We perceive the world around us, including objects like trees, cars, and buildings, because of **light**. In the absence of light, such as in a dark room, we cannot see anything. Light enables us to see objects.

Sources of light, like the Sun, a torch, or headlights, emit light that travels outwards. When this light falls on objects and then enters our eyes, we are able to see those objects.

Observations of beams of sunlight entering a room or beams from headlights suggest something about how light travels.

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Light Travels Along A Straight Line

The way light forms sharp shadows and beams indicates that it follows a particular path. A simple experiment can demonstrate this:

If you try to look at a lighted candle flame through a **straight pipe**, you can see the flame. However, if you try to look through a **bent pipe**, the flame is **not visible**. This shows that light cannot bend around corners; it travels in a **straight line**.

Knowing that light travels in straight lines, can we alter its path? Yes, the direction of light can be changed when it encounters certain surfaces.

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Reflection Of Light

When light falls on a polished or shiny surface, its direction changes. This phenomenon is called **reflection of light**. Any smooth, shiny surface can act as a **mirror**, causing reflection.

Common examples of surfaces that cause reflection include a shining stainless steel plate, a new steel spoon, and the surface of still water. You might have seen the reflection of trees, buildings, or the sky in a pond or lake.

A mirror specifically changes the direction of light rays that fall on it. We can investigate this with a simple activity.

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Activity 15.1

Using a torch covered with a chart paper having three narrow slits (to create narrow beams of light) and a plane mirror strip fixed vertically on a board. Direct the light beam from the torch onto the mirror at an angle. Observe how the light is bounced back from the mirror. The direction of the outgoing light (reflected light) is different from the direction of the incoming light (incident light).

When you look into the mirror along the path of the reflected light, you see an image of the slits. This activity demonstrates that a mirror changes the direction of light by reflecting it.

Paheli's question suggests that objects are visible to us because light from a source (luminous object) or light reflected from an object (non-luminous object) reaches our eyes.

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Activity 15.2

Placing a lighted candle (**object**) in front of a plane mirror allows you to see its likeness behind the mirror. This likeness is called the **image** of the candle formed by the mirror. Observe the image formed by a plane mirror. It appears as if a similar candle is placed behind the mirror.

In the case of a plane mirror, the image formed is always **erect** (upright, not upside down) and appears to be of the **same size** as the object.

Can this image be captured on a screen? If you place a screen (like a piece of paper) behind the mirror where the image appears, or in front of the mirror, you will find that the image of the candle flame **cannot be obtained on the screen**. An image that cannot be obtained on a screen is called a **virtual image**. An image that can be obtained on a screen is called a **real image**.

The image formed by a plane mirror is always **virtual**.

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Activity 15.3

Using a chessboard or a grid drawn on paper, with a plane mirror placed vertically along the middle line. Place a small object (like a sharpener) at the boundary of a square a certain distance from the mirror. Observe the position of its image behind the mirror. Repeating this for different distances (e.g., placing the object at the boundary of the third square, then the fourth square from the mirror) reveals a consistent relationship: the **image is formed at the same distance behind the mirror as the object is in front of the mirror**.

Summary of characteristics of the image formed by a **plane mirror**:

• The image is formed **behind the mirror**.
• The image is **erect** (upright).
• The image is of the **same size** as the object.
• The image is at the **same distance** behind the mirror as the object is in front.
• The image is **virtual** (cannot be obtained on a screen).

Right Or Left!

While a plane mirror forms an image that is erect and the same size, there is one key difference between you and your image: there is a **lateral inversion**.

**Lateral inversion** means that in the image formed by a plane mirror, the **left side of the object appears on the right side** in the image, and the **right side of the object appears on the left side**. The image is not flipped upside down, just side-to-side.

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Activity 15.4

Standing in front of a plane mirror and raising your left hand, you observe that your image in the mirror raises its **right hand**. Touching your right ear in front of the mirror, your image touches its **left ear**. This confirms the left-right reversal (lateral inversion) by a plane mirror.

This property of lateral inversion explains why the word 'AMBULANCE' is often written in reverse on the front of ambulances. When the driver of a vehicle in front sees the ambulance in their rear-view mirror (which is usually a plane mirror), the word 'AMBULANCE' appears correctly written due to lateral inversion, making it easily readable and prompting them to give way.

Side mirrors of scooters and cars, however, show images that are smaller than the actual objects. These are not plane mirrors.

Playing With Spherical Mirrors

Many curved shining surfaces also act as mirrors. The most common type of curved mirror is a **spherical mirror**, which is a part of a hollow sphere.

Spherical mirrors can be classified based on their reflecting surface:

• **Concave mirror:** If the **inner** surface of the spherical segment is the reflecting surface (like the inner side of a spoon). The reflecting surface is curved inwards.
• **Convex mirror:** If the **outer** surface of the spherical segment is the reflecting surface (like the outer side of a spoon). The reflecting surface is curved outwards.

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Activity 15.5

Using a stainless steel spoon to observe images. Looking at your image in the **outer side (convex surface)** of the spoon shows an image that is **erect** but **smaller** in size than your face. Looking at your image in the **inner side (concave surface)** of the spoon shows an image that can be **erect and larger** (when close to the spoon) or **inverted and smaller/same size** (when further away from the spoon). This shows that curved mirrors form different types of images than plane mirrors.

A concave mirror can form a **real image**, unlike a plane mirror. We can demonstrate this by focusing sunlight.

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Activity 15.6

Holding a concave mirror to face the Sun and directing the reflected sunlight onto a sheet of paper. By adjusting the distance of the paper from the mirror, a **sharp, bright spot** can be obtained on the paper. This bright spot is a **real image** of the Sun formed on the screen (paper). Holding the setup steady can even cause the paper to burn due to the concentrated heat energy in the focused image. This confirms that a concave mirror can form a real image that can be projected onto a screen.

Caution: Never look directly at the Sun or its image formed by a mirror or lens, as it can severely damage your eyes.

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Activity 15.7

Using a concave mirror to form the image of a lighted candle flame on a screen. By placing the candle at different distances from the concave mirror and adjusting the screen position, you can observe that a concave mirror can form images that are:

• **Real** (can be obtained on screen) or **virtual** (cannot be obtained on screen).
• **Inverted** (upside down) or **erect** (upright).
• **Smaller** than the object, **larger** than the object, or of the **same size**.

When the candle is placed very close to the concave mirror, a **virtual, erect, and magnified (larger)** image is formed, which cannot be obtained on the screen.

Table 15.1: Characteristics of image formed by a concave mirror at different object distances (Example Structure):

Distance of the object from the mirror	Smaller/larger/same size than the object	Inverted/ erect	Real/virtual
Very far	Smaller	Inverted	Real
Between far and close	Smaller, Same size, or Larger	Inverted	Real
Very close to mirror	Larger	Erect	Virtual

Concave mirrors are used by doctors to examine eyes, ears, nose, and throat (to get magnified views), by dentists to see enlarged images of teeth, and as reflectors in torches and vehicle headlights to direct light into a concentrated beam.

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Activity 15.8

Repeating the image formation activity using a **convex mirror**. No matter where the object (candle) is placed in front of a convex mirror, you will find that the image formed is always **virtual, erect, and smaller in size** than the object. It is also formed behind the mirror and cannot be obtained on a screen.

Convex mirrors form images of objects spread over a large area, making them useful as **side mirrors (rear-view mirrors)** in automobiles to help drivers see the traffic behind them.

Images Formed By Lenses

A **lens** is a transparent optical device that refracts (bends) light. Lenses are commonly used in spectacles, magnifying glasses, telescopes, and microscopes.

Lenses can be identified by their shape:

• **Convex lens:** Thicker in the middle than at the edges.
• **Concave lens:** Thinner in the middle than at the edges.

Since lenses are transparent, light passes through them and gets bent (refracted), forming images on the side opposite to the object.

Caution: Never look at the Sun or a bright light source through a lens, as it can focus the light and cause severe eye damage. Also, avoid focusing sunlight with a convex lens on your body or flammable materials.

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Activity 15.9

Using a convex lens to focus sunlight on a sheet of paper produces a sharp, bright spot, similar to using a concave mirror. This is a real image of the Sun, indicating that a **convex lens converges (bends inwards) light rays** generally falling on it. Hence, it's also called a **converging lens**. A concave lens, when used in a similar way, does not form a bright spot because it **diverges (bends outwards) light rays**; it's called a **diverging lens**.

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Activity 15.10

Using a convex lens to form images of a lighted candle flame on a screen. By placing the candle at different distances from the convex lens and moving the screen on the other side, you can obtain sharp images. A convex lens can form images that are:

• **Real** (can be obtained on screen) or **virtual** (cannot be obtained on screen).
• **Inverted** or **erect**.
• **Smaller**, **larger**, or of the **same size** as the object.

When the candle is placed very close to the convex lens, a **virtual, erect, and magnified** image is formed (Fig. 15.26). This image cannot be obtained on a screen. This is precisely how a convex lens is used as a **magnifying glass** to see enlarged images of small objects.

When using a **concave lens** to form images, you will find that the image formed is always **virtual, erect, and smaller in size** than the object, regardless of the object's position (Fig. 15.27).

Sunlight — White Or Coloured?

We often see a **rainbow** in the sky after rain, appearing as a large arc with multiple colours. A rainbow is evidence that sunlight is composed of different colours.

When observed carefully, a rainbow typically shows **seven distinct colours**: Red, Orange, Yellow, Green, Blue, Indigo, and Violet (VIBGYOR). Other phenomena like colourful soap bubbles or colours seen on a CD surface under light also suggest that white light is not pure but a mixture of colours.

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Activity 15.11

Allowing a narrow beam of sunlight to pass through a glass **prism** in a dark room. When the light emerges from the other side of the prism and falls on a white screen, it is split into a band of seven colours, similar to a rainbow spectrum. This experiment demonstrates that **sunlight**, which appears white, is actually a composite of seven different colours.

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Activity 15.12

Creating a circular disc (Newton's disc) divided into seven segments painted with the seven rainbow colours. When this disc is rotated rapidly (e.g., fixed on a spinning top or pen refill), the individual colours blend together, and the disc appears to be **whitish**. This experiment demonstrates the reverse process: mixing the seven colours of the spectrum results in white light. This confirms that white light is composed of these seven colours.

In summary, light travels in straight lines and its direction can be changed by reflection from mirrors or refraction through lenses. Plane mirrors form virtual, erect, same-size, laterally inverted images. Curved mirrors (concave and convex) and lenses (convex and concave) form various types of images depending on the object's position. White sunlight is a mixture of seven colours.

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Choose the correct option in questions 11–13

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