Chapter 15: LIGHT ENERGY

Chapter 15: Light Energy Class 10 Science and Technology (212) NIOS Textbook Solutions

Chapter 15. LIGHT ENERGY

INTEXT QUESTIONS 15.1

1. In column A, some sources of light are given. In column B, you have to write whether these are lumineus or non-lumineus.

Source (A)	Nature of source (B)
1. Glowing bulb 2. Burning candle 3. Moon 4. Fire fly 5. Shining steel plate	1.  ……………………. 2.   …………………… 3.   …………………… 4.  ……………………. 5.  ……………………..

Answer:

Source (A)	Nature of source (B)
1. Glowing bulb 2. Burning candle 3. Moon 4. Fire fly 5. Shining steel plate	1. Luminous 2. Luminous 3. Non-Luminous 4. Luminous 5.  Non-Luminous

2. Write two differences between real and virtual image.

Answer: Two differences between Real Image and Virtual Image

Real Image	Virtual Image
A real image is formed when light rays actually meet at a point.	A virtual image is formed when light rays appear to meet at a point.
A real image can be obtained on a screen.	A virtual image cannot be obtained on a screen.

3. When you are standing infront of a plane mirror, a virtual and correct image of you is formed. If some one is taking a photograph of it using camera, what will be the nature of image on photograph?

Answer: The image formed on the camera film/sensor is a real image, because a camera lens always forms a real image on the film or sensor. The photograph will show the same laterally inverted image as seen in the plane mirror.

4. A light ray is falling on a plane mirror at 30° as shown in the diagram. If plane mirror is rotated by 30° without changing the direction of incident ray, by what angle the reflected ray will rotate?

Fig. 15.9

Photo

Answer:

5. An object of height 10 cm is placed infront of a plane mirror of height 8 cm. What will be the height of image formed? Taking the distance of object from the mirror 6 cm, draw the ray diagram.

Answer: In a plane mirror, the height of the image is always equal to the height of the object. Therefore, the height of the image formed is 10 cm.

Photo

6. The image of an object placed at 10 cm from the mirror is formed at 10 cm behind the mirror. If the object is displaced by 4 cm towards the mirror, by what distance will the image be displaced with respect to the (i) mirror (ii) object?

Answer: The object is moved 4 cm towards the mirror, so its distance from the mirror becomes 6 cm. In a plane mirror, the image is formed at the same distance behind the mirror as the object is in front.

(i) With respect to the mirror: The image moves 4 cm towards the mirror.

(ii) With respect to the object: The object-image distance changes from 20 cm (= 10 + 10) to 12 cm (= 6 + 6). Therefore, the image is displaced by 8 cm with respect to the object.

7. An object is moving with velocity 6 m/s  towards a plane mirror, what will be the velocity of image towards the (i) mirror (ii) object?

Answer: When an object moves towards a plane mirror, the image also moves towards the mirror with the same velocity.

(i) Velocity of image towards the mirror = 6 m/s

(ii) Since both object and image move towards each other at equal speeds, the velocity of the image with respect to the object = 6 + 6 = 12 m/s.

8. Some letters are given in following boxes. Make the meaningful words related to reflection of light choosing the horizontal and vertical sequencing.

N	E	P	R	E	C	T
O	P	X	V	R	T	U
R	L	V	I	R	T	U
M	A	L	R	E	A	L
A	N	I	T	C	A	R
L	E	O	U	T	A	E
A	I	M	A	G	E	J
N	K	N	L	E	N	C

Answer: The meaningful words related to reflection of light are:

(i) Real (horizontal),

(ii) Erect (horizontal),

(iii) Plane (vertical),

(iv) Virtual (vertical),

(v) Image (horizontal).

9. The distance and height of an object placed infront of a plane mirror are given in column A and B respectively. In column C and D the distance of image and height of image are given but not in same order. Correct the order.

Distance of object (A)	Height of object (B)	Distance of image (C)	Height of image (D)
10 cm 5 cm 6 cm	5 cm 10 cm 8 cm	10 cm 5 cm  6 cm	10 cm 8 cm 5 cm

Answer: In a plane mirror, Distance of image = Distance of object  and Height of image = Height of object

The correct order is:

Distance of object (A)	Height of object (B)	Distance of image (C)	Height of image (D)
10 cm	5 cm	10 cm	5 cm
5 cm	10 cm	5 cm	10 cm
6 cm	8 cm	6 cm	8 cm

INTEXT QUESTIONS 15.2

 

1. An object is placed infront of a concave mirror as shown in the Fig. 15.16. Write the position and nature of the image. What is the focal length of the mirror?

Fig. 15.16

Answer: s

2. In what condition, the image formed by concave mirror is virtual?

Answer: A concave mirror forms a virtual image when the object is placed between the pole (P) and the focus (F) of the mirror. The image formed is erect, enlarged, and appears behind the mirror.

3. At what position will the reflected ray shown in Fig. 15.17 intersect the principal axis beyond focus or before focus?

Fig. 15.17

Answer:

4. What type of image will be formed if an object is placed beyond centre of curvature infront of a concave mirror?

Answer: If an object is placed beyond the centre of curvature (C) in front of a concave mirror, the image formed is real, inverted, and diminished (smaller in size). It is formed between the focus (F) and the centre of curvature (C).

5. Find the position of the object placed infront of a concave mirror of focal length 20 cm if image is formed at the distance of 30 cm from the mirror.

Answer: Given:  f = –20 cm , v = –30 cm , u = ?

Using mirror formula,

 1f=1v+1u

 ⇒1f-1v=1u

⇒1u=1-20-1-30

⇒1u=-120+130=-3+260=-160

⇒1u=-160

⇒u=-60 cm

Therefore, the object is placed 60 cm in front of the concave mirror.

6. Write two uses of concave mirror.

Answer: Concave minor is used

(i) as a reflector in searchlight, head light of motor cars and projectors etc.,

(ii)in flood lights to obtain a divergent beam of light to illuminate buildings.

7. Write the nature of image formed in convex mirror.

Answer: The image formed by a convex mirror is always virtual, erect, and diminished (smaller in size). It is formed behind the mirror.

8. Find the position of the image formed in convex mirror of focal length 12 cm when object is placed at the distance of (i) 8 cm, (ii) 12 cm and (iii) 18 cm from the mirror.

Answer: Given, f = +12 cm

Using mirror formula:    

(i) Here, u=−8 cm

1f=1v+1u

⇒1v=1f-1u

⇒1v=112-1-8

⇒1v=112+18=2+324=524

⇒1v=524

⇒v=245=4.8  cm

(ii) Here, u=−12 cm

⇒1v=1f-1u

⇒1v=112-1-12=112+112=212=16

⇒v=6 cm

(iii) Here, u=−18 cm

⇒1v=1f-1u

⇒1v=112-1-18=112+118=3+236=536

⇒1v=536⇒v=365

⇒v=7.2 cm

9. Complete the following table with corresponding positions of object and image in case of concave mirror.

Position of object	Position of image
(i) at F (ii) between F and 2F (iii) (iv) (v) beyond 2F	(i)  ……………… (ii)  …………….. (iii) between F and 2F (iv) beyond 2F (v) ……………………..

Answer: Answer:

Position of object	Position of image
(i) at F	at infinity
(ii) between F and 2F	beyond 2F
(iii) at 2F	between F and 2F
(iv) between P and F	beyond 2F (behind the mirror)
(v) beyond 2F	between F and 2F

10. Write two uses of convex mirror.

Answer: Convex mirror is used:

(i) in vehicle for rear view (ii) as safety viewers at dangerous corners

11. Does concave mirror always converges the light rays?

Answer: Answer: No, a concave mirror does not always converge light rays. When the object is placed between the pole (P) and focus (F), the reflected rays diverge and form a virtual image behind the mirror.

12. Write the conditions to produce a magnified image in concave mirror.

Answer: Answer: A concave mirror produces a magnified image when the object is placed:
(i) between the focus (F) and centre of curvature (2F) → image is real, inverted, and magnified.
(ii) between the pole (P) and focus (F)→ image is virtual, erect, and magnified.

INTEXT QUESTIONS 15.3

1. Name the type of lens which always produces virtual image.

Answer: A concave lens always produces a virtual, erect, and diminished image.

2. Draw the ray diagram for the image formation in convex lens where object is placed at (i) F (ii) between F and 2F (iii) beyond 2F.

Answer:

3. Draw the ray diagram for image formation in concave lens.

Answer:

4. The sizes of the image and object are equal in a lens of focal length 20 cm. Name the type of lens and distance of object from the lens.

Answer: The lens is a convex lens.

When the size of the image and object are equal, the object is placed at 2F (twice the focal length).

Given, f = 20 cm

The distance of object from the lens = 2f=2×20=40 cm

Therefore, the object should be placed at a distance of 40 cm from the lens.

5. An object of size 10 cm is placed infront of convex lens of focal length 20 cm. Find the size of the image formed.

Answer: kkkkkkkkkkkkkk

INTEXT QUESTIONS 15.4

1. When light passes from air to a medium its speed reduces to 40%. The velocity of light in air is 3×108ms-1 . What is refractive index of the medium?

Answer: Here, 3×108 ms-1

Speed of light in medium = 40% of speed in air.

v=40100×3×108 ms-1=410×3×108 ms-1

 v=1210×108 ms-1=1.2×108 ms-1

Refractive index μ=3×1081.2×108=3012=52=2.5  

Therefore, the refractive index of the medium = 2.5.

2. When sunlight is passed through prism, it splits into seven colours as shown in Fig. By numbers write corresponding colours .

Fig. 15.27

Answer:

3. 3. How do r and δ change for same angle of incidence i if the prism shown in Fig. is immersed in water

Fig. 15.28

Answer:

4. Why does white light split into seven colours when it passes through a prism?

Answer: White light splits into seven colours when it passes through a prism due to dispersion of light. Different colours have different wavelengths, so they bend by different amounts while passing through the prism and separate into seven colours.

5. Write a natural phenomenon of dispersion of light.

Answer: A natural phenomenon of dispersion of light is the formation of a rainbow. It occurs due to the dispersion of sunlight by water droplets in the atmosphere.

INTEXT QUESTIONS 15.5

1. Identify the eye having defective vision from the following diagrams. Write the type of defect in vision. How this defect can be removed?

Fig. 15.33

Answer:

2. Three students Riya, Tiya and Jiya in a class are using sphericals of power +2D, +4D and –2D. What type of defect in vision they have?

Answer:  Riya (+2D): Has long-sightedness (Hypermetropia) because a convex lens (+ power) is used.

Tiya (+4D): Has long-sightedness (Hypermetropia) because a convex lens (+ power) is used.

Jiya (–2D): Has short-sightedness (Myopia) because a concave lens (– power) is used.

3. How does the focal length of the eye changes when a lens is used to correct the defect of vision in case of (i) short sightedness and (ii) long or for sightedness.

Answer: (i) Short-sightedness (Myopia): The focal length of the eye is increased by using a concave lens so that the image forms on the retina.

(ii) Long or far-sightedness (Hypermetropia): The focal length of the eye is decreased by using a convex lens so that the image forms on the retina.

TERMINAL EXERCISE

1. What happens to the speed of light when it goes from (i) denser medium to rarer medium (ii) rarer medium to denser medium?

Answer: (i) When light goes from a denser medium to a rarer medium, its speed increases.

(ii) When light goes from a rarer medium to a denser medium, its speed decreases.

2. Can angle of incidence be equal to the angle refraction? Justify.

Answer: Yes, the angle of incidence can be equal to the angle of refraction when light passes normally (perpendicularly) to the surface. In this case, both angles are 0°, so no bending of light occurs.

3. Does a convex lens always converge light? Explain.

Answer: No, a convex lens does not always converge light. When the object is placed within the focal length of the convex lens, the refracted rays appear to diverge and form a virtual, erect, and magnified image.

4. Write the nature of the image formed by concave lens.

Answer: The image formed by a concave lens is always virtual, erect, and diminished (smaller in size). It is formed between the optical centre and the focus on the same side of the object.

5. In horizontal and vertical boxes of the letter grid some meaningful words regarding the properties of light are placed in different rows and column in the table below. Find at least three and define them?

Photo

Answer:

6. What will be the nature of the image formed in a convex mirror and in a concave mirror each of focal length 20 cm and object is placed at the distance of 10 cm.

Answer: Given, Focal length = 20 cm and object distance = 10 cm

(i) Convex mirror: The image formed is virtual, erect, and diminished. It is formed behind the mirror.

(ii) Concave mirror: Since the object is placed within the focal length (10 cm < 20 cm), the image formed is virtual, erect, and magnified. It is formed behind the mirror.

7. Find the position of the image formed in concave mirror of focal length 12 cm when object is placed 20 cm away from the mirror. Also find magnification.

Answer: Here,  f = –12 cm , u = –20 cm and v = ?

Using mirror formula:

1f=1v+1u

⇒1v=1f-1u=1-12-1-20

⇒1v=-112+120

⇒1v=-5+360=-260=-130

⇒v=-30  cm

Therefore, the image is formed 30 cm in front of the mirror.

Now

m=-vu=-(-30)-20=-32=-1.5

Therefore, magnification = –1.5 (image is real, inverted and enlarged)

8. In which of the following media, the speed of light is maximum and in which it is minimum.

Medium	Refractive index
A B C D	1.6 1.3 1.5 1.4

Answer:

9. The image of a candle formed by a convex lens is obtained on a screen. Will full size of the image be obtained if the lower half of the lens is printed black and completely opaque? Illustrate your answer with a ray diagram.

Answer:

10. Can a single lens ever form a real and errect image?

Answer: No, a single lens cannot form a real and erect image. A real image formed by a lens is always inverted, while an erect image is always virtual.

11. What is dispersion of light? What is the cause of dispersion of light?

Answer: Dispersion of light is the phenomenon of splitting of white light into its seven constituent colours when it passes through a prism.

 

Dispersion occurs because different colours of light have different wavelengths and are refracted by different amounts in the prism.

12. Why do distant object appear to be smaller and closer to each other?

Answer: Distant objects appear smaller and closer to each other because the angle subtended by them at the eye decreases with increase in distance. Therefore, they form smaller images on the retina and seem closer together.

13. A person looking at a net of crossed wires is able to see the vertical direction more distinctly than the horizontal wires. What is the defect due to? How is such defect of vision corrected?

Answer: The defect is astigmatism. In this defect, a person sees lines in one direction more clearly than lines in another direction due to unequal curvature of the eye lens or cornea. It is corrected by using a cylindrical lens of suitable power.

14. A person can see the objects placed at a distance of 30 cm clearly but cannot see the objects placed 30 m away. What type of defect of vision he has? How is this defect of vision corrected?

Answer: The person has short-sightedness (Myopia) because he can see nearby objects clearly but cannot see distant objects clearly. This defect is corrected by using a concave lens (diverging lens) of suitable focal length.

15. Distinguish visible, ultraviolet and infrared light.

Answer:

Visible Light	Ultraviolet Light	Infrared Light
It can be seen by human eyes.	It cannot be seen by human eyes.	It cannot be seen by human eyes.
Wavelength is 400–700 nm.	Wavelength is less than 400 nm.	Wavelength is more than 700 nm.
Produces vision.	Causes chemical effects and sterilisation.	Produces heating effect.

16. Which of the following quantities remains constant during reflection of light? (i) speed of light

(ii) frequency of light

(iii) wavelength of light

Answer: During reflection of light, the following quantities remain constant:

(i) Speed of light
(ii) Frequency of light
(iii) Wavelength of light

Therefore, all (i), (ii) and (iii) remain constant during reflection of light.

17. Write the value of angle of reflection at both the reflecting surfaces M1 and M2 held perpendicular to each other as shown in Fig. 15.34

Photo

Answer:

18. An object is placed infront of a plane mirror. The mirror is moved away from the object with the speed of 0.25 m/s . What is the speed of the image with respect to the mirror and with respect to the object?

Answer: Given, the speed of mirror = 0.25 m/s away from the object

In a plane mirror ,the image moves by the same distance as the mirror but on the opposite side.

So, Speed of image with respect to the mirror = 0.25 m/s

And speed of image with respect to the object = 0.25 + 0.25 = 0.50 m/s

19. Size of the image in a plane mirror of height 12 cm is 20 cm. What is the size of the object?

Answer: Given. Height of image = 20 cm

In a plane mirror, the size of the image is equal to the size of the object.

Therefore, the size (height) of the object is 20 cm.

---

Posted 3 weeks ago