LIGHT:REFLECTION and REFRACTION

 1.What is light?

Ans:Light is a form of energy which produces the sensation of vission.

2. What is beam?
Ans: A collection of rays moving in an organized manner is called beam.

3. What is photon?
Ans: Photon is a particle of light energy.

4.What is optics?What are the different kinds of optics?
Ans: The branch of physics that deals with the nature of light,it’s source, properties and effects is called optics. There are two kinds of optics.they are –
a) Physical optics.
b) geometrical optics.

5. How many types  are there?
Ans: There are three types of rays.they are-.
a) Divergent rays.
b) Convergent rays.
c) Parallel rays.

6. What do you mean by reflection of light?
Ans: The phenomenon in which a ray of light travels in a medium,comes back into the same medium when incident on a second medium is known as reflection of light.

7. State the laws of reflection.
Ans: a)The incident ray,the reflected ray and the normal at the point of incidence,all lie in the same plane.
b) The angle of incidence is always equal to the angle of reflection.

8. What is an image?what are the types of image and explain it.
Ans: When a number of rays starting from a point after reflection or refraction actually meet at or appear to do so at another point.the second point is called the image of the first.
There are two types of image.they are- Real image – The image formed by actual intersection of the reflected rays or refracted rays is called a real image. Virtual image – The image formed by imaginary intersection of reflected rays or refracted rays is called virtual image.

9. Mention differences between real and virtual image.
Ans: a) The real image is formed by actual intersection of the reflected rays.on the other hand,The virtual image is formed by imaginary intersection of the reflected rays.
b) Real image can be projected on a screen.but,The virtual image can not be projected on a screen.
c) Real image is always inverted with respect to an object.on the other hand, virtual image is always erect with respect to an object.

10. What is spherical mirror? What are the types of spherical mirror?
Ans: A spherical mirror is a mirror whose reflecting surface forms a part of a hollow sphre of glass.
spherical mirror are of two types -.
a) Concave mirror – If the reflecting surface is curve inwards and the outer bulged surface is silvered then the mirror is a Concave.
b) Convex mirror – If the reflecting surface is bulged outwards and inner hollow surface is silvered then the mirror is a convex.

11. Define the following terms with a respect to spherical mirror.
a)Pole – The central point of the reflecting surface is called the pole of the spherical mirror.
b) Centre of curvature – The centre of the sphere of which the mirror surface is a part is called the centre curvature of the spherical mirror.
c) Radius of curvature – The radius of the sphere of which the mirror surface is a part is called the radius of curvature of the spherical mirror.
d) Principal axis – The line that joints the centre of curvature of the spherical mirror with its pole is called the principal axis of the spherical mirror.
e) Principal focus – For a Concave mirror it is a point on the principal axis such that rays of light parallel to the principal axis converge to it after reflection in the mirror. This point is called principal focus.
f) Focal length – The distance from the principal focus to the pole of the spherical mirror is called the focal length of the mirror.
g) Focal plane – A plane perpendicular to the principal axis and containing the principal focus is called the focal plane of the spherical mirror.

12. Write the uses of Concave mirror and Convex mirror.
Ans: Concave mirror – a) Concave mirrors are commonly used in torches, search lights,and vehicles headlights to get powerful parallel beams of light.
b) Concave mirrors are used by dentists to see large images of the teeth of patients.
c) Concave mirrors are used as shaving mirrors to see a larger image of the face. d) Large concave mirrors are used to concentrate sunlight to produce heat in solar furnaces.
Convex mirror – a) Convex mirrors are commonly used as rear view mirrors in vehicles.
b) Convex mirrors are used as security mirrors in public buildings.
c) Convex mirrors are used in magnifying glass.
d) Convex mirrors are also used in making lenses of sunglasses.

13. What do you mean by magnification of an image?
Ans: Magnification produces by a spherical mirror gives the relative extent to which the image of an object is magnified with respect to the object size. It is expressed as the ratio of the height of the image to the height of the object.It is usually represented by the letter ‘m’
m = – Height of the image (h’) / Height of the object (h).
The Magnification m is also related to the object distance (u) and image distance (v).It can be written as :.
Magnification (m) = – h’/h = -v/u
sign convention for magnification
(i) Magnification is negative for real images (since such images are inverted). (ii) Magnification is positive for virtual images (since such images are erect).

14. What do you mean by refraction of light?
Ans: When light travels from one medium to another medium,it speed is changed . consequently,at the interface of two medium,the direction of the passing ray changed. The change of direction of the incident ray is called refraction of light.

15. State the laws of refraction.
Ans: Laws of refraction
(a) The incident ray, the refracted ray and the normal to the interface of two transparent media at point of incidence all lie in the same plane.
(b) For a particular pair of media the angle of incidence’ i’ in medium l is related to the angle of refraction ‘r’ in medium 2 by the relation-
n Sin i = n sin r or,
Sin i / sin r =constant.
The constant value is called the refractive index of the second medium with respect to the first.
The second law of refraction is called Snell’s law of refraction.

16. What do you mean by refractive index of medium?
Ans: The refractive index (n) of a transparent medium is the relation of the speed of light in vaccum (c) to the speed of light in that medium (v).
Refractive index = speed of light in vaccum / speed of light in medium.
* n= c/v.

17. What is a lens? Name two types of lenses.
Ans: A lens is a transparent medium bounded by two surfaces of which at least one is spherical.
Lenses can be classified into two categories –
(i) Convex or converging lenses:A lens which is thick at the centre but thinner at the edges is a convex lens. There are three types of convex lens-
a) Double convex lens or biconvex lens. b) plano-convex lens.
c) Convex meniscus.
(ii) Concave lens or diverging lenses: A lens which is thin at the centre but thicker at the edges is a Concave lens. There are three of concave lens –
a) Double concave lens or biconcave lens b) Plano concave lens.
c) Concave meniscus.

18. Define the following term in connection with lens.
a) Centre of curvature : The centre of curvature of the surface of a lens is the centre of the sphere of which it forms a part because a lens has two surfaces so it has two centres of curvature.
b) Radius of curvature : The radius of curvature of the surface of a lens is the radius of the sphere of which the surface forms a part.
c) Principal axis : It is the line passing through the two centres of curvature of the lens.
d) Optical centre : It is a point situated within the lens through which a ray of light passes undeviated.
e) Focal length : It is the distance between the principal focus and the optical centre of the lens.
f) Aperture : It is the diameter of the circular boundary of the lens.

19. Differences between Concave and a convex lens.
Ans: (i) Concave lens is thinner at the centre than the edges.and Convex lens is thicker at the centre than the edges.
(ii) Concave lens diverges a parallel beam of light on reflection through it.and Convex lens converges a parallel beam of light on refraction through it.
(iii) concave lens has a virtual focus.and convex lens has a real focus.

20. What do you mean by magnification of lens?
Ans: The magnification produced by a Lens, similar to that for spherical mirrors,is derived as the ratio of the height of the image and the height of the object.It is represented by the letter ‘m’ .If h is the height of the object and h’ is the height of the image given by lens,then the magnification produced by the lens is given by,
m = Height of the image / Height of the object = h’ / h.
Magnification produced by a lens is also related to the object (u) and the image distance (v).
This relationship is given by Magnification (m) = h’ / h = v / u.

21. Define the power of lens. What is its SI unit?
Ans: The ability of a convex lens to converge a parallel beam of light to point near the lens or the ability of a Concave lens to make the beam appear to diverge from a nearby point known as the power of the lens . If P denotes the power of. Lens of focal length f meter, then.               
P = 1 / f The SI unit of power lens is dioptre. It is denoted by the letter D. If f is expressed in metres, then power is expressed in dioptre.






TEXTUAL QUESTIONS




1. Define the principal focus of a concave mirror .
Ans: It is a point on the principal axis where the rays of light parallel to principal axis meet.

2. The radius of curvature of spherical mirror is 20 cm. What is its focal length?
Ans: Focal length=1/2 × Radius of curvature
=1/2 × 20 cm
=10 cm

3. Name a mirror that can give an erect and enlarged image of an object.
Ans : Concave miror.

4. Why do we prefer a convex mirror as a rear-view mirror in vehicles?
Ans: Convex mirror can cover a wider range and give erect and diminished image Hence convex mirror is used as a rear-view miror to get wider field of view.

5.Find the focal length of a convex mirror whose radius of curvature in 32Cm.
Ans: As the radius of curvature and focal length of a convex mimor are positive, so
R = +32 cm
f  = R/2
   =16 cm.

6. A concave mirror produces three times magnified (enlarged) real image of an object placed at 10 cm in front of it. Where is the image located?
Ans : Object distance, u =-10 cm (concave mirror),
magnification, m=-3
                  m=-v/u
therefore,v = – mu
                      =- (-3) (-10)
                      =-30cm.
Image formed, v = 30 cm in front of the concave mirror.

7. 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?
Ans: Light trends faster in rarer medium (air) and slower in denser medium (water). As a ray of light traveling in air enters obliquely into water, it slows down and bends towards the normal.

8. Light enter 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 3×10⁸ m/s.
Ans: The speed of light in vacuum is 3×10⁸ m/s.
Refractive index of glass = 1.50
n= C/V
speed of light in glass, v ,= C/h
                                              =3×10 g /1.50
                                              =2 × 10⁸ m/s.

9. Find out, from table 10.3 the medium having highest optical density. Also find the medium with lowest optical density.
Ans: Diamond has highest refractive index ( 2.42), so it has largest optical density.
Air has lowest refractive index (= 1.0003), so it has lowest optical density.

10. You are given Kerosene, turpentine and water In which of these does the light travel fastest? Use the information given in table 10.3.
Ans: Refractive index of Kerosene=1.44
Refractive index of Turpentine.    =1.47 Refractive index of water               =1.33 Lower the refractive index faster in the speed of light in that medium. Hence, light will travel fastes in water.

11. The refractive index of diamond is 2.42. What is the meaning of this statement?
Ans: The refractive index of diamond is 2 .42 It indicates that the ratio of speed of light in air to that in diamond is 2.42.

12. Define 1 dioptre of power of a lens.
Ans: 1 dioptre is the power of a lens whose focal length is 1 metre.
1 D = 1 m–¹

13. A convex lens forms a real and inverted image of a needle at a distance of 50 cm from it. Where is the needle placed in front of the convex lens if the image is equal to the size of the object? Also find the power of the lens.
Ans: Here, v = +50 cm [vis positive for real image]
As the real image is of the same size as the object, so
m= h’/h = V/U =1
or,u = -v
       =-50 cm
Now,1/f = 1/v-1/u
               = (1/+50) – (1/-50)
               = + 2 / 50
               = + 1/ 25
or, f = 25 cm
        = 0.25 m
therefore, P = 1/f
                       =1 / +0.25 m
                       = +4 D

14.Find the power of a concave lens of focal length 2m.
Ans: Focal length of concave lens, f = -2m Power of concave lens, P = 1/f
                       therefore, P = 1/-2
                                             = – 0.5 D

15. (i) Which one of the following materials cannot be used to make a lens?
(a) water (b) glass (c) plastic (d) clay
Ans: (d)clay.

(ii) The image formed by a concave mirror is observed to be virtual, erect and larger than the object. Where should be the position of the object?
(a) Between the principal focus and the centre of curvature.
(b) At the centre of curvature.
(c) Beyond the centre of curvature.
(d) Between the pole of the mirror and its principal focus.
Ans: (d) between the pole of the mirror and its principal focus.

(iii) A spherical mirror nd a then spherical lens have each focus length of 15 cm. The mirror and the lens are likely to be
(a) both Concave
(b) both convex
(c)the mirror is concave and the lens is convex,
(d) the mirror is convex,but the lens is concave
Ans : (a) both concave.

(iv) No matter how far you stand from a mirror, your image appears erect. The mirror Is likely to be.
(a) Plane (b) Concave (c) convex (d) either plane or convex.
Ans. (d) either plane or convex. 

(v) 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.
Ans. (C) Convex lens of focal length 5 cm.

16. We Wish to obtain an erect image of an object, using a concave mirror of focal length 15 cm. What should be the range of distance of the object from the mirror? What is the nature of the image? Is the image larger or smaller than the object?Draw a ray diagram to show the image formation in this case.
Ans. Focal length of concave miror=15 cm.
Object should be placed in front of green concave
lmage formed is virtual and erect.
image size is larger than object.

17. Name the type of mirror used in the following situations,
(a) Head light of a car.
(b) Side/rear view mirror of a vehicle.
(c) Solar furnace. Support your answer with reason.
Ans: (a) For head light of a car : Use Concave mirror, to get a powerful beam of light after reflection.
(b) A convex mirror is used a side/rear view mirror of a vehicle. Convex mirror forms an erect and diminished image of vehicles and gives wider view of rear.
(C) In solar furnance concave mirror is used as a reflector, it concentrates sun light at a point where the temperature increases sharply to 180°C-200°C.

18. One-half of a convex lens is covered with a black paper. Will this lens produce a complete image of the object? Verify your answer experimentally? Explain your observation.
Ans. One-half of a convex lens when covered with a black paper, the lens produces a complete, full image of an object.
To verify experimentally:Take a convex lens, cover half part of it as shown in the figure, with a paper. Place it on a stand. Focus a distant object on a screen, the image obtained on the screen is complete.
Observation and conclusion : Image formed on the screen does not depend on the size of the lens. The brightness of the image decreases as less number of rays pass through the lens.

19. 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.
Ans:






20. A concave lens of local length 15 cm forms an image 10 cm from the lens. How far is the object placed from the lens? Draw the ray diagram.
Ans. Concave lens
f= – 15 cm
V= -10 cm
We know,
1/v – 1/u = 1/f
=>1/-10 – 1/u = 1/-15
1/u = -1/30
u = -30 cm
Ray diagram,






21. 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.
Ans:






The image is formed 6 cm behind the mirror virtual image is formed.

22. The magnification produced by a plane mirror is +1. What does this mean?
Ans. Magnification,m=+ 1
+indicates virtual image.
1 indicates that the size of the object and size of the image is same.

23. An object 5.0 cm in length is placed at a distance of 20 cm in front of a convex mirror of radius of curvature 30 cm. Find the position of the image, its nature and size.

Ans:






24. 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.
Ans:






25. Find the focal length of a lens of power -2.0 D. What type of lens is this?Ans. P=-2.0 D.
P=1/f
f=1/P
=1/-2.0D = -0.5 m.
The lens is concave lens as f= -ve.

26. A doctor has prescribed a corrective lens of power +1.5 D. Find the focal length of the lens. Is the prescribed lens diverging or converging?
Ans.p=+1.5 D,
therefore,p=1/f
Focal length of the lens, f =1/P
=1/+1.5D
=+0.67 m
Power of the lens is ve, and it is converging lens i.e., Convex lens.




–Assam Study Hub