PHYSICAL SCIENCE LESSON-1 REFLECTION OF LIGHT AT CURVED SURFACE

 PHYSICAL SCIENCE

LESSON-1

REFLECTION OF LIGHT AT CURVED SURFACE

Reflections on concepts

Question 1

            Where will the image be formed when we place an object, on the principal axis of a concave mirror at a point between focus and center of curvature?

Answer

     When we place an object on the principal axis of concave mirror at a point between focus and center of curvature, the image is formed beyond center of curvature.

Question 1

 The image so formed is real, inverted and magnified

Question 2

State the differences between convex and concave mirrors.

Answer

Convex mirror	Concave mirror
1. A parallel beam of light falling on this mirror appears to diverge from a point after reflection.	1. A parallel beam of light falling on this mirror converges at a point after reflection.
2. The reflecting surface of convex mirror is bulged out.	2. The reflecting surface of a concave mirror curve inward.
3. Radius of curvature and focal length are negative.	3. Radius of curvature and focal length are positive.
4. It’s magnification has positive only.	4. It’s magnification has both positive and negative.
5. Magnification of convex mirror is in between zero and one.	5. Magnification value of concave mirror having all values except zero to one.
6. The image formed by convex mirror always diminished.	6. The image formed by concave mirror may be magnified or diminished.

Question 3
Distinguish between real and virtual images.

Answer

Real image	Virtual image
1. Real image is formed if light after	1. Virtual image is formed when rays reflection or refraction converges after reflection appear to be comto a point. ing from a point.
2. Here the rays actually meet at the from the image point.	2. Here the rays appear to diverge image point.
3. It can be captured on screen.	3. It cannot be captured on screen.
4. It is always inverted.	4. It is always erect.

 

Question 4
How do you get a virtual image using a concave mirror?

Answer
A. When an object is kept between pole and focus of a concave mirror virtual image is formed behind the mirror.

Question 4

Question 5
What do you know about the terms given below related to spherical mirrors?
(a) pole
(b) centre of curvature
(c) focus
(d) Radius of curvature
(e) Focal length
(f) principal axis
(g) object distance
(h) image distance
(i) Magnification

Answer
(a) Pole: The point on the principal axis of spherical mirror with respect to which all the measurements are made. Usually, it is the mid point of the curvature of mirror.

(b) Centre of curvature: The centre of the sphere of which the curved surface of the mirror is a part.

(c) Focus: The light rays coming from a source parallel to the principal axis converge at a point after reflection. This point is called focus or focal point.

(d) Radius of curvature: The radius of the sphere of which the curved surface is a part is called radius of curvature.

(e) Focal length: The distance between the pole of the mirror and focus is called focal length of mirror.

(f) Principal axis: The straight line passing through the centre of curvature and pole of curved mirror is called principal axis.

(g) Object distance: The distance between the pole of the mirror and object position of object is known as object distance.

(h) Image distance: The distance between the pole of the mirror and position of image is called image distance.

(i) Magnification: Magnification of a spherical mirror is the ratio between size (height) of image to the size (height) of object. Also, m= v/u.

Question 6
What do you infer from the experiment which you did to measure the object distance and image distance?

Answer
Inference from the experiment which I did with concave mirror

As the object moves away from the mirror the image approaches the mirror.

As the object moves away from mirror the size of the image becomes smaller.

Question 7.

Write the rules for sign convention.

Answer

Rules for sign convension:

 

1.AN distances should be measured from the pole.

2.The distances measured in the direction of incident light, are taken positive and the opposite direction of incident light are taken negative.

3.Height of object (H0) and height of image (H1) are positive if measured upward

from the axis and negative if measured downward.

4.For a concave mirror ‘f’ and ‘R’ are negative and for a convex mirror these are positive.

Application of concepts

 

Question 1.

Find the distance of the image, when an object is placed on the principal axis, at a distance of 10 cm in front of a concave mirror whose radius of curvature is 8 cm.

Answer:

Object distance u = -10 cm

Radius of curvature (r) = -8cm

Image distance v=?

The image distance (v) = 6.7 cm.
i.e., Real image is formed at same side of the mirror.

Question 2
The magnification product by a plane mirror is +1. What does it mean?
Answer

 

1. Magnification =  height of the image / height of the object = distance of the image/  distance of the object 
2. The magnification produced by a plane mirror is +1 means then the size of the image is equal to the size of the object.
3. + sign indicates that the image is erect. Magnification ‘+1’ indicates the image is erect and size of the image is equal to size of the object.

Question 3
If the spherical mirrors were not known to human beings, guess the consequences.

Answer
If spherical mirrors are not known to human beings

1. Many optical instruments would not have been invented.
2. We cannot increase the size of images of the objects.
3. The problem of lateral inversion of images will not be solved.
4. Now a days spherical mirrors are used as shaving mirrors, head mirrors for ENT specialists, in headlights of motor vehicles, in solar furnaces and as rearview mirror. If spherical mirrors are not known all these are not possible.

Question 4.
Draw suitable rays by which we can guess the position of the imge formed by a concave mirror.
Answer:
The following rays are used to guess the position of the image formed by a concave mirror.
(i) A ray parallel to the principal axis passes through principal focus (F) after reflection from a concave mirror.

(ii) A ray passing through ‘F’ becomes parallel to principal axis after reflection from a concave mirror.

iii) A ray passing through ‘C’ is reflected back along the same path after reflection from a concave mirror.

(iv) A ray incident obliquely to the principal axis towards the pole P, on the concave mirror is reflected obliquely, following the laws of reflection.

Question 5
Show the formation of image with a ray diagram, when an object is placed on the principal axis of a concave mirror away from the centre of curvature.

Answer
When an object is placed on the principal axis of a concave mirror and beyond (away from) its centre of curvature ‘C’, the image is formed between the focus (F) and the centre of curvature (C). The ray which is parallel to principle axis will pass through focus after reflection and the ray which passes through focus will travels parallel to principal axis after reflection. These two rays will converge between F and C of the mirror.

The image is real, inverted and diminished in size.

Question 6
Why do we prefer a convex mirror as a rear-view mirror in the vehicles?

Answer
We use convex mirror as a rear-view mirror in the vehicles because

1. Convex mirror always forms virtual, erect, and diminished images irrespective of distance of the object.
2. A convex mirror enables a driver to view large area of the traffic behind him.
3. Convex mirror forms very small image than the object. Due to this reason convex mirrors are used as rear-view mirrors in vehicles.

Multiple choices questions

Question 1
If an object is placed at ‘C’ on the principal axis in front of a concave mirror, the position of the image is ………………. . [ ]
(a) at infinity
(b) between F and C
(c) at C
(d) beyond C

Answer
(c) at C

Question 2
We get a diminished image with a concave mirror when the object is placed ………………………. . [ ]
(a) at F
(b) between the pole and F
(c) at C
(d) beyond C

Answer
(d) beyond C

Question 3
We get a virtual image in a concave mirror when the object is placed …………………….. . [ ]
(a) at F
(b) between the pole and F
(c) at C
(d) beyond C

Answer
(b) between the pole and F

Question 4
Which of the following represents Magnification? [ ]
(i) vu
(ii) −vu
(iii) hih0
(iv) h0hi
(a) i, ii
(b) ii, iii
(c) iii, iv
(d) iv, i

Answer
(d) iv, i

Question 5
ray which seems to be traveling through the focus of a convex mirror, path of the reflected ray of an incident ……………… . [ ]
(a) parallel to the axis
(b) along the same path in opposite direction
(c) through F
(d) through C

Answer
(a) parallel to the axis

 

Question 6
Size of image formed by a convex mirror is always …………………… .[ ]
(a) enlarged
(b) diminished
(c) equal to the size of object
(d) depends on position of object

Answer
(b) diminished

Question 7
An object is placed at a certain distance on the principal axis of a concave mirror. The image is formed at a distance of 30 cm from the mirror. Find the object distance if radius of curvature R = 15cm. [ ]
(a) 15 cm
(b) 10 cm
(c) 30 cm
(d) 7.5 cm

Answer
(c) 30 cm

Question 8
All the distances related to spherical mirror will be measured from …………………… .[ ]
(a) object to image
(b) focus of the mirror
(c) pole of the mirror
(d) image to object

Answer
(c) pole of the mirror

Question 9
The minimum distance from real object to a real image in a concave mirror is ………………….. . [ ]
(a) 2f
(b) f
(c) 0
(d) f/2

Answer
(c) 0