Light is a form of energy which enables us to see objects.

Activity to show that light travels in a straight line

If we try to see at a lighted candle through a straight pipe with one eye (keeping the other eye closed), then we can see the light of candle flame and if we look at the lighted candle through a bent pipe in a similar way, then we will not be able to see the light of candle flame. It means that the light of candle flame can travel through the straight hole of a straight pipe but not through the bent hole of a bent pipe.

Reflection of Light

There are some certain situations in which a mirror or shiny surfaces like stainless steel plate, shining steel spoon, or tile floor, act as a mirror, can change the direction of light that falls on it.

This process of change in direction of light by a mirror is called reflection of light.

The surface of water can also act like a mirror and can also change the path of light and that is why we see the reflection of trees or buildings in water.

Activity to show the reflection of light by a plane mirror

Hold a torch and take a black chart paper. Now, make three long and narrow cuts (slits) in it and try to cover the glass of the torch with this black chart paper having long and narrow cuts.

Now, spread another chart paper on a drawing board and fix a rectangular plane mirror strip vertically on this chart paper as shown in figure.

Switch ON the torch, this will produce three rays of light. Put the torch in such a way that its light rays can be seen along the chart paper fixed on the drawing board. Try to adjust the position of the torch in such a way that all the three rays of light coming from the three slits fall on the plane mirror at an angle to its surface.

We observe that the plane mirror varies the direction of the three rays of light falling on it.

Image of An Object

When we see our face in a mirror then our face is the object and what we see in the mirror is the image of this object.

The image of our face seen in the mirror is formed where light rays, after reflection from the mirror, seem to originate from the image of our face behind the mirror.

There are two types of images:

(i) Real image: It is an image which can be obtained on a screen, e.g. the image formed on a cinema screen. When the light rays coming from an object actually meet at a point after reflection from the mirror, then it results in the formation of a real image.

(ii) Virtual image: Virtual image is an image which cannot be obtained on a screen, e.g. image formed by a plane mirror. When the light rays coming from an object appear to meet after reflection from the mirror, then it results in the formation of virtual image. It is not possible to form a virtual image on the screen because light rays actually do not pass the screen or cannot be received on a screen.

Activity to observe the characteristics of image formed by a plane mirror

We put a lighted candle in front of a plane mirror (see figure). In this case the candle is the object. When we look into the plane mirror, it appears as if a similar candle is placed behind the mirror. The candle which appears to be behind the mirror is the image of the candle formed by the plane mirror and this image is formed as follows.

Light rays from the candle travel in straight lines to the mirror and the same mirror reflects (sends back) these rays of light. Some of the reflected light is entering into our eyes. As, the light seems to be coming from behind the mirror, we can see the image of candle at that position. The image of candle seen in the plane mirror is virtual.

Characteristics of the Images Formed by a Plane Mirror

(i) When we see the mirror, the image of candle appears to be formed behind the mirror.

(ii) Now, put a vertical screen behind the plane mirror (where the image of candle appears to be situated), then we will notice that the image of candle cannot be formed on the screen. Even if the screen is placed in front of the plane mirror, then the image of candle cannot be formed on the screen. Since, the image of candle formed in the plane mirror cannot be formed on a screen, which means that the image of candle in the plane mirror is a virtual image.

(iii) If we see the figure, then we will find that the length and breadth of the image of the candle and its flame to be the same as that of the original candle and its flame. The image of candle in the plane mirror is of the same size as the original candle.

(iv) The candle has a flame at the top and the image of candle also has a flame at the top. The top of candle remains at the top in the image. In the same way, the bottom of candle remains at the bottom in an image. Such an image is called an erect image (or upright image). Therefore, the image formed by a plane mirror is erect.

Activity to show that image formed in a plane mirror is at the same distance behind the mirror as the object is in front of it

We take a chess board and fix a plane mirror vertically along any one edge of it, as shown in the figure.

Now we put any small object such as sharpener at the boundary of the fourth square counting from the mirror. Carefully, note down the position of the image.

Now, try to shift the position of the sharpener at the boundary of fifth square and again note the position of the image.

We will notice that the image is at the same distance behind the mirror as the object is in front of it.

Therefore, the image formed in a plane mirror is at the same distance behind the mirror as the object is in front of the mirror.

Spherical Mirrors ( Curved Mirrors )

All the mirrors are not straight like plane mirror as some of the mirrors are curved mirror. There is a common example of a curved mirror, i.e. spherical mirror.

A mirror whose reflecting surface is the part of a hollow sphere of glass is known as a spherical mirror.

Image Formed by Spherical Mirror

It is a fact that spherical mirrors form images of the objects placed in front of them. These images are formed, when light rays coming from the object fall on the mirror, get reflected and converge or diverge. We can use a spoon in order to understand the image formation by spherical mirror.

Activity to show the formation of an image from the inner side or outer side of a spoon.

We take a stainless steel spoon and bring the outer side of the spoon (which forms a convex mirror) near our face and look into it.

We observe that our image in the spoon is erect and smaller than our face, i.e. object.

Now, we look into the spoon using the inner side (which forms a concave mirror) of it. We observe that if our face is close to the spoon, then the image is erect and larger in size. If we increase the distance of the spoon from our face, we observe that our image is inverted.

From the above activity, we can conclude that,

·      In a convex mirror, the image is erect and smaller than our face.

·      In a concave mirror, image of our face is

o  erect and larger in size if we are close to the mirror. In this case the image is said to be virtual as it is formed behind the mirror and can not be seen on a screen. Hence, in this case the image is virtual and erect.

o  inverted if we are away from the mirror. In this case size of the image can be larger or smaller depending on how far the object (face) is from the mirror. In this case the image is said to be real as it is formed in front of the mirror and can be seen on a screen. Hence, in this case image is real and inverted.

Concave Mirror (Converging Mirror)

The mirror whose reflecting surface is concave (and polished surface is convex) is called a concave mirror.

The concave mirror reflects the parallel rays of light in such a way that after reflection, all the rays converge (or meet) at one point called focus in front of the mirror.

Since, a concave mirror converges a beam of parallel light rays, it is also known as a converging mirror.

Image Formed by a Concave Mirror When the Object is Far Off

A concave mirror forms a real image of the sun. We can understand the formation of image by a concave mirror when the object is far off by an activity.

Activity to observe the formation of image

Take a concave mirror and hold it facing the sun (as the sun is a source of light which is very far off and here, we can use the sun as a far off object), so that sun's rays fall on it as shown in figure.

Hold a sheet of paper in the other hand and try to get the light rays reflected by concave mirror on the sheet of paper. Now, we will adjust the distance of paper from concave mirror until we get a sharp and bright spot of light on paper. This bright spot of light on paper is the image of the sun formed by the concave mirror.

Hold the concave mirror and the sheet of paper in this position for few minutes, we will notice that after sometime, the paper burns at the spot where the sun’s image was formed because concave mirror converges the sun's parallel rays and when lot of sun’s heat rays are concentrated on a small point of paper, then the paper gets heated too much at that point and burns to form a hole.

Therefore, we can conclude that the image formed by concave mirror is much smaller than the object (highly diminished) and real because it can be obtained on a sheet of paper (which is a kind of screen).

When an object is placed at a far off distance from a concave mirror, then image formed by concave mirror is

(i) real

(ii) inverted

(iii) much smaller than the object.

Image Formed by a Concave Mirror When the Object is Placed Close to Concave Mirror

Let us perform an activity to understand the formation of image by a concave mirror when the object is placed close to concave mirror.

Activity to observe the formation of image

We take a concave mirror, a candle, a matchstick and a white sheet of paper. Place the concave mirror on the stand so that its reflecting surface is perpendicular to the ground.

Light a candle and put it at a distance of about 50 cm from the reflecting surface of the mirror. Take the white sheet of paper and hold it on the other side of the candle. The plane of the sheet should be parallel to the surface of the mirror.

Move the white sheet towards or away from the candle till we obtain a sharp image of the flame on it. Record the nature of the image formed. Now, move the candle by a distance of about 10 cm towards the mirror and again adjust the position of the white sheet to obtain the image of the candle flame on it.

 

We observe that (except for when the candle is very close to the mirror) the image is always real, inverted, and formed on the same side as the object. Also, depending on the distance between the object and the mirror, the image size can be diminished, enlarged, or the same size as the object.

When the object is very close to the mirror, the image is formed behind the mirror. It is virtual, erect, and larger in size than the object. It cannot be obtained on the white sheet.

Considering the figure given below, Image Formation by a Concave mirror can be summarized as follows:

Position of the object	Position of the image	Size of the image	Nature of the image
At infinity	At focus, F	Highly diminished and pointed in size	Inverted and Real
Beyond C	Between F and C	Diminished	Inverted and Real
At C	At C	Same size	Inverted and Real
Between C and F	Beyond C	Enlarged	Inverted and Real
At F	At infinity	Highly enlarged	Inverted and Real
Between F and P	Behind the mirror	Enlarged	Erect and virtual

Conclusion

Hence, we can conclude that when an object is placed close to a concave mirror, the image formed by concave mirror is

(i) virtual

(ii) erect

(iii) larger than the object (enlarged or magnified).

Uses of Concave Mirrors

(i) To see the large image of teeth of patient, concave mirrors are used by dentist.

(ii) In torches, headlights of vehicles and search lights to get a strong, straight beam of light, etc., concave mirrors are used as reflectors.

(iii) To see a large image of face, then concave mirrors are used as shaving mirrors.

Convex Mirror (Diverging Mirror)

The mirror whose reflecting surface is bulging or convex (polished surface is concave) is called convex mirror. After reflection from convex mirror, the parallel rays of light are spreading out. When the parallel rays of light spread out, we can say that the rays of light are diverging.

We can say that a beam of parallel light rays diverges (spreads out) after reflection from a convex mirror.

Since, a convex mirror diverges a beam of parallel light rays, therefore, it is also known as a diverging mirror.

Image Formed By a Convex Mirror

Activity to observe that the image formed by a convex mirror is always virtual, erect and smaller than the object

Take a convex mirror, fix it on a stand and keep it on a table. Now, place a lighted candle (as object) at any distance from the convex mirror. Now, put a screen in front of the convex mirror in such a way that it should not obstruct the path of light rays coming from the candle flame and falling on the convex mirror. Move the screen towards or away from the convex mirror and try to obtain the image of candle flame on the screen.

We observe that in this case, we cannot form the image of candle flame on the screen. Now, see the convex mirror directly. We will observe a small image of candle and its flame in the convex mirror.

The image of candle can be seen only by looking into the convex mirror and cannot be formed on a screen. It is a virtual image. If we look at the image in the convex mirror, we will find that it is same side up as the candle. So, the image is erect. And if we compare the size of the candle and its image, the image appears to be smaller. Therefore, the image is smaller in size than the object (or diminished). Even if we change the distance of candle (object) from the convex mirror, we will notice that in every case, the image of candle formed by the convex mirror remains virtual, erect and smaller in size than the candle.

Position of Object	Position of Image	Size of Image	Nature of Image
At Infinity	At the focus F, behind the mirror	Highly diminished	Virtual and Erect
Between Infinity and the Pole	Between P and F, behind the mirror	Diminished	Virtual and Erect

Conclusion

We can conclude that whatever be the distance of object from a convex mirror, the image formed by a convex mirror is always

(i) virtual

(ii) erect

(iii) smaller than the object (or diminished).

Uses of Convex Mirrors

(i) To see the traffic at the rear side or backside on the road, convex mirrors are used as rear view mirrors or side view mirrors in vehicles such as cars, scooters, buses, etc.

(ii) Big convex mirrors are used as shop security mirrors. By installing a convex mirror in the shop, the shop owner can keep an eye on the customers.

Image Formed by Lenses

Since, a lens is a piece of transparent glass bound by the two spherical surfaces. Lenses are transparent so that light can pass through lenses. Lenses are of two types:

(i) Convex lens

(ii) Concave lens

Convex Lens (Converging Lens)

Convex lens is the lens which is thicker in the middle than at the edges. A beam of parallel rays of light falls on a convex lens from the left side. After passing through the convex lens, the beam of parallel rays of light converges at a point as shown in figure given below. Hence, a convex lens is a converging lens.

Image Formed by a Convex Lens

The nature and size of the image formed by a convex lens depends on the distance of the object from the convex lens.

Activity to observe formation of image by a convex lens, when the object is far off

We take a convex lens, fix it on a stand and keep it on a table. Put a lighted candle (as object) at a distance of about 100 cm (or more) in order to make it a far off object. Now, place a screen at some distance behind the convex lens. Move the screen towards or away from the convex lens until you get a sharp image of the candle flame on the screen (see figure). Since, the image of candle flame can be obtained on a screen kept behind the convex lens, therefore, the image is real. If we look at the image of the flame, we will observe that the image is upside down. So, the image is inverted. And if we compare the size of flame in the candle and its image, we will find that the image is much smaller. Therefore, the image is much smaller than the object (or the image is highly diminished).

Thus, we can conclude that when an object is placed at a far off distance from a convex lens, then the image formed by the convex lens is real, inverted and much smaller than the object (or highly diminished).

Now, change the distance of the candle from the lens and try to obtain the image of the candle flame every time on the screen by moving it. So, is it possible to get in any position of the object for which image was erect and magnified? Yes, it is possible when the candle is placed very close to the convex lens.

Activity to observe formation of image by a convex lens, when the object is close to convex lens.

Take a convex lens, fix it on a stand and keep it on a table. Put a lighted candle (object) close to the convex lens. Now, place a screen at some distance behind the convex lens. Move the screen backward and forward and try to get the image of candle flame on the screen. We will find that in this case, we cannot obtain the image of candle flame on the screen (see figure). Therefore, when the candle (or object) is close to convex lens, it is not possible to obtain its image on the screen. Now, look through the convex lens by keeping your eye on the back side of the convex lens. We will observe a large image of the candle and its flame through the convex lens as shown in figure.

Since, the image can be seen only by looking through the convex lens and cannot be obtained on the screen, therefore, the image is virtual. If we look at the image, we find it to be same side up as the candle, so the image is erect. And if we compare the size of candle and its image, then we will notice that the image is larger than the candle.

Uses of Convex Lenses

(i) Convex lenses are used as a magnifying glass.

(ii) In the manufacturing of spectacles, camera, microscope, telescope and binoculars, convex lenses are used.

Concave Lens (Diverging Lens)

Concave lens is the lens which is thinner in the middle than at the edge. A parallel beam of light falls on a concave

lens as shown in figure. After passing through the concave lens, the rays of light are diverging (or spreading out).

Since, a concave lens diverges light rays falling on it, therefore, it is also called a diverging lens.

Image Formed by a Concave Lens

In the case of a convex lens, we have studied that the nature of image formed depends on the distance of the object from the convex lens. But this is not followed in the case of a concave lens.

Let us perform an activity to understand the formation of image by a concave lens.

Activity to show that the image formed by a concave lens is always virtual, erect and smaller than the object

Take a concave lens, fix it on a stand and keep it on a table. Place a lighted candle (as object) at any distance (say, 15 cm) from the concave lens. Now, place a screen at some distance behind the concave lens and try to move the screen backward and forward and try to get the image of candle flame on the screen.

The image of candle flame on the screen kept behind the concave lens (see figure). Even if we place the screen on the front side of lens, then the image of candle flame cannot be formed on it. Now, look through the concave lens by keeping our eye behind the lens.

We observe a small image of the candle and its flame through the concave lens and cannot be obtained on a screen, therefore, the image is virtual. If you look at the image, we find it to be the same side up as the candle. So, the image is erect. And if we compare the size of candle and its image, we will find that the image is smaller than the candle.

Therefore, we can conclude that whatever be the distance of an object from a concave lens, the image formed by a concave lens is always (i) virtual (ii) erect and (iii) smaller than the object (or diminished).

Uses of Concave Lenses

(i) In order to see the image of the person standing outside, concave lenses are used in the peep holes in the door of hotel rooms.

(ii) Concave lenses are used in making spectacles.

Sunlight : White or Coloured

We might have noticed a rainbow which usually appears after the rain when the sun is low in the sky. An arc of seven colours seen in the sky is known as the rainbow. The seven colours of a rainbow are red, orange, yellow, green, blue, indigo and violet. We might also have seen that when we blow soap bubbles, they appear colourful. Similarly, when light is reflecting from the surface of a Compact Disc (CD), we can see many colours. The rainbow is produced by the dispersion of sunlight by tiny rain drops suspended in the atmosphere.

Dispersion of Light

In the year 1665, Newton discovered by his experiments with glass prisms that white light (like sunlight) consists of a mixture of lights of seven colours. Newton found that if a beam of white light is passed through a glass prism, then the white light splits to form a band of seven colours on a white screen . The band of seven colours formed on a white screen, when a beam of white light is passed through a glass prism, is known as spectrum of white light. The seven colours of the spectrum are Red, Orange, Yellow, Green, Blue, Indigo and Violet.

So, dispersion of light is phenomenon of splitting up of white light into seven colours on passing through a transparent medium like a glass prism. The formation of spectrum of seven colours indicates that white light is a mixture of seven colours. White light can be sunlight. So, now we can say that the sunlight consists of seven colours.

We can mix these colours to get white light. This can be done by using Newton's disc, let us try this.

Activity to show that when seven colours are mixed together, then white colour is formed.

We take a circular cardboard disc of about 10 cm diameter, then divide this disc into seven equal segments and paint the seven colours of white light (red, orange, yellow, green, blue, indigo and violet) on these segments (as shown in figure given ).

Instead of painting the segments, we can also paste seven coloured papers on these segments. This disc painted with the seven colours of spectrum is called Newton's disc. Now, make a small hole at the centre of the disc.

Fix the disc loosely on the tip of a refill of a ball pen. Make sure that the disc can rotate freely. Now, rotate the disc quickly in day light. When the disc is rotated very fast, then all the seven colours mix together due to which the disc appears to be white (see figure). Therefore, the rapid rotation of Newton's colour disc tells us that mixing of seven colours of the spectrum can give us white light.