Hello friends,
In this video, we will revise tenth chapter of Science – Light – Reflection and Refraction.
You can now see on your screen the concepts that we will discuss in this video.
Let us now quickly revise this chapter.
Note that these laws of reflection are obeyed by all types of reflecting surfaces.
Spherical Mirrors: The reflecting surface of a spherical mirror may be curved inwards or outwards.
You can see these mirror on your screens.
4. Aperture: The diameter of the reflecting surface of spherical mirror is called aperture.
For spherical mirrors of small apertures, the radius of curvature is found to be equal to twice the focal length. We put this as R = 2f .
Let us now revise the ray diagrams and properties of images formed by spherical mirrors. For this, we always consider an object of extendable size, and two light rays emerging from it. The intersection of any two rays can help us locating the image of a given object.
A ray parallel to the principal axis, after reflection, will pass through the principal focus in case of a concave mirror or appear to diverge from the principal focus in case of a convex mirror as you can see in this diagram.
3.2 A ray passing through the principal focus of a concave mirror or a convex mirror, after reflection, will emerge parallel to the principal axis as shown in the diagram.
A ray passing through the centre of curvature of a concave mirror or a convex mirror, after reflection, is reflected back along the same path.
3.4 A ray incident obliquely to the principal axis, towards a point P (pole of the mirror), on the concave mirror or a convex mirror, is reflected obliquely.
Note that at the point of incidence, the incident ray is reflected in such a way that the angle of reflection equals the angle of incidence. Using any two of the above rules, you can draw ray diagrams for both concave and convex lenses, for any position of the object. You can see these diagrams on your screen. These are the images formed by concave mirror.
Similarly you can draw ray diagrams for images formed by convex mirror.
Now, different images can have different characteristics. The nature, position and size of the image formed depends on the position of the object in relation to points P, F and C. When an image is formed by actual intersection of the rays, it is real. And when rays appear to intersect at a point, the image formed is virtual. The image is either magnified, reduced or has the same size, depending on the position of the object.
On your screen now you can see the nature, size and position for objects kept at different positions for concave and convex mirror.
Concave Mirrors –
Convex Mirrors –
The object distance (u), image distance (v) and focal length of mirror (f) follow a relationship given as:
1/f =1/v +1/u
Magnification produced by a spherical mirror is given as –
m=h’/h =-v/u where h’ is the height of the image and h is the height of object.
You must use the New Cartesian Sign Convention while substituting numerical values for u, v, f, and R in the mirror formula for solving problems.
You can see them in this diagram –
Now, let us revise another phenomena of light i.e. refraction. When travelling obliquely from one medium to another, the direction of propagation of light in the second medium changes. This phenomenon is known as refraction of light.
(i) The incident ray, the refracted ray and the normal to the interface of two transparent media at the point of incidence, all lie in the same plane.
(ii) The ratio of sine of angle of incidence to the sine of angle of refraction is a constant, for the light of a given colour and for the given pair of media.
This law is also known as Snell’s law of refraction.
Mathematically, sin i / sin r =constant
This constant value is called the refractive index of the second medium with respect to the first.
To read about refractive index in detail, look at this video:
Just like mirrors, we have two types of lenses – concave and convex. We will study some rules for image formation for both of them.
1. A ray of light from the object, parallel to the principal axis, after refraction from a convex lens, passes through the principal focus on the other side of the lens. In case of a concave lens, the ray appears to diverge from the principal focus located on the same side of the lens,
2. A ray of light passing through a principal focus, after refraction from a convex lens, will emerge parallel to the principal axis. A ray of light appearing to meet at the principal focus of a concave lens, after refraction, will emerge parallel to the principal axis.
3. A ray of light passing through the optical centre of a lens will emerge without any deviation for both the lenses.
The ray diagrams for the image formation in a convex and concave lens for different positions of the object can now be seen on your screen.
For lenses, we follow sign conventions, similar to the one used for spherical mirrors. All measurements are taken from the optical centre of the lens. The focal length of a convex lens is positive and that of a concave lens is negative. The nature, position and size of the image formed by a convex and concave lens are shown in your screen.
As we have a formula for spherical mirrors, we also have formula for
spherical lenses, which relates u, v and f –
1/f =1/v-1/u
And magnification is given as –
m=h’/h =v/u
Take proper care of the signs of different quantities, while putting numerical values for solving problems related to lenses and mirrors
It is the ability of a lens to diverge or converge the light rays. The power of a lens is defined as the reciprocal of its focal length. Its SI unit is Dioptre.
We can simply write –
P=1/f
If f is expressed in metres, then, power is expressed in dioptres. Remember to convert the f in metres while solving the numerical problems.
Note that the power of a convex lens is positive and that of a concave lens is negative.
Let us now understand this chapter’s weightage. Generally, questions of 1, 2, 3 and 5 marks are asked from this chapter.
Some question types which have been repeatedly asked in the previous year papers are:
1. Based on finding the nature, size and position of the image for an object kept in front of a mirror or a lens, along with its ray diagram or vice-versa.
2. Another type of question can be a numerical based on mirror formula or lens formula and magnification or power of the lens. There is a high probability of a 3 marks or a 5 marks question on this topic.
Let us now solve a sample question based on this concept-
Q. At what distance should an object be placed from a convex lens of focal length 18 ?? to obtain an image at 24 ?? from it on the other side. What will be the magnification produced in this case? [CBSE,2010]
Solution-
We are given:
f= +18 cm
v= +24 cm
u=?
By lens formula,1/f =1/v-1/u
∴1/u=1/v-1/f
=1/((+24))-1/((+18))
=1/24-1/18=(3-4)/72=(-1)/72
∴ u = -72 cm
m=h’/h=v/u
So m=((+24))/((-72))
m=-1/3
3. Remember that practising ray diagrams is very important for this chapter as you are generally asked to draw them for most of the questions.
You can now see on your screen the previous years’ questions from this chapter.
1. Draw the following diagram in your answer-book and show the formation of image of the object ?? with the help of suitable rays. [CBSE,2008]
2. Explain why a ray of light passing through the centre of curvature of a concave mirror gets reflected along the same path. [CBSE,2010]
3. What is the nature of the image formed by a concave mirror if the magnification produced by the mirror is +3? [CBSE,2010]
1. Draw ray diagrams to represent the nature, position and relative size of the image formed by a convex lens for the object placed :
2. The absolute refractive indices of glass and water are 4/3 and 3/2 respectively. If the speed of light in glass is 2 108 ?/?, calculate the speed of light in
3. Name the type of mirrors used in the design of solar furnaces. Explain how high temperature is achieved by this device. [CBSE, 2016]
4. An object is placed at a distance of 30 ?? from a concave lens of focal length 15 ??. List four characteristics (nature, position, etc.) of the image formed by the lens. [CBSE, 2017]
1. For which position of the object does a convex lens form a virtual and erect image? Explain with the help of a ray diagram. [CBSE, 2009]
2. At what distance should an object be placed from a convex lens of focal length 18 ?? to obtain an image at 24 ?? from it on the other side. What will be the magnification produced in this case? [CBSE, 2010]
3. If the image formed by a mirror for all positions of the object placed in front of it is always erect and diminished, what type of mirror is it? Draw a ray diagram to justify your answer. Where and why do we generally use this type of mirror? [CBSE, 2015]
4. The image of an object formed by a mirror is real, inverted and is of magnification −1. If the image is at a distance of 40 ?? from the mirror, where is the object placed? Where would the image be if the object is moved 20 ?? towards the mirror? State reason and also draw ray diagram for the new position of the object to justify your answer. [CBSE, 2016]
5. If the image formed by a lens for all positions of an object placed in front of it is always erect and diminished, what is the nature of this lens? Draw a ray diagram to justify your answer. If the numerical value of the power of this lens is 10 ?, what is its focal length in the Cartesian system? [CBSE, 2017]
1. What is meant by power of a lens? Define its S.I. unit.
You have two lenses ? and ? of focal lengths +10 ?? and −10 ?? respectively. State the nature and power of each lens. Which of the two lenses will form a virtual and magnified image of an object placed 8 ?? from the lens? Draw a ray diagram to justify your answer. [CBSE, 2015]
2. One half of a convex lens of focal length 10 ?? is covered with a black paper. Can such a lens produce an image of a complete object placed at a distance of 30 ?? from the lens? Draw a ray diagram to justify your answer.
A 4 ?? tall object is placed perpendicular to the principal axis of a convex lens of focal length 20 ??. The distance of the object from the lens is 15 ??. Find the nature, position and size of the image. [CBSE, 2015]
3. It is desired to obtain an erect image of an object, using concave mirror of focal length of 12 ??.
Show the positions of pole, principal focus and the centre of curvature in the above ray diagrams. [CBSE, 2016]
4. a) Define optical centre of a spherical lens.
b) A diverging lens has a focal length of 20 ??. At what distance should an object of height 4 ?? from the optical centre of the lens be placed so that its image is formed 10 ?? away from the lens. Find the size of the image also.
c) Draw a ray diagram to show the formation of image in above situation. [CBSE, 2016]
5. a) In the image formed by a mirror for all positions of the object placed in front of it is always diminished, erect and virtual, state the type of the mirror and also draw a ray diagram to justify your answer. Write one use such mirrors are put to and why.
b) Define the radius of curvature of spherical mirrors. Find the nature and focal length of a spherical mirror whose radius of curvature is +24 ??. [CBSE, 2017]
6. Analyse the following observation table showing variation of image-distance (?) with object-distance (?) in case of a convex lens and answer the questions that follow without doing any calculations :
| S.No. | Object-Distance
u (cm) |
Image-Distance
v (cm) |
| 1 | -100 | +25 |
| 2 | -60 | +30 |
| 3 | -40 | +40 |
| 4 | -30 | +60 |
| 5 | -25 | +100 |
| 6 | -15 | +120 |
1. A student obtains a sharp image of the distant window (?) of the school laboratory on the screen (?) using the given concave mirror (?) to determine its focal length. Which of the following distances should he measure to get the focal length of the mirror?
2. A student used a device (?) to obtain/focus the image of a well illuminated distant building on a screen (?) as shown below in the diagram. Select the correct statement about the device (?).
3. A student traces the path of a ray of light through a rectangular glass slab for the different values of angle of incidence. He observes all possible precautions at each step of the experiment. At the end of the experiment, on analyzing the measurements, which of the following conclusions is he likely to draw?
4. To determine the approximate value of the focal length of a given concave mirror, you focus the image of a distant object formed by the mirror on a screen. The image obtained on the screen, as compared to the object is always :
5. Suppose you have focused on a screen the image of candle flame placed at the farthest end of the laboratory table using a convex lens. If your teacher suggests you to focus the parallel rays of the sun, reaching your laboratory table, on the same screen, what you are expected to do is to move the :
6. In your laboratory, you trace the path of light rays through a glass slab for different values of angle of incidence (∠?) and in each case measure the values of the corresponding angle of refraction (∠?) and angle of emergence (∠?). On the basis of your observations your correct conclusion is :
7. Study the given ray diagrams and select the correct statement from the following :
8. 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
9. A student very cautiously traces the path of a ray through a glass slab for different values of the angle of incidence (∠?). He then measures the corresponding values of the angle of refraction (∠?) and the angle of emergence (∠?) for every value of the angle of incidence. On analysing these measurements of angles, his conclusion would be
1. A 4 ?? tall object is placed on the principal axis of a convex lens. The distance of the object form the optical centre of the lens is 12 ?? and its sharp image is formed at a distance of 24 ?? from it on a screen on the other side of the lens. If the object is now moved a little away from the lens, in which way (towards the lens or away from the lens) will he have to move the screen to get a sharp image of the object on it again? How will the magnification of the image be affected? [CBSE, 2015]
2. An object of height 2.5 ?? is placed at a distance of 15 ?? from the optical centre ‘?’ of a convex lens of focal length 10 ??. Draw a ray diagram to find the position and size of the image formed. Mark optical centre ‘?’, principal focus ? and height of the image on the diagram. [CBSE, 2016]
3. A student focuses the image of a candle flame, placed at about 2 ? from a convex lens of focal length 10 ??, on a screen. After that he moves gradually the flame towards the lens and each time focuses its image on the screen.
1. For which position of the object does a convex lens form a virtual and erect image? Explain with the help of a ray diagram. [CBSE, 2009]
2. At what distance should an object be placed from a convex lens of focal length 18 ?? to obtain an image at 24 ?? from it on the other side. What will be the magnification produced in this case? [CBSE, 2010]
3. If the image formed by a mirror for all positions of the object placed in front of it is always erect and diminished, what type of mirror is it? Draw a ray diagram to justify your answer. Where and why do we generally use this type of mirror? [CBSE, 2015]
4. The image of an object formed by a mirror is real, inverted and is of magnification −1. If the image is at a distance of 40 ?? from the mirror, where is the object placed? Where would the image be if the object is moved 20 ?? towards the mirror? State reason and also draw ray diagram for the new position of the object to justify your answer. [CBSE, 2016]
5. If the image formed by a lens for all positions of an object placed in front of it is always erect and diminished, what is the nature of this lens? Draw a ray diagram to justify your answer. If the numerical value of the power of this lens is 10 ?, what is its focal length in the Cartesian system? [CBSE, 2017]