed towards the principal focus of a convex mirror. Mark on it the angle of incidence and the angle of reflection.
Answer:

∠i = Angle of incidence
∠r = Angle of reflection

Question. Name the type of mirror used (i) by dentists and (ii) in solar furnaces. Give two reasons why such mirrors are used in each case.
Answer: (i) Concave mirrors are used by dentists to see the large images of the teeth of patients because when a tooth is within the focus of a concave mirror, then an enlarged image of the tooth is seen in the concave mirror. Thus, it becomes easier to locate the defect in the tooth.
(ii) Large concave mirrors are used in solar furnaces as reflectors. Solar furnace is placed at the focus of the concave reflector which focusses the Sun’s heat rays on the furnace due to which the solar furnace gets very hot. Even steel can be melted in this solar furnace.

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:

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: Focal length of concave lens (OF1), f = – 15 cm
Image distance, v= – 10 cm
According to the lens formula,

Question. 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: Focal length of convex mirror, f= +15 cm
Object distance, u= – 10 cm
According to the mirror formula,

The positive value of magnification indicates that the image formed is virtual and erect.

Question. “A ray of light incident on a rectangular glass slab immersed in any medium emerges parallel to itself.” Draw a labelled ray diagram to justify the statement.
Answer:

Angle of incidence = 55°
Angle of refraction = 40°
Angle of emergence = 55°
FO Incident ray
GH emergent ray

Question. The absolute refractive indices of glass and water are 3/2 and 4/3 respectively. If the speed of light is 2 × 10⁸ m/s, calculate the speed of light in
(i) vacuum, (ii) water.
Answer: (i) Given: v_g = 2 × 10⁸ m/s (Speed of light in glass)
We know, Absolute Refractive Index of a Medium = Speed of light in Vacuum (c) / Speed of light in the Medium

Long Answer Type Questions

Question. What is the principle of reversibility of light? Show that the incident ray of light is parallel to the emergent ray of light when light falls obliquely on a side of a rectangular glass slab.
Answer: Principle of reversibility of light. Refractive index for light going from medium 1 to medium 2 is equal to the reciprocal of refractive index for light going from medium 2 to medium 1.

According to Snell’s law
aη sin i = 8η sin r₁ …….(i)
Similarly, 8η sin r₂ = aη sin e ……..(ii)
r₂ = r₁ because glass slab is rectangular and therefore the two normal are parallel.
r₁ and r₂ are alternate interior angles.
Comparing (i) and (ii), we get
aη sin i = aη sin e
sin i = sin e ⇒ i = e

Question. (a) It is desired to obtain an erect image of an object, using a concave mirror of focal length 20 cm.
(i) What should be the range of distance of the object from the mirror?
(ii) Will the image be bigger or smaller than the object?
(iii) Draw a ray diagram to show the image formation in this case.
(b) One half a convex lens of focal length 20 cm is covered with a black paper.
(i) Will the lens produce a complete image of the object?
(ii) Show the formation of image of an object placed at 2F1 of such covered lens with the help of a ray diagram.
(iii) How will the intensity of the image formed by half-covered lens compare with nn-covered lens?
Answer: