Light – Reflection and Refraction CBSE Class 10 Revision & Important Questions

Spherical Mirrors: The reflecting surface of a spherical mirror may be curved inwards or outwards.

1. A spherical mirror, whose reflecting surface is curved inwards, that is, faces towards the centre of the sphere, is called a concave mirror.
2. A spherical mirror whose reflecting surface is curved outwards, is called a convex mirror.

You can see these mirror on your screens.

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 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.

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.

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.

Laws of Refraction

(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:

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,

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.

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

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.

1 Mark Questions

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?

• ??
• ??
• ??
• ??−?? [CBSE, 2015]

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 (?).

• This device is a concave lens of focal length 8 ??.
• This device is a convex mirror of focal length 8 ??.
• This device is a convex lens of focal length 4 ??.
• This device is a convex lens of focal length 8 ??. [CBSE, 2015]

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?

• ∠?=∠?<∠?
• ∠?<∠?<∠?
• ∠?>∠?>∠?
• ∠?=∠?>∠? [CBSE, 2015]

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 :

• Laterally inverted and diminished
• Inverted and diminished
• Erect and diminished
• Erect and highly diminished [CBSE,2016]

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 :

• lens slightly towards the screen
• lens slightly away from the screen
• lens slightly towards the sun
• lens and screen both towards the sun [CBSE, 2016]

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 :

• ∠? is more than ∠?, but nearly equal to ∠?
• ∠? is less than ∠?, but nearly equal to ∠?
• ∠? is more than ∠?, but nearly equal to ∠?
• ∠? is less than ∠?, but nearly equal to ∠?  [CBSE, 2016]

7. Study the given ray diagrams and select the correct statement from the following :

• Device ? is a concave mirror and device ? is a convex lens, whose focal lengths are 20 ?? and 25 ?? respectively.
• Device ? is a convex lens and device ? is a concave mirror, whose focal lengths are 10 ?? and 25 ?? respectively.
• Device ? is a concave lens and device ? is a convex mirror, whose focal lengths are 20 ?? and 25 ?? respectively.
• Device ? is a convex lens and device ? is a concave mirror, whose focal lengths are 20 ?? and 25 ?? respectively. [CBSE, 2017]

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

• away from the screen
• towards the screen
• to a position very far away from the screen
• either towards or away from the screen depending upon the position of the object. [CBSE, 2017]

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

• ∠?>∠?>∠?
• ∠?=∠?>∠?
• ∠?<∠?<∠?
• ∠?=∠?<∠? [CBSE, 2017]

2 Marks Questions:

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.

• In which direction does he move the lens to focus the flame on the screen?
• What happens to the size of the image of the flame formed on the screen?
• What difference is seen in the intensity (brightness) of the image of the flame on the screen?
• What is seen on the screen when the flame is very close (at about 5 ??) to the lens ? [CBSE, 2017]

3 Mark Questions

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]