CBSE Class 10 Science Light Reflection and Refraction Assignment Set E

Question. A student obtains a blurred image of a distant object on a screen using a convex lens. To obtain a distinct image on the screen he should move the lens
(a) away from the screen
(b) towards the screen
(c) to a position very far away from the screen.
(d) either towards or away from the screen depending upon the position of the object.
Answer : D 

Question. A student very cautiously traces the path of a ray through a glass slab for different values of the angle of incidence (∠i). He then measures the corresponding values of the angle of refraction (∠r) and the angle of emergence (∠e) for every value of the angle of incidence. On analysing these measurements of angles, his conclusion would be :
(a) ∠i > ∠r > ∠e
(b) ∠i = ∠e > ∠r
(c) ∠i < ∠r < ∠e
(d) ∠i = ∠e < ∠r
Answer : B

Question. Three students A, B and C focussed a distant building on a screen with the help of a concave mirror. To determine focal length of the concave mirror they measured the distances as given below :
Student A : From mirror to the screen
Student B : From building to the screen
Student C : From building to the mirror
Who measured the focal length correctly ?
(a) Only A
(b) Only B
(c) A and B
(d) B and C
Answer : A

Question. A student has focussed on the screen a distant building using a convex lens. If he has selected a blue coloured building as object, select from the following options the one which gives the correct characteristics of the image formed on the screen. 
(a) Virtual, erect, diminished and green shade
(b) Real, inverted, diminished and in violet shade
(c) Real, inverted, diminished and in blue shade
(d) Virtual, inverted, diminished and in blue shade
Answer : C

Question. An optical device has been given to a student and he determines its focal length by focusing the image of the sun on a screen placed 24 cm from the device on the same side as the sun. Select the correct statement about the device.
(a) Convex mirror of focal length 12 cm
(b) Convex lens of focal length 24 cm
(c) Concave mirror of focal length 24 cm
(d) Convex lens of focal length 12 cm
Answer : C

Question. A student determines the focal length of a device ‘X’ by focusing the image of a distant object on a screen placed 20 cm from the device on the same side as the object. The device ‘X’ is
(a) Concave lens of focal length 10 cm
(b) Convex lens of focal length 20 cm
(c) Concave mirror of focal length 10 cm
(d) Concave mirror of focal length 20 cm
Answer : D

Question. A teacher sets up the stand carrying a convex lens of focal length 15 cm at 42.7 cm mark on the optical bench. He asks four students A, B, C and D to suggest the position of screen on the optical bench so that a distinct image of a distant tree is obtained almost immediately on it. The positions suggested by the students were as :
A. 12.7 cm B. 29.7 cm
C. 57.7 cm D. 72.7 cm
The correct position of the screen was suggested by
(a) A
(b) B
(c) C
(d) D
Answer : C

Question. In an experiment to trace the path of a ray of light through a triangular glass prism, a student would observe that the emergent ray : 
(a) is parallel to the incident ray.
(b) is along the same direction of incident ray.
(c) gets deviated and bends towards the thinner part of the prism.
(d) gets deviated and bends towards the thicker part (base) of the prism.
Answer : D

Question. Match the following
Column I                           Column II
(a) Shaving mirror            (i) Convex mirror
(b) Diverging lens             (ii) Concave lens
(c) Converging lens          (iii) Concave mirror
(d) Shop security mirror   (iv) Convex lens
Answer : (a) (iii), (b) (ii), (c) (iv), (d) (i).

Question. Fill in the blanks
(a) The ratio of speed of light in vacuum to the speed of light in a medium is called ................... .
(b) When a ray of light travelling in glass enter into water. It is refracted .................. the normal.
(c) A .................. is used as a rear-view mirror.
Answer :
(a) Refractive index, (b) away from, (c) diverging mirror.

Assertion and Reason Type MCQs :

Following questions consists of two statements – Assertion (A) and Reason (R). Answer these questions selecting the appropriate option given below:
A. Both assertion and reason are true, and reason is the correct explanation of assertion.
B. Both assertion and reason are true, but reason is not the correct explanation of assertion.
C. Assertion is true but reason is false.
D. Assertion is false and reason is true.

Question. Assertion (A): Large Concave mirrors are used to concentrate sunlight to produce heat in solar cookers.
Reason (R): Concave mirror converges the light rays falling on it to a point.

Answer: A

Question. Assertion (A): The angle of incidence for a ray of light having zero angle of reflection is two.
Reason (R): Refracting surfaces follow Snell’s Law.

Answer: B

Question. Assertion(A) : Concave mirrors are used as make-up mirrors.
Reason (R) : When the face is held within the focus of a concave mirror, then a diminished image of the face is seen in the concave mirror.

Answer: A

Question. Assertion(A): Light travels faster in water than air
Reason (R): Water is denser than Air.

Answer: D

Question. Assertion(A): The mirrors used in search lights are concave spherical.
Reason (R) : In concave spherical mirror the image formed is always virtual.

Answer: C

Question. Assertion (A): Light changes its speed when it passes from one medium to another.
Reason (R): When a ray travels from vacuum to a medium, then refractive index is known as absolute refractive index.

Answer: B

Question. Assertion(A): Virtual images are always erect.
Reason (R) : Virtual images are formed by converging lenses only.

Answer: C

Question. Assertion (A): The emergent ray is parallel to the direction of incident ray.
Reason (R): The extent of bending of the ray of light at the opposite parallel faces (air- glass interface and glass- air interface) of the rectangular glass slab is equal and opposite.

Answer: A

Question. Assertion(A) : The formula connecting u, v and f for a spherical mirror is valid in all situations for all spherical mirrors for all positions of the object.
Reason (R) : Laws of reflection are strictly valid for plane surfaces.

Answer: C

Very Short Answers Type Questions

Question. An object is placed 75 cm from a converging lens of focal length 25 cm. What is the nature of the image ?
Answer : The image is real, inverted and diminished as the object is placed beyond 2f.

Question. Define the term angle of refraction.
Answer : The angle between the refracted ray and the normal at the point of incidence is called angle of refraction.

Question. Define the term angle of incidence.
Answer : The angle between an incident ray and the normal at the point of incidence is called angle of incidence.

Question. What is absolute refractive index ?
Answer : Refractive index of a medium with respect to vacuum is called absolute refractive index.

Question. What is relative refractive index ?
Answer : Refractive index of a medium with respect to another medium is called relative refractive index.

Question. Define power of a lens. 
Answer : The power of a lens is a measure of the degree of convergence or divergence of light rays falling on it.It is also defined as the reciprocal of its focal length in metres.

Question. Write two different uses of concave mirrors.
Answer : The concave mirror is used as a shaving mirror and headlights of vehicles.

Question. Define the following terms related to spherical mirrors :
(a) Centre of curvature, (b) Pole and (c) Aperture.
Answer : (a) The centre of curvature of a spherical mirror is the centre of the sphere of which the mirror is a part.
(b) The pole of a spherical mirror is the geometric centre of the spherical surface of the mirror.
(c) The plane surface area of the mirror through which the light rays enter and fall on the mirror is called its aperture.

Question. (a) What is meant by magnification ? Write its expression.
(b) What is its sign for (i) real, (ii) virtual image ?
Answer : (a) The ratio of height of the image to the height of the object is called linear magnification.
If the height of the image is ‘I’ and that of the object is ‘O’, then the magnification ‘m’ is : ‘m‘ = I/O
(b) (i) For a real image, magnification is negative.
(ii) For a virtual image, magnification is positive.

Short Answer type Questions

Question. The absolute refractive indices of glass and water are 3/2 and 4/3 respectively. If the speed of light in glass is 2 × 108 ms-1, calculate the speed of light in (i) vacuum and (ii) water.
Answer : Given, ng= 4/3 and nw = 3/2
Speed of light in glass ( vg) = 2 × 10⁸ ms^-1
(i) Speed of light in vacuum, c = ng × vg =3/2 × 2 × 10⁸ = 3 × 10⁸ ms^-1
(ii) Speed of light in water, vw = c/nw =3 × 10⁸ / (4/3) = 2.25 × 10⁸ ms^-1

Question. “The magnification produced by a spherical mirror is -3”. List all information you obtain from this statement about the mirror/ image.
Answer : Negative sign of magnification indicates that the image is real and inverted. Since the image is real and inverted, the mirror is concave and magnification of -3 indicates that the image is magnified.

Q21. Draw ray diagrams to show the image formed by a concave lens for the object placed at
(i) infinity (ii) Between f and 2f of the lens.
Ans. (i) object at infinity (ii) object between f and 2f

Question. A convex lens of focal length 25 cm and a concave lens of focal length 10 cm are placed in closed contact with each other. Calculate the lens power of the combination.
Answer : f1=25cm=0.25 m f2= -10cm= - 0.1m
Power of convex lens, P1 = 1/f1=1/0.25=+4D
Power of concave lens, P2 = 1/f2=1/ -
0.1m=-10D
power of combination, P = P1 +P2 = 4D – 10D = -6D

Q22. Draw a ray diagram to show the path of light when it travels through glass slab.
Ans.

Incident ray I enters the glass slab forms an angle of incidence ‘i’. Its bends towards the normal and forms an angle of refraction ‘r’.
The emergent ray is parallel to the incident ray.

Question. An object is placed at a distance of 12 cm in front of a concave mirror of radius of curvature 30 cm. List four characteristics of the image formed by the mirror
Answer : Radius of curvature (R) = 30 cm, object distance is 12 cm in front of the mirror. Thus we can say that object is placed between focus and pole. Four characteristics of the image formed by die given concave mirror when object is placed between pole and focus are:
(i) Virtual (ii) Erect
(iii) Enlarged (iv) Image is formed behind the mirror

Question. Define power of a lens. What is its unit? One student uses a lens of focal length 25 cm and another of – 25 cm. What is the nature of the lens and its power used by each of them?
Answer : Power of a lens is its ability to converge or diverge the rays of light falling on it. P = 1/f where f is in metre. Its unit is Dioptre. ! Diopter is the
power of a lens whose focal length is 1meter Lens is convex in the first case as f is positive and concave in the second case as f is negative. P1=1/0.25= 4D
P2=1/-0.25=-4D

Q23. Draw the following diagram in your answer book and show the formation of image of the object AB with the help of suitable rays.

Ans.

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 : A concave mirror can produce a magnified image of an object when object is placed:
(1) In between its pole and its focus
(2) In between its focus and its centre of curvature.
Difference,between these two images:
The image produced in first case will be virtual and erect. The image produced in second case will be real and inverted 

Q26. (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 of 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
non-covered lens? (CBSE 2008)
Ans. (a) (i) Range of the object distance is 0 to 20 cm from the pole.
(ii) Image will be bigger than the object.
(iii) Ray diagram:

Question. A convex lens of focal length 2.0 m 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 : Yes, it is correct.
If the object is placed within 2.0 m from the lens in the it forms magnified virtual image Between 2 m and 4 m it will form a real inverted and magnified image.

Q24. Draw ray diagrams to represent the nature, position and relative size of the image formed by a convex lens for the object placed:
(a) At 2F
(b) Between F1 and the optical centre O of lens
Ans. 

(iii) Intensity will be reduced as the light falling on the lower (covered) portion will not reach the position of image.

Creating Based Questions

Question. Using the following information form a pathway showing the reflection of a spherical mirror. And also include information that is not mentioned below to complete it. Reflecting surface, spherical mirror,curved inwards, curved outwards, pole, centre of curvature.
Answer : Spherical mirror reflections → curved inwards or outwards. The reflecting mirrors → are curved inwards → called concave mirror. The reflection surface → of convex mirror → is curved outwards. Pole is the centre of reflecting surface. The centre of spherical mirror → known as centre of curvature.

Question. Using the following information form a pathway showing the formation of image by a convex mirror. And also include information that is not mentioned below to complete it. Convex, Infinity, position of image, size of image, Nature of image, between infinity and pole.
Answer : Two positions of objects → considered in the formation of image → by a convex mirror.
Position 1 : The position of object → at infinity → position of image is at the focus of F behind the mirror, size of the image is → highly diminished and point sized. The nature of image is → virtual and erect.
Position 2 : The position of object → between infinity and pole P → displays the image position between → P and F behind the mirror. Size of the image → is diminished. Nature of the image is → virtual and erect.

Question. Using the following information form a pathway showing the process of magnification in spherical mirror. And also include information that is not mentioned below to complete it. Magnification, spherical mirror, object image, ratio, height of image, positive, negative.
Answer : The spherical mirror magnifies → objects image to a relative extent with respect to object size. It is the ratio of → height of image → to the height of object. The positive sign in magnification value → denotes the virtual image. The negative sign in magnification value denotes → real image.

Question. Using the following information form a pathway showing the representation of an image in a spherical mirror. And also include information that is not mentioned below to complete it. Ray, centre of spherical mirror, reflection, light rays, reflecting surface.
Answer : In both concave and convex mirror → for the ray passing through the centre of curvature → the reflection is along same path. The reflection of light rays → is along same path → since the rays incident on the mirror → is normal to the reflecting surface.

Question. Using the following information form a pathway defining refractive index. And also include information that is not mentioned below to complete it. Ray of light, second medium, refractive index,speed, media, air.
Answer : The refractive index → is the extent of change in direction → in second medium of given media pair.
The propagation of light travels → with different speed in different media. The speed of light in air is → comparatively less when compared to vacuum.

Q1. An object is placed at a distance of 30cm from a concave mirror of focal length 20cm. Where will the image be formed?

Q2. A 2.0-cm-high object is placed perpendicular to the principal axis of a concave mirror. The distance of the object from the mirror is 30cm, and its image is formed 60cm from the mirror, on the same side of the mirror as the object. Find the height of the image formed.

Q3. A 1.2-cm-long pin is placed perpendicular to the principal axis of a convex mirror of focal length 12 cm, at a distance of 8cm from it.

(a) Find the location of the image.

(b) Find the height of the image.

(c) Is the image erect or inverted?

Q4. Sunlight is incident on a concave mirror, parallel to its principal axis. The image is formed at a distance of 12cm from the pole. Find the radius of curvature of the mirror.

Q5. An object is placed at a distance of 20cm from a convex mirror of focal length 25cm.Calculate the position of the image. Discuss its nature.

Q6. Find the position, size and the nature of the image formed by a spherical mirror from the following data. u= -20cm f= -15cm ho= 1.ocm .

Q7. A 2-cm-high object is placed at a distance of 32cm from a concave mirror. The image is real, inverted and 3cm in size. Find the focal length of the mirror and the position of the image.

Q8. A concave mirror forms an inverted image of an object placed at a distance of 12cm from it. If the image is twice as large as the object, where is it formed?

Q9. A concave mirror forms an erect image of an object placed at a distance of 10cm from it. The size of the image is double that of the object. Where is the image formed?

Q10. An object is placed at a distance of 12cmfrom a concave mirror of radius of curvature 16cm.Find the position of the image.

Q11. An object of height 2cm is placed at a distance of 15cm from a concave mirror of focal length 10cm.Draw a scale diagram to locate the image. From the diagram, find the length of the image formed.