Worksheet - Light

 Light - Solved Worksheet

Remember:

Reflection:

1.      Angle of Incidence = Angle of Reflection i.e.  <i  = <r (Law of Reflection)

2.      For Plane Mirror – Image formed is -

·         Virtual & Erect

·         Same Size

·         Laterally Inverted

·         Behind the Mirror

·         Same distance as that of the object from the Mirror

 

3.      For Spherical Mirrors

1.      Concave Mirror

·         Focal Length (f) is Negative (as Focus is to the left of Pole)

·         Converging Mirror

·         Forms Real and Inverted Image. (Except when object is between F and P)

2.      Convex Mirror

·         Focal Length (f) is Positive (as Focus is to the right of Pole)

·         Diverging Mirror

·         Always forms Virtual, Erect & Diminished Image.

Mirror Formula:

1/f = 1/u + 1/v

Magnification m = hi/ho = -v/u

m = 1 –> Object and Image equal in height

m > 1 -> Image is Enlarged

m < 1 -> Image is diminished

m is positive indicates Virtual and Erect Image

m is negative indicates Real and Inverted Image

 

4.      Lens

1.      Convex Lens

·         Focal Length (f) is positive

·         Converging Lens

·         Forms Real and Inverted Image except when object is between F1 and O

·         Power is also positive

2.      Concave Lens

·         Focal Length (f) is negative

·         Diverging Lens

·         Always forms Virtual, Erect and Diminished Image

·         Power is also Negative

Lens Formula:

1/f = 1/v - 1/u

Magnification m = hi/ho = v/u

1. When a plane mirror is rotated through a certain angle, the reflected ray turns through twice as much and the size of the image: 

(a) is doubled 

(b) is halved 

(c) becomes infinite 

(d) remains same

Answer: d 

1. The size of the image does not change when the mirror is rotated 

2. It depends on the magnification which is independent of rotation. 

3. So, there will be no change in the size of the image due to the rotation of the mirror

2. If an object is placed symmetrically between two plane mirrors, inclined at angle of 72 degree, then total no. of images formed: 

(a) 5 

(b) 4 

(c) 2 

(d) infinite

Answer: b 

Given: Angle between the mirrors, θ = 72° So, 

n = 360/ θ 

   = 360°/72° 

   = 5 

As n is odd and object is placed symmetrically (i.e.; the object is placed at the equal distance from each mirror in between them or in other words object is placed at the line of symmetry in between the two mirrors), 

hence, the number of images, 

N = n −1 

   = 5 −1 

   = 4

3. Which statement is true for the reflection of light? 

(a) The angle of incidence and reflection are equal. 

(b) The reflected light is less bright than the incident light. 

(c) The sum of angle of incidence and reflection is always greater than 900. 

(d) The beams of incident light after reflection diverge at unequal angles.

Answer: a

4. Focal length of a plane mirror is 

(a) 0 

(b) infinite 

(c) 25 cm 

(d) -25

Answer : b 

The focal length of a plane mirror is considered to be "infinity" or "undefined." 

In optics, a plane mirror is a flat reflecting surface where light undergoes reflection. Unlike curved mirrors (concave and convex mirrors) that have a measurable focal point and focal length, a plane mirror has a virtual focus located at a theoretically infinite distance behind the mirror surface. 

Therefore, for practical purposes and in optical calculations involving plane mirrors, the focal length is considered infinite, and no real focal point exists.

5. The image shows the path of incident rays to a concave mirror. Where would the reflected rays meet for the image formation to take place? 

(a) behind the mirror 

(b) between F and O 

(c) between C and F 

(d) beyond C

Answer c 

When incident rays parallel to the principal axis strike a concave mirror, they converge and meet between the center of curvature (C) and the focal point (F) to form a real and inverted image.

6. ) A beam of light incident on a plane mirror forms a real image on reflection. The incident beam is : 

(a) parallel 

(b) convergent 

(c) divergent 

(d) not certain

Answer: b 

Convergent beam of light when incident in a plane mirror forms real image.

7. An object is placed at a distance of 40cm in front of a concave mirror of focal length 20cm. The image produced is: 

(a) virtual and inverted 

(b) real and erect 

(c) real, inverted and of the opposite size as that of the object 

(d) real, inverted and of the same size as that of the object

Answer : d 

Since focal length of concave mirror given is 20 cm. 

Object is at 40 cm distance which means it is at centre of curvature. 

Hence, image will be formed of same size and at centre focus only but real and inverted.

8. A student conducts an experiment using a convex lens. He places the object at a distance of 60 cm in front of the lens and observes that the image is formed at a distance of 30 cm behind the lens. What is the power of the lens? 

(a) 0.005 diopter 

(b) 0.05 diopter 

(c) 5 diopter 

(d) 50 diopter

Answer: c 

Given: u = -60 cm, v = +30 cm 

We know, Lens formula 

1/f = 1/v – 1/u 

     = 1/30 – 1/-60 

     = 1/30 + 1/60 

   f = 30 x 60/90 

     = 20 cm 

     = 0.2 m 

P = 1/f 

   = 1/0.2 

   = +5 D

9. An image of an object produced on a screen which is about 36 cm using a convex lens. The image produced is about 3 times the size of the object. What is the size of the object? 

(a) 12 cm 

(b) 33 cm 

(c) 39 cm 

(d) 108 cm 

Answer: a 

Solution: v = - 36 cm, m = -3 since real and inverted 

m = v/u 

u = v/m 

   = -36//-3 

   = 12 cm 

10. Image formed by a convex spherical mirror is: 

(a) virtual 

(b) real 

(c) enlarged 

(d) inverted

Answer: a 

Regardless of the position of the object reflected by a convex mirror, the image formed is always virtual, upright, and reduced in size.

11. A student studies that a convex lens always forms a virtual image irrespective of its position. What causes the convex mirror to always form a virtual image? 

(a) because the reflected ray never intersects 

(b) because the reflected ray converges at a single point 

(c) because the incident ray traces its path back along the principal axis 

(d) because the incident ray of a convex mirror gets absorbed in the mirror

Answer: c 

Convex mirror always form virtual images because the focal point and the centre of curvature of the convex mirror are imaginary points and that cannot be reached. So image is formed inside the mirror and cannot be projected on a screen.

12. A 10 mm long awl pin is placed vertically in front of a concave mirror. A 5 mm long image of the awl pin is formed at 30 cm in front of the mirror. The focal length of this mirror is? 

(a) -30 cm 

(b) -20 cm 

(c) -40 cm 

(d) -60 cm 

Answer: d 

ho = 10 mm = 1 cm, u = -30 cm and hi = 5 mm= -0.5 cm 

m = hi/ho = -v/u 

hi/ho = -v/u 

-0.5/1 = - -30/v 

v = 30/0.5 

 = -60 cm

13. Rahul conducts an experiment using an object of height 10 cm and a concave lens with focal length 20 cm. The object is placed at a distance of 25 cm from the lens. Can the image be formed on a screen? 

(a) yes, as the image formed will be real 

(b) yes, as the image formed will be erect 

(c) no, as the image formed will be virtual 

(d) no, as the image formed will be inverted

Answer: a 

The object is between F1 and 2F1, A Real and inverted image is formed that can be formed on screen

14. Magnification produced by a rear view mirror fitted in vehicles: 

(a) is less than one 

(b) is more than one 

(c) is equal to one 

(d) can be more than or less than one depending upon the position of the object in front of it

Answer : a 

Magnification produced by a rear view mirror fitted in vehicles is less than one. m < 1. Because convex mirror is used as a rear view mirror

15. A student conducts an activity using a concave mirror with focal length of 10 cm. He placed the object 15 cm from the mirror. Where is the image likely to form? 

(a) at 6 cm behind the mirror 

(b) at 30 cm behind the mirror 

(c) at 6 cm in front of the mirror 

(d) at 30 cm in front of the mirror

Answer: d 

f = -10 cm u = -15cm 

By using mirror's formula, 

1/f = 1/v + 1/u 

1/-10 = 1/v + 1/-15 

1/v = 1/-10-(1/-15) 

1/v = -1/30 

Therefore, v = -30 cm

16. The image of an object placed in front of a convex mirror is formed at 

(a) the object itself 

(b) twice the distance of the object in front of the mirror 

(c) half the distance of the object in front of the mirror 

(d) behind the mirror 

Answer : d

17. A full length of image of a distant tall building can definitely be seen using: 

(a) a concave mirror 

(b) a convex mirror 

(c) a plane mirror 

(d) both concave as well as plane mirror

Answer : b 

We know that a convex mirror forms a virtual and smaller image irrespective of position of the object. So, a full length image of a distant tall building can definitely be seen by using a convex mirror.

18. A student conducts an activity using a flask of height 15 cm and a concave mirror. He finds that the image formed is 45 cm in height. What is the magnification of the image? 

(a) -3 times 

(b) -1/ 3 times 

(c) 1/ 3 times 

(d) 3 times

Answer: d 

The magnification of an image formed by a mirror can be calculated using the formula: Magnification = Height of the Image / Height of the Object 

Given: Height of the object (h) = 15 cm Height of the image (h') = 45 cm 

Using the formula for magnification: 

Magnification = ℎ′/h 

Magnification = 45 cm/15 cm 

Magnification = 3

19. Which of the following can make a parallel beam of light from a point source incident on it? (a) Concave mirror as well as convex lens 

(b) Convex mirror as well as concave lens 

(c) Two plane mirrors placed at 90 degree to each other 

(d) Concave mirror as well as concave lens

Answer: a 

A type of optical device that can make a parallel beam of light from a point source incident on it is a convex lens or a concave mirror. Both of these optical devices have the ability to converge incoming light rays to a single focal point. 

When the source of light is positioned at the focal point of a convex lens or a convex mirror, the outgoing light rays will be parallel. In the case of a convex lens, if the light source is at its focal point, the rays that come out on the other side of the lens will be parallel. Similarly, a convex mirror, when the light source is positioned at its focal point, will reflect the light rays in such a way that they become parallel to each other after reflection. Both these devices have the capability to converge light rays to a focal point and then diverge them in a way that they become parallel again.

20. A student studies that the speed of light in air is 300000 kms/ sec where that of speed in a glass slab is about 197000 kms/ sec. What causes the difference in speed of light in these two media? 

(a) difference in density 

(b) difference in temperature 

(c) difference in amount of light 

(d) difference in direction of wind flow

Answer: a 

The difference in the speed of light between air and a glass slab is primarily due to the variation in the optical density of these mediums. Optical density refers to how much a material can slow down the speed of light as it passes through it. 

In simpler terms, it's related to how easily light can move through a substance. When light passes through a different medium (like air to glass), its speed changes because of the change in the medium's optical density. In more optically dense materials like glass, light tends to move slower compared to less dense mediums like air. This change in speed occurs because light interacts with the atoms and molecules of the material it's passing through. 

In denser materials like glass, these interactions are more frequent, causing the light to slow down as it moves through the medium. This phenomenon is often referred to as refraction—the bending of light as it passes from one medium to another due to the change in its speed. The difference in the optical density of air and a glass slab is the reason behind the variation in the speed of light between these two mediums.